Insulating Basement Walls
Best Methods For Insulating Basement Walls
I’ve written several articles on insulating basement walls and evolved some of my methods over time. Basements provide for a very challenging insulation problem. Basements are a challenge because of high moisture levels and cooler temperatures. In this article I’d like to summarize some of my experience with insulating basement walls.
Basement Insulation Reference
Most of my methods are based on information from Building Science.com. They have a really great publication that you can download called: “Renovating Your
Basement” Research Report – 0308, 2003 (revised 2007) by Building Science Corporation. This publication is really concise and full of great information.
Understanding Basement Insulation Problems
I can’t tell you how many finished basements I’ve seen with wood framed walls directly against the concrete and the cavities of the framing filled with fiberglass insulation in direct contact with the concrete foundation walls. This type of details almost always leads to a serious mold problem and obviously a potentially risky health concern for your family.
Most people don’t understand the microscopic composition of concrete and often overlook the significant moisture levels present in concrete. I often have people say to me “the concrete walls are very dry”. I hate to be the bearer of bad news but ALL concrete has significant levels of moisture present in it. If you were to look at concrete under a microscope you’d be surprised to see how many small voids or “pores” exist. Concrete actually acts like a sponge and those pores contain lots of water. So you MUST assume that your concrete is fully saturated and full of water in order to properly insulate your basement walls.
Insulation Materials Are The Key
The key to successfully insulating basement walls is selecting insulating materials that stop moisture movement and prevent mold growth. Basements are the perfect location for foam type insulation products. Cellulose is also an option for basements but it’s not a product that I’ll cover here. I wrote Insulating A Basement with Cellulose which uses both foam and cellulose to insulate a basement for a Energy Star Home.
Spray foam is probably the best possible insulation material for basements and damp areas. Closed cell spray foam is perfect for locking “out” the water vapor that inherintely wants to migrate from your damp basement walls to your finished basement rooms. Spray foam offers several benefits including additional structural stability, great vapor barrier, easily covers pipes, wires and other utilities and it provides exception R values. The problems with spray foam are it’s substantial price tag (although this will get better with time) and it’s a very messy task. However, if you can afford the cost and mess then this is the ultimate insulation product.
The next best basement insulating method is using foam board products. This is the method that we use most often and it’s the method that most DIY folks can handle. It’s also quite a bit cheaper than spray foam so it’s an attractive alternative. There are lots of foam board products on the market so I suggest you read Foam Board Insulation Types and R Values for a quick education on the different products available.
The key to using foam board is choosing the correct thickness and sealing it properly to create an effective vapor barrier. If you’re just going to use foam board then you’ll most likely need 2 to 4 inches in thickness depending on local energy code requirements. I recommend sealing all the joints with Tyvek (or similar) house wrap tape. You can also use “Great Stuff” spray foam in a can to seal around all your utilities and also along the bottom of the foam board. For more information on using foam board I recommend you read How To Insulate Basement Walls with Polystyrene Insulation.
Hybrid Foam & Fiberglass Insulation
The last method that I’ve used is a hybrid system of foam board and fiberglass. This method is the least expensive yet I believe it will perform well in basements that don’t have visual signs of water infiltration. This method should NOT be used if you have a history of water, even small amounts.
In this method you’ll be installing a layer of foam board, sealing it as noted above. Then you’ll frame a wall (wood or steel as I don’t believe there is much difference) in front of the foam board. Finally you’ll install fiberglass insulation in the stud cavities. Again the key here is to come up with the proper R value based on local energy codes. Most references say that you should NOT use a vapor barrier over the fiberglass insulation in this method. I’m not sure if I agree with that but it’s certainly a gray area.
There have been tons of questions about whether or not to use a vapor barrier. The discussion is complicated so we wrote another article about the topic. Please check out: Vapor Barriers For Basement Insulation.
Basement Insulation Summary
The bottom line is not all basements are acceptable for finished space. Too often people try and “force” a finished basement when it’s not practicle due to flooding and water problems. All of these methods assume you don’t have any serious water problems in your basement. The key is using foam insulation effectively to separate the damp concrete or block from framing and insulation products that promote mold growth. I hope this article will help you decide which basement insulation method is best for your home.
Do you know if insulating the basement walls with foam board cuts the humidity level? I run my dehumidifier throughout the summer and was wondering if insulation would help cut the humidity in the basement. I’m planning on insulating anyway (time to take advantage of tax credits), but was wondering if there is an additional benefit.
@ J. – Using foam to insulate the walls and properly sealing the insulation will definitely cut down on the humidity levels.
I have an older basement but do not have an issue with water. Humidity is the only issue, and insulating the walls should fix that. Then would finish the walls afterward with the stud wall and fiberglass. should I use the paperbacked fiberglass for this application?? I’m hearing varied answers on this. I have a low ceiling so I’m just going to spray paint everything above. How should I seal around the top and the bottom of the finished walls?? I slo have a small window to deal with. How do insulate and frame around this and ensure airtight??
@ Rob A. – You’ll want to run the foam board up all the way tight to the bottom side of the framing/floor above. Be sure to seal it all with spray foam (Great Stuff in a can works very well ). The vapor barrier issue is one that really depends on the moisture levels (in my opinion). If you’re basement is really damp then I’d skip the paper faced fiberglass and focus on sealing the foam REALLY well.
This is great info. I live in NJ. How thick would the foam board need to be??
Then how thick should the unfaced fiberglass be for insulating between the studs??
I also have a small window to deal with. How do I insulate and frame around this and ensure airtight??
@ Rob A. – If you’re basement is really damp I’d use at least 1-1/2 inches of foam board. I’m not sure what the energy code requires in NJ but you’d want a minimum of 3 inches of fiberglass after that. If you’re basement is REALLY humid you might want to just install 2 inches of foam and neglect using the fiberglass.
The window isn’t that big of a deal. When you frame your wall frame an opening that’s larger than the window. The dimensions depend on how you’ll finish the opening. You can then create a wood or drywall return back to the window sash.
I was wondering what you opinion was for using spray foam in an attic?
Our medium term goal is to finish the space, take advantage of the rebates now and insulate it. However, I’ve read and heard conflicting reports on using foam directly on the roof.
@ Jeff – There are lots of conflicting reports on this topic because of ventilation. However, the longer the debate goes on and the more installations of spray foam in attics has been happening I’ve bought into this idea. If you spray a sufficient amount of foam directly against the roof, the roof becomes insulated enough that ice dam issues go away. I say if you can afford the spray foam go for it.
hi, i’m looking to properly refinish my basement and i have 2 questions. my basement doesnt suffer from water problems its pretty dry.
right now i’m thinking of placing 1.5″ thick R7 pink owens corning rigid insulation panels right agains the concrete walls. i will tape the seams with the tyvek tape. on top of these panels i will put 2.5″ metal studs. between the studs i will put R13 3.5″ thick unfaced insulation. then on top of all that i’m thinking of putting 1/2″ paperless drywall. this is all in hopes of achieving a moisture free and mold free basement. i understand that i will have to use a dehumidifier regardless.
1. is use of a vapor barrier recommended against the concrete walls? or is my approach ok?
2. am i missing anything vital in my approach? any suggestions are welcome.
3. can someone recommend blown in foam insulation like great stuff that comes in a bigger package?
@ Smash – The thickness really depends on where you’re located. Your approach seems solid. Just be sure your metal studs are not exposed to cold air that can cause condensation problems. If you install the foam board properly and seal it well then there is no need for a vapor barrier. Good luck.
After reading your article and the building science article a few questions came to mind. Since it is very apparent that concrete basement walls change in moisture level based on seasonal changes every basement not having an outside membrane (older houses) will have moisture issues. If the walls are properly sealed from the interior space then how can a dehumidifer help this issue in-between the concrete and finished wall? Has anyone ever examined using a dehumidifer to maintain low moisture in-between the concrete and finished wall (wall out 2-3 inches, moisture resistant batts, vapor barrier with sealed joints) so that mold and odour do not occur? This is where all moisture issues originate in basements. Would using this method simply cause the concrete on the interior to wick moisture from the outside to maintain an equalibrium in terms of vapor pressures. Using this method of treatment would give the consumer the best of both worlds. The intake and return for the dehumidifer would both be sealed into the wall and the unit itself could be located in a utility room etc out fo site but still accessible to empty the pan etc. Let me know what your feelings are on a setup of this nature.
@ Jason – Your concept certainly sounds good. It really comes down to finding a solution that is reasonably easy to build. Even with properly sealed walls the slab itself creates a tremendous amount of moisture. I personally am not worried about moisture between the concrete and a vapor barrier such as 2 inch thick foam. Mold needs a food source in order to grow so it seems like a small problem to me. Definitely an interesting idea though.
Todd, another quick question….
i’m looking to put a bathroom in the basement. one of the walls of the bathroom will be against the concrete founation. what would you recommend as far as spacing between concete and bathroom wall and how if at all should this area be insulated?
@ Smash – I would insulate the bathroom the same way. Just be sure that you seal the foam really well. I typically put the framing up against the foam.
I have an old bungalow in Chicago and I’m planning to “partially” finish the basement. Based on other sources I have read and combining your information, I am planning to adhere foam board direct to the existing basement walls, tape & seal joints, then adhere mold resistant drywall direct to the foam board. No vapor barriers. I am also skipping any framing on the exterior walls, as I have no need for structural support or utilities (electric). Does this sound reasonable? What adhesives would you recommend for both against the existing walls and for the drywall to the foam board? Lastly, any reason for leaving a gap at the floor?
Appreciate any input!
@ Brian – Thanks for the compliment. First off be sure you use probably 2 inches of foam seeing that you’re in Chicago. Most home centers sell foam panel adhesive that will adhere it to concrete/block. The trick is to put the adhesive on the panel, let it set for a few mins until it tacks up, then adhere it to the wall. Unfortunately the adhesive doesn’t always hold it over extended periods of time. That’s why framing the walls is so nice, it helps hold the foam in place. I’d rather see you install the foam, then attached 1×3 pine strapping to the surface with a Hilti Gun (or similar). Then you can just screw the drywall to that. The gap is important to keep any moisture that might get on the floor from wicking up into the drywall.
Thanks Todd! Got more information from your article and you than the dozen plus “experts” at the big box home improvement centers.
As far as the framing advice, I might try to minimize the approach primarily due to limitations for allowable thickness in which I can make the wall due to existing piping systems (which I’m not about to relocate for cost reasons). Bottom line is that I can not go any thicker than 2 inch overall.
I’m thinking of directly attaching a horizontal 2×4 or 2×6 to the upper portion of the basement wall, say 12 to 18 inches below the ceiling, but remaining above the exterior grade line. This would act as my drywall securement for the “extended period” as you recommend. This in addition to glueing the drywall to the foam insulation would hold the drywall in place. I would glue 1-1/2 inch insulation above and below the 2x and seal all gaps. Sound reasonable? SHould I put anything between the 2x and the wall?
@ Brian – This approach may work ok. I think it’s a bit optimistic to think the foam and drywall will hold tight to the wall below. What is your plan at the bottom of the wall?
One important point I failed to mention, this specific portion of the basement wall is only about 12 feet in width. Other surrounding walls will be framed in front of the foam.
As far as the bottom, I plan to run the foam as tight as possible to the floor then fill any voids with spray foam. The drywall will be cut short an inch or so from the floor.
Maybe an alternative would be to place a treated 2x at the floor?
Brian – Is this going to be a finished living space? You might want to go with a slightly thinner foam, 1-1/2″, 3/4″ strapping shot to the concrete, then 1/2″ drywall.
It will be a kids playroom, so semi-living.
My luck thus far with shooting anything into the old concrete, floor or wall, ends up making a divit more than anything. It’s an old aggregate with river rock type stone that seem to be extremely hard.
If I do go the route of the strapping, I assume these should be set on a treated baseplate?
@ Brian – What type of pipes are on that wall? Can you send me a photo?
Pipes are 80+ year old schedule 80 steel horizontal steam supply and condensate return mains suspended at the basement ceiling. They supply the risers throughout the house, one of which is nearby this wall. Pipes are 3 and 2 inch diameter. They are far too costly to relocate for the partially finished basement project. If they were copper hot water pipes, I’d consider relocating, but steam is much more involved. Plus, the ceiling has a lath & plaster finish, so my hopes is to retain this as much as possible.
@ Brian – I only asked because I was curious if you could build in front of them and leave an access hole for inspection.
Ah. Unfortunately that wouldn’t be possible. The return pipe is 2 inches away from the basement wall (hence the 2 inch maximum new wall thickness) and the supply pipe about a foot or so away. Inconveniently located for creating new insulated walls.
Thanks again for all your recommendations! I plan to begin after the Holidays.
Have a great Holiday!
First of all i would like to say that this site is filled with great information and will be visiting back every so often to see the new post. Ok now to the nitty gritty. I have been researching about finishing basements for some tme now and have only seen pictures, posts and comments about drying in solid concrete basement wall construction… What about Concrete Block walls? Can the same techniques be used? Right now my basement is a square hole dug out of the ground with earth on all sides that has a floating slab floor with a drain channel around the perimeter to a corner point where a drain is. This is a 50 year old house, effloresence is apparant throughout the basement. In the two years that i have lived their i have never seen the drain channel filled with water. Water is present near the drain but never standing… Is the cinder block wall more porus than the slab wall my fear is that the hollow cors in the block are filled with water and i would need to go abvout drying in my own basement a different way. Does anyone have any thought on this matter?
House is located in central North Carolina.
@ Michael – Thanks for the kind words. Block walls for are really the same animal when it comes to how they perform as a foundation. They may be slightly more porous but the fact is concrete and block both hold a tremendous amount of water. The details that we’ve shown will work for both concrete and block walls. The reality is you can never fully dry out concrete or block and that’s why it’s so important to carefully seal it off.
I appreciate your response and will post again with the results. Thank you.
Todd, is spray-on foam insulation a good idea in a basement with laid-up stone walls and dirt floor? This basement is always damp but has had no pools or moisture buildup excepting when we had a federal disaster-class flood in 1996. The house is civil war-era. — Lester
@ Lester – Spray foam is a great application for field stone foundations. The foam helps control moisture and also helps keep stones stabilized and in place.
Awsome site. Full of detail and explanation. My question is this…..I am looking to re-do my finished lower level. Currently, it is paneling. I pulled a section out to see what i might find and as i expected, it is a shotty half — job. There are 1×2 furring strips that were glued to the cinder block foundation. No insulation. looks like they stapled saran wrap to the furring strips for a vapor barrier. The house was built in the sixty’s and is a tri level. I want to insulate and drywall. I’ll have to use furring strips instead of 2×4 because i will lose too much space. I will use foam panel insulation as you recommend but am not sure on the thickness. The new wall can be a max of 4″ total. I want to achieve the best R value i can with this project. What do you recommend as far as material and will i need to screw the furring strips to the wall or can i assemble a frame? Are there any alternatives to anchoring at the bottom. Im a little leary of putting any type of hole in the wall below the surface line. I’m fine with doing that above. Currently no moisture problems. House is in Northwest Indiana. Your input would be greatly appreciated.
@ Jason – In your case I’d recommend 2 inches of foam board. I would install 2 inches of foam, then install 1×3 pine strapping (furring) over the foam, either shooting long masonry nails or screwing them, then install drywall. This will give you less than 4 inches.
Hello, I recently moved into a 40 year old ranch style home. It had a couple leaks, one from around the septic exit pipe and one from a crack below a window. The weaping tile around the house was plugged due a very large willow tree located near the house. I have cut down the willow tree, excavetd around the house, replaced all weaping tile and added new 3/4″ crushed stone(except under garage, where I cleaned out the plugged tile. Then went on to wrap the whole exterier concrete wall with “blueskin” (60mil poly with a tar coating on one side), and then wrapped that with delta membrane. Since then i have had no leaks except one little one in the cold cellar probably due to a crack in the step above and there is no vent in their. I now want to insulate my interior walls to an R24 value. I was planning on putting a 5/8″ foamboard against the interior wall, then a 1-inch air space followed by 2/6 framing with 6 bats of fiberglass type insulatoin. I noticed you comment “This method should NOT be used if you have a history of water, even small amounts. I live in Southern Ontario, please help, should I being doing it this way or is there a better way????
@ STU – First off I’m glad to see you fixed the water problems! Most people take the easy way and ignore that!
I think you’re ok to insulate your basement, however, I would highly recommend you use a min of 2″ of foam board, followed by the fiberglass of sufficient thickness to get the desired R value.
Todd thanks for the reply, this site is very helpfull. Why do you reccomend 2 inch ove the 5/8. With THE 2 my wall will be very thick by the time iget it too R24. ALo I am concerned with all the mixed advice on vapour barriers and moisture build up with this combonation method. What do you think, thanks. Stu
@ STU – You really need 1.5 to 2 inches of foam to create a vapor barrier to keep all that moisture in the foundation from drying towards the finished space and getting trapped in the fiberglass. If you can afford the 2 inches, then be sure you don’t use a vapor barrier on the fiberglass, the painted drywall makes a good vapor inhibitor which will keep vapor from moving into the cavity but also let it escape. The 2″ of foam will keep that surface warm so any vapor from the finished side won’t condensate. Hope that helps.
Wow, so your saying with two inches of foam board directly glued against the concrete wall will keep the moisture whithin the concrete and it wont try and penetrate the foam board. I heard foam board is capable of retaining water. Also with this methed should their be any air space between the foamboard and the bats of insulation?
@ STU – You need to use a closed cell foam product. An air space isn’t absolutely necessary but it sure is nice if you can spare the space.
One last thing . . in this case are you saying a vapour barrier would cause more harm than good if used on the inside of the framing? I was not planning on drywalling it right away and may even used pannelling later on. Thanks again. Stu
@ STU – If you’re not going to drywall then you should use a kraft faced insulation and be sure to seal all the seams well.
Wont that 60mil poly I stuck all around the exterior walls help keep moisture our, reducing the need for 1.5-2 inch foamboard. Also is it ok to just stuff the 6 inch bats into the header? Thanks
@ STU – The 60Mil will keep water from entering in from the outside, however, concrete has a TREMENDOUS amount of water in it’s microstructure already, that water and water vapor wants to go somewhere. Not sure what header you’re talking about.
Todd, I see you mention that you usually frame right up to the foam board. But in my case you siad that leaving an air space would be a good Idea. Do you think if I do not frame up to foam that over time the glue adhearing the foam to the concrete will break down and the the foam board come loose? Also I have already purchased 5/8 foam board, should I try and return it or can I just double it up on the wall? Stu
@ STU – The air space is a great idea, however, sometimes it’s not practical. It really depends on how well the foam board sticks to the wall. Yesterday at a trade show I saw some spray foam adhesive from DOW that is supposed to work VERY well between concrete and foam. I’ll try to track down the actual name and write about it at some point.
The double layer of 5/8″ will probably cost more and be difficult to work with…i’d try to return it.
Thanks, Ill do that then, Ill check back later to see if you put the name of that stuff on here. Stu
First off I want to let you know what a great website you have. It has a lot of quality information. I do have a few questions for you regarding finishing my basement. I have 3/4″ furring strips up right now. So I was going to install that foam board insulation betweeen the furring strips. Do you recommend using 1/2″ foam insulation board so I have a 1/4″ gap for airspace? Or do you recommend using a 3/4″ insulation and putting the drywall up firm against the foam insulation board? I plan on putting up a plastic vapor barrier. Do I need to put up tape between the furring strips and insulation, or can I just staple plastic to the furring strips to block out moisture. What are you recommendations? I do not get water in my basement. It has cylinderblock walls. One more thing. Do you recommend installing insulation all the way to the ground or should I leave a little space at the bottom.
@ Bry – Thanks for the nice compliment.
1. Much of what I’ll say next is predicated on living in a cold region.
2. If you place insulation between the furring strips and then screw drywall to the furring strips the strips will still be cold. Once moist/damp air get’s behind the drywall and hits either the strip or even the plastic it will condensate.
3. So, the idea is you need to install sufficient insulation to stop that from happening. You also need to install sufficient insulation or a vapor barrier to keep the water that’s in the blocks (no matter how dry the blocks appear to be they are FULL of water) from getting into both the wood and drywall.
4. Having said all that I really do believe you need a minimum of 1-1/2″ of foam board (preferably 2″) installed on the block before the drywall, seams taped well.
5. Insulation should go all the way to the floor.
Yes Todd it definitely makes sense. Thanks for the great input Todd. It will be very helpful. I live in Ohio, so do you still recommend the 2″ insulation now knowing where I live? When I put the insulation between the furring strip do you think I should tape, or caulk it to seal in space between the insulation board and the furring strip? If tape do you think duct tape would work ok? Do you think it is necessary to take this step if I install a plastic vapor barrier and staple it to the furring strips. I also want your opinion on if I should leave a space between the insulation board and the drywall. Thanks in advance.
@ Bry – I would install 2″ foam directly over the furring strips, use long screws and another layer of furring strips to hold the foam in place, then screw the drywall to the new furring strips. Be sure to tape all the seams with Tyvek tape.
Which method do you think is better installing the insulation directly to the block walls, or just putting it over the furring strips? Any certain type of insulation foam board you recommend for basement walls? I am not sure of all the different types like Extruded Polystyrene, Expanded Polystyrene, Polyisocyanurate Rigid Foam, T+g Foamular. Do these really matter, or should I just base my decision on the R-Value?
@ Bry – Just be sure to get a closed cell foam, EPS is fine. I think it’s best to apply it to the wall so you have uniform thickness throughout.
Todd thanks for sharing all your knowledge. All your info was greatly appreciated.
Well I thought I was done with questions. Guess not. Is it a good idea to get foil backed foam board if it is about the same price as a non foil back foam board? If so I am assuming you face the foil side towards the drywall. Is that the case?
Yes and yes! :) Good luck.
I had a contractor come out and give me a quote on the drywall, just to see how much he would charge compared to what I could get the materials for. He said if I were you I would just put the drywall up over the paneling and not even put insulation? I said really? Do you think that is a good idea? Do you have a general idea if I put up insulation how much warmer it might possibly make it. A few degrees? I am going to install carpet also. The paneling does not really feel cold, but the basement floor is pretty cold. Currently it is about 10 degrees colder in the basement now compared to the first floor. Then I started thinking by the time I cut all this insulation and piece it together between the furring strip is it really going to insulate that well and make a noticable difference? Is it really worth the extra money to insulate?
@ Bry – These are questions you need to evaluate yourself. Insulated basements are healthier, warmer, more valuable and frankly better for the environment. Paneling is one thing, paper faced drywall in that environment is another….I myself wouldn’t recommend it.
I had also thought about just putting up insulation over the paneling I currently have down there. Then drywalling. Which way do you thing would be the better way to insulate. Insulate right over the paneling or take down the paneling and install between the furring strips? The furring strips are ran horizontal, so I will have to make a lot of cuts. Will the insulation lose a lot of its R-Value if I cut the insulation to put it between the furring strips. Putting the foam board over the paneling would be the easiest, but is it as well insulated that way? Which method do you think would insulate better between the 2 options I listed? Thanks again for all your advise. It is greatly appreciated.
@ Bry – Either method will insulate just as well as the other. The only issue really is water/mold. You may create a situation where the water vapor is trapped between the paneling and concrete wall.
In your ‘Hybrid Foam and Fiberglass’ method you state – “This method should NOT be used if you have a history of water, even small amounts”.
Why is this the case ? Doesn’t the foam board sealed with tape and spray foam adequately protect against water issues? Can you elaborate?
– I have started using this method, and do have a history of water (a problem I THINK i’ve fixed by recently excavating and repairing a section of weeping tile…but so far only one winter of monitoring to gage success). I live in Toronto Area.
@ JeffB – I made the statement because some people finish their basements without fixing the water problem (sounds like you have). Then they are upset when the basement floods a bit and all that fiberglass insulation starts to mold. I guess I’m trying to point out the fact that not all basements are sufficiently dry enough to finish.
Todd – thanks for the clarification. Last item for follow up; I am thinking about Roxul product in place of Fiberglass (mineral product which is SUPPOSED to be water repelant). Any experience with this and/or comment?
@ JeffB – I haven’t used it but lots of folks do and I haven’t heard anything negative. I say go for it.
Excellent website. I’m finishing a basement that has a new addition with 1 1/2 inch thick foam on the exterior walls that are in the ground. The 2-3 feet of the foundation that is above ground does not have anything on the exterior walls. Do you still recommend 1 1/2 inches of foam on the interior followed by insulation between the studs? (I’m in NJ) Also, there are french drains in the new basement with plastic running between the wall and the floor to collect any water coming down the walls. Otherwise the floor is poured concrete and runs right up to the plastic at the base of the wall. Would I install the foam over the plastic and seal it against the floor? Thanks in advance.
@ Chris – I would just install 1-1/2″ of foam seeing you live in NJ and there is some insulation on the outside. Not exactly sure of the floor drain detail…maybe you could email me a photo?
Very interesting site, thank you for all the info. I haven’t seen any mention of using DryLock or any other type of waterproofing on the concrete block. Is this a bad idea? Also, has anyone used or have any knowledge on magnesium board or MgO board instead of sheetrock. There are 2 places that advertise their use one has an all in one system with insulation between 2 pieces of magboard and the other site sells just straight sheet of magboard that can be installed like sheetrock. I have websites if interested but don’t want to advertise for them, just want thoughts. Thanks
Oh yes, there is another question I forgot to ask. In my basement there is one wall that has a sill in it about 3/4 the way up the wall. the sill is about 4 inches deep. (Why is this, were there 2 different size blocks used?) If I were to use the foam board from the floor up, would I need to fill in the gap of 4 inches wide and approximately 2 feet high, or can I just go straight up from the floor with the insulation board and leave an air gap behind the insulation board?
@ George – No issues with the space, just run the foam up over it. I haven’t heard about the MgO board….maybe you could share the link with us so we can read about it. Thanks.
Please let me know what you think.
Looks pretty cool…any idea on cost and availability?
Magnesiacore will ship to NJ for about $60 for 12mm 4 x 8 sheet. Alligator’s product is much more expensive and runs about $225 for a 4 x 8 panel, but their panel includes 2 sheets of mgo board surrounding a closed cell insulation. The installation of the product is simple. I didn’t get a quote from magnumbp. I think the market is still tight with few suppliers and that price will come down in the future as more start to offer the product.
I plan to finish the basement and I am concerned with the concrete walls that I have had some water leaking problems in the past due to cracks on the walls. I repaired the cracks from the interior using a product called Sanitred which seems to have done the job. The contractor plans to first put a French drain in, then I plan to have him use the foam board insulation on the walls, then frame and insulate using R11 fiberglass insulation. My question is, if the walls have a leak sometime in the future, is the foam insulation that is glued to the wall going to be a problem? Do we need to put something on the wall like plastic sheeting first so any potential water will run down and into drain without touching the foam? There are also small amounts of efflorescence present throughout the basement on the bottom of the walls. I live in NJ.
RJ – If you use at least 1-1/2″ of foam it will act as a vapor barrier which means you really don’t need plastic.
A follow up to George’s question about running the foam to the top of the 3/4 concrete wall w/ sill on top. What if the upper wall is studded, insulated (fg) with vapor barrier. Can I still run the foam board up to the joist or will be vapor barrier be an issue. Should I pull the barrier off?
John – I wouldn’t run it all the way up. I would stop it where there framing starts.
should i install the interior walls in my basement on the concrete floor with atreated bottom plate then install my subfloor tiles, or should i install my subfloor tiles throughout the basement then build my interior walls. your website is great.
Matt – If you do the sub-floor first…your bottom plate will be high enough to attach base trim, otherwise you need to add blocking. Also, unless the walls are load bearing..there’s no reason to frame them first.
one more question, when i put these interior walls up how should i mount them to the floor? if they were put up directly on top of concrete i would secure them to the concrete, now the concrete is covered. should i still mount them all the way down to the concrete? if i do mount them all the way down to the conrete will this leave a avenue for moisture to penetrate through my subfloor.
Matt – I would just use a good quality construction adhesive and a few screws.
I have a question slightly off topic. I’ll be finishing off a room, in an above grade concrete block construction, with a room above it built with standard framing. I’ve read the above thread and it sounds like there is a lot to consider, at least in insulating a basement. What method(s) should I use in insulating an above ground space built with concrete block? It is located in Lewisburg, PA
Jerry – The same methods really apply to above grade block construction. I would either use spray foam or rigid foam as your insulating material.
I am building a fish room in my basement. This room will be kept at 87 deg. I live in PA. i would like to efficiently insulate this rooms walls,ceilings and water proof the floor. I was thinking of using 2″ thick Polyisocyanurate sheets against the cinder block wall then furring out on top with 1×3″ On top of that i would use reflectix creating an air space between the foam sheet and reflectixinc material. Is this the proper way to insulate a heated area in a cold and damp environment?
Kenny – If you’re going to keep the basement at that temperature you may want to consider additional thickness of foam. The cost of foam will be far less than heating that space over time. I’m not really sure why you want to install the reflectix? If you use a foil faced polyiso I think you’d achieve the same result.
Thanks Todd. If i install 2″ thick polyiso directly to the wall with adhesive should i leave an air space of 3/4″ between the next layer. would it make sense to use the same thickness or maybe the outer layer can be 1″thick?
Kenny – No airspace required. The thickness really comes down to cost now vs cost later. You should look at the total R value and make a decision.
last question Todd… Does the polyiso sheets with the silver face act as a radiant barrier? Since i am heating this room to a high temp how can i keep the heat in. Is 4″ (2 layers of 2″) going to keep the heat in or would i have to install a radiant barrier on top of this insulation?
Thanks for all you help!
Kenny – the foil face will help as a radiant barrier. 4″ of polyiso will give you an R value of 28 to 32 which is great for walls. If you can afford 4″ then you’ll have a very well insulated room.
just reading up on the different types of sheeting insulation material available. Some are saying not to use polyiso material below grade since it is not water proof. would it make sense to use a water proof insulation board against the wall and a foil faced polyiso towards the interior of the room? I know i said “this is my last question” i just want to do it right.
Kenny – This is what you should use: http://building.dow.com/na/en/products/insulation/supertuffr.htm
Thank you very much Todd. You have been a great help.
I have a quick question for basement insulation. My house is old (1940’s) from Seattle. I was not sure what I m suppose to do for putting insulation; there is approx 3 – 4 feet high concrete block from base, then wood stud above… Should I put form board on concrete block, then figerglass insulation between wood stud and vapor barrier?? After done insulation; just put 7 feet stud for the wall?? I m trying to find any articles but didnt find,,, hope you could help me Thanks!\\\
Terry M – Sounds like you’re on the right path. Do you have photos of it you can email? If so it would be easier to tell exactly with photos. This detail isn’t uncommon for walk-out style basements. There are typically two options.
1. Insulate lower 3-4 with foam board including the top of the block wall. Seal all joints with tape and/or spray foam.
2. Insulate wall cavity with fiberglass, install proper vapor barrier and seal very well with tape.
3. Now you can either frame a short knee wall and then cap it and tie it into the higher wall or you can just frame a new wall in front of the block all the way to the ceiling.
Do you feel DryLock is necessary to use before gluing the foam boards to basement walls? Will PL300 adhere properly to it?
Joe – I really don’t think it’s necessary but then again it can’t hurt. PL300 is a good adhesive but it takes quite awhile to “set” and makes it difficult to get the foam to stay in place. We’ve been using the Great Stuff Pro foam adhesive lately with very good results.
Great site and thanks for your help. Would welcome your guidance on whether or not to: (1) place the XPS foam insulation directly on the foundation walls or to leave a 1/2-2 inch gap to channel occasional water seepage due to cracks in the wall; (2) place the foam directly on the header joist — i.e. from base plate to the ceiling; and (3) Build in place additional features to channel occasional seepage water away from the insulated foundation walls. Details follow.
I live in a 1930’s corner row house in Washington DC with a cement basement and brick foundation walls. In the basement is about 7 feet tall with 8 inch header joists/joists. The header joists that about the foundation wall jut out about 2 inches from the wall. The basement is damp and along one wall we occasionally (every 3 years or so) get some water seepage coming through cracks in the foundation wall/bricks that leads to minor basement flooding (less then 1/16 an inch) which I quickly vacuum up. I have sealed the cracks from the inside with hydrostatic cement, but we still occasionally get water seepage when it rains a lot or a neighbors pipes bust.
Given the above and based on your guidance/blog, on the walls that have water seepage I was planning to leave a 2 inch gag between the wall and the XPS foam. I was also going to run the foam all the way from the bottom plate and over the header joist. I was also going to rest the foam on a 3/4 inch composite board bottom plate in which I was going to rout 1/8 to 1/4 channels every 4 feet or so to allow any flooding water to seep out into the basement where I can then vacuum it up. I would welcome your advice on the three questions/issued presented above. Thanks.
Rob – Sounds like you’ve given this some thought. Your plan seems reasonable but I’m wondering if you might want to consider having an interior french drain installed around the perimeter first. Then any water that seeps in can be collected by the drain before it runs out onto your floor. If you leave the gap be sure you secure the foam really well. I recommend using Great Stuff Pro for the adhesive. It works very well.
Thanks. Am considering putting in a french drain. Should I run the foam all the way up to the wood ceiling (i.e. over the wooden header joist)?
Rob – The more you insulate the better. It’s always a matter of what’s practical.
I was wondering if formaldehyde free fiberglass that JM sells is any better than the standard stuff? They claim its more mold resistant and fairs better in basements then the old stuff. I’m in the process of building a home theater and originally planned to stay with all xps. However fiberglass is said to help improve acoustics.
Not sure if its worth the mold risk?
Chris – I’d stay away from fiberglass unless you’ve install foam first as a barrier. All fiberglass insulation is now formaldehyde free as far as I know..sounds like marketing glory :)
I have xps against the foundation already. My basement is also partially above ground as I mentioned in another question to you before. I put 2 inch xps in these studded above foundation areas. I’m just wondering if r10 will be enough for chicago area.
John Mansville makes it out like their insulation is the only formaldehyde free stuff on the market. They also say it has some sort of epa approved mold inhibitor. Then again the history of mold problems with fiberglass insulation deters me.
Many of the fiberglass companies sell formaldehyde free. Mold inhibitors are just that, inhibitors, it’s certainly better than none but my experience is that they don’t work well. Not mention once the insulation gets wet it loses it’s R value and will likely not dry out.
R10 is probably at the low end. However, it really depends on the energy code in your area. Some of them are based on the entire house as a whole, so if you’ve got good insulation and windows you might be able to get away with a lower value in the basement.
Thanks for the tips. Unfortunately, I had already finished half the basement before I found it. So my question is about what I already have done. I painted the concrete with a thick concrete sealer paint then proceded with a wood stud frame and fiberglass insulation. Considering I sealed the concrete do you think I will have a problem?
I don’t have a damp basement and water runs away from the house pretty good. I was aware of the porous concrete and water vapor. Do I need to tear it down to avoid a mold problem?
I am also painting the floor to completely seal the concrete.
Thanks for the help
Todd – The chances of having a problem are quite high if you live in a cold environment. The reality is that the concrete wall will be cold, any moist air that gets in the wall will hit that surface and condense, the fiberglass will trap it, and mold will grow. At the very least I would not recommend fiberglass against that wall.
I live in a split foyer, in northern Virginia. The basement, with the exception of the laundry area in finished. I have two rooms, a bathroom and a recreation room in the basement. The two rooms and and bathroom have drywall on them and are as livable as the rooms I have upstairs. The rec room is surrounded by a thin paneling, which apparently are hung directly on a 1×2″ stud installed by wide face against the wall. The rec room always has a musky smell and, if we leave the house for 1-2 days, upon return we smell something like a dead animal (this has been going on for a long time, so it is not actually that!). Anyhow, I’ve been looking at removing the paneling and installing drywall. The contractors, whom I have seen, have suggested applying a hydraulic cement against the concrete blocks to seal any minor issues (once the paneling is removed), applying some kind of cement sealant (like drylok) on the concrete, followed by installation of plastic sheeting, frame, (may be insulation – although not strongly suggested), and dry wall…
What do you think about drylok or other liquid applicators on the net, which claim to penetrate concrete up to 4 inches to close all the pores?
My concern and problem is with humidity and not much with heat / cold. I recently installed a dehumidifier, which has remedied the smell issue, but dries the air too much and is too noisy.
PS. The previous owner installed a french system. But, I have no water sipping in and the pumps have not had to work in almost 12 years I have lived here.
Thank you for your advise.
Joe – Sounds like you’ve done some of your homework already. Obviously moisture is an issue after hearing about your dehumidifier experiment.
Drylok and other similar products use some interesting science for such claims. While I have no doubt that those products can seal the surface of concrete I’m almost certain they cannot penetrate 4 inches into concrete just based on the pore structure of concrete. At any rate they can make a difference in water vapor, just don’t expect them to lock out water that’s under hydrostatic pressure because it’s not going to happen.
So, if this were my basement and I lived in VA I would recommend installing an inch of closed cell foam board followed by strapping and drywall. The foam makes a good vapor barrier (you would use Drylok plus the foam) and it would help insulate the space during extreme temperature swings. This may seem like overkill but it’s money well spent considering today’s energy prices and the possibility of mold/mildew.
Thank you, Todd.
When you say “closed cell foam board”, what type of board are you referring to? Lowes have many types of board that appear to match, but prices vary greatly and I am not sure whether the difference is simply the extra cost of brand recognition.
For your application any of those will work. They all have different pros and cons, but mostly you’re dealing with the differences between higher R values and some with radiant barrier foil. Take your pick based on what you want to spend.
Thank you! ;-)
This is a great site, Todd. I’ve read quite a few comments, and I apologize if my questions have been covered and I missed them.
I’ve read “Basement Insulation Systems”, a research report, (C)2002 by Building Sciences Corporation. Their diagrams indicate foil-faced polyisocyanurate rigid foam (or XPS) for insulating the rim joists and the top of the concrete foundation wall, then XPS rigid foam on the concrete wall face.
Would you see any objection to using the foil-faced isocyanurate entirely…including on the walls? The basis of this question is price and availability. My product sourcing research concludes that the isocyan is actually cheaper than equivalent thickness XPS, and has better availability in my area (southeastern Mass). I’m planning to install 2″ foam over the concrete, covered by a 2×3 partition of drywall…with no further insulation and no vapor barrier.
Secondly, what is your preferred method/product for “sticking” the foam to the concrete walls? I believe I read that you use Great Stuff Pro as an adhesive for XPS foam (and also to seal seams and edges against wood/concrete/penetrations). Will this product work as well on foil-faced polyisocyanurate? And if not, what alternative product would you recommend?
Gregg – You did some great research….and your proposal is right on track. The Great Stuff Pro will work nicely. Good luck.
Thanks, Todd. I’ll advise with a progress report.
Nice links, helpful. Also clear that Todd is reading the recent science/findings on basement insulation, which are in conflict with the old (incorrect).
The best single resource I’ve found is Building Science. It echoes what I’ve seen here by Todd (though I haven’t read all his comments!).
The foam people want, by the way, is (if you’re not doing spray) _unfaced extruded_. Unfaced means no membrane on either side, and extruded is either DOW blue foam at lowes or pink owen cornings at home depot (there are other brands, too). These are the ones to use, don’t go thicker than 2″ (R10).
I don’t think drylok is a great idea, I just haven’t read much to support it. If you go with building science they are saying you want whatever moisture is coming into the basement to dry inwards (it can slowly through foam as long as it’s unfaced and not too thick and the correct type). If you use drylok you’re trying to basically trap it in the concrete. Well it will eventually get through, whether the concrete cracks or not. Also the stuff has a lifespan to it in any case.
As always if you have major water ingress you need to solve it first. Don’t bother finishing the basement until it’s addressed.
“Would you see any objection to using the foil-faced isocyanurate entirely…including on the walls?” Yes. check on its permeability. I think you cannot have drying to the interior with that, which is why Building Science isn’t recommending it. If they are for the rim joists it’s because they are above grade and can dry to the exterior.
I tried great stuff for “glue” a little. I think it worked ok but PL300 is specifically made for it (get it at lowes or home depot). The stuff sticks like mad and will break the foam before it pulls off it. I used liberal amounts of it (something like one tube every two sheets (2X8′ sheets)–probably over kill! I mainly used Great Stuff around rim joists and a nice trick I came up with was buying some flexible tubing, then taping it to a stick to make it rigid. Poke a can of great stuff’s shorter straw into this and you have a range extender. I needed that extra range around some of the joist areas I just could not get a hand into otherwise.
I’d probably spray foam joists if I had to do it again. It is an incredible pain and hassle to do cutting all those sheets, gluing them, foaming around, it took me forever! I’m not a professional, but I read a lot about this before doing my basement. Way too much, in fact ;)
hey Todd, a previous post almost anwered my question, but then veered off. i was wondering how to tie in the rigid foam board with the french drain(drain tiles) that have been installed around the interior perimiter of my basement. it is 100 yr old brick two flat in chicago where i am fininishing the basement. i beleive all water seapage issues have been delt with from the outside( tuckpointing, caulking ect.) but want to make sure that anything i am doing to finish will work with the existing drain tiles if there is a problem in the future.
so do you tuck the foam board into the edge? do you use the great stuff pro on the bottom, or leave it open?
thank you for all your great advice so far!
Mike – Each situation is a bit different. The key is getting the foam down as low on the wall as possible yet leaving a space at the bottom for water to escape (assuming there is a chance for water, your history suggests that). The idea is to stop water from getting trapped.
so just to be clear, foam board as low as possible with no adhesive or great stuff filler along the bottom edge. i have sourced the 1″ dow xps blue board tonge and grove that seems like it would fit snuggly inside the plastic shell of the drain tile system that is exposed as there was 3/4″ furing strips tucked in there before. any water that pentrates the foundation or condensates of the back of the xps should settle into the drain tile? right?
also what do you suggest for the top of the xps? gs pro all along the top edge to seal it?
Mike – Sounds good.
We just had our walk-out basement spray foamed approx 1 – 1 1/2 inches. We then put up fiberglass insulation. We now have ice forming between the spray foam and the fiberglass insulation on our exterior walls. Should we replace the fiberglass insulation with beathable foam board? We have a dehumidifier running also. Why is this happening? Any help is greatly appreciated!!
Emilie – Do you happen to know if the contractor used open or closed cell foam? Does the fiberglass insulation have a vapor barrier?
The contractor used closed cell foam and the fiberglass insulation does not have a vapor barrier.
Emilie – One inch of foam just doesn’t have sufficient R value to keep it’s own surface from being so cool that vapor ridden air won’t come in contact with it and condense. Without a vapor barrier over the poly the damp air in your basement passes through it, hits the cold foam and condenses and freezes. Bad detail.
So would spraying another inch of spray foam on the walls help? Or would it be better to dry out the walls, remove the fiberglass insulation, add a closed cell foam board over the spray foam and then add the fiberglass insulation with a vapor barrier over the fiberglass insulation?
What is the best way to solve this problem and make sure that the ice does not return?
Emilie – The problem is keeping water vapor from the conditioned space from getting into the stud bay where it hits the cold foam. That can be accomplished by adding a proper vapor barrier to the warm side. You need to dry it all out first though.
I am in the process of getting some quotes for basement remodeling. I am in northeast ohio. So it gets fairly cold up here. Do you think if I i can afford spray foam, I should go with that over alternative methods? I have done extensive research but everyone has their own opinion on this matter. Our basement had some cracks a while back that were sealed by the builder using cement. One of the contractors wants to put fiberglass and other has mentioned spray foam. I am really unsure.
Rick – If you can afford spray foam use it. But beware, there are contractors trying to sell open cell foam for basements because it’s cheaper. DO NOT go that route. Also, DO NOT go for fiberglass unless you have a minimum of 1-1/2″ of XPS installed first.
The contractors that are coming out are saying they are just going to stud the walls and put fiberglass on the bare concrete wall. However I believe this is inadequate. Is there a brand of spray foam you prefer over another? I am definitely going to ask for closed cell if I go that route.
How about the floor of the basement? Any recommendations on that?
Rick – Any contractor that proposed fiberglass against concrete should be escorted to your front door with a huge no thank you.
Honestly the brand of foam isn’t important. What you need to do is find a local reputable foam contractor that has lots of references. Have an honest discussion that you only want to consider closed cell foam.
As far as floors you might want to read our Basement Floor Insulation article.
my basement stairs is against a wall that is getting wet, i can’t move the stairs. should i dig it out on the outside, locate the problem, re-parge the wall, then stick some xps onto it? will this keep the stairs from molding, then put xps 1 1/2 thick on all walls, adhered with great stuff?
Can you be more specific? What type of foundation? where are you going to dig? etc?
My basement that I am finishing has a poured foundation that makes up the bottom 40″ of wall height, then the rest is your typical framing. I plan on putting up wood paneling on the bottom section, and dry wall on the upper section. The foundation makes a convenient shelf for social gatherings, and I would like to keep that aspect in the finished version, just in a more polished look.
I would like to put up the 1.5″ or possibly 2″ foam board which ever is required (manchester, NH area) with strapping over it. The problem being that I would really like to add electrical outlets in the wall, which would require cutting holes in the foam board in order to fit the boxes inside the wall. Is it possible to seal around the electrical boxes and still have a properly insulated wall? I want to avoid building a traditional studded wall on the lower half since it would make the shelf nearly 8″ wide, looks awkward. Thoughts or suggestions?
Here in NH you’ll likely have to do an energy code calculation to determine the thickness. It’s based on looking at the entire home including insulation, windows, doors and HVAC equipment.
I think you can likely run the wiring in the 3/4″ space created by the strapping. The you can probably use shallow rough-in boxes.
I live in Michigan and have a poured basement. Reading the articles above, I am likely to use the foam board, while using spray foam in the cavities between the floor joists. My question is the following: I plan on building a 2×4 wall, but no planning on insulating the wall. If I use the proper thickness of foam board, do I need to insulate between the wall studs?
Not at all. You only need additional insulation if you need to get to a certain R value for energy code requirements. Good luck.
First of all thanks for all the great info, really helpful but it brings up a lot of questions. I live in South Carolina just below Charlotte, NC and am planning on finishing my basement in the very near future. A few years back we had a small water problem; however in preperation for finishing the basement, we have piped all the downspouts away from the house and regraded/concreted the driveway leading to the basement. We have had no problems since. We plan to have a bathroom, bedroom and family/game room. We presently have a garage door that I plan to remove,brick up and install a 3′ exterior door and window in the space. About 6′ of my basement is underground on all sides except where the existing garage door is of course. I am planning to install 3/4″ blue foam board, tape the seams and build a 2 x 4 stud wall directly in front of the blue board. I then plan on finishing the walls with moisture/mold resistant drywall. Do you recommend installing fiberglass insulation w/o paper in the 2 x 4 stud wall? I see where you recommend leaving a 1/2″ to 1″ space between wall and foam board, how critical is it to do this? How do you recommend handling the concrete floor?
Craig – Thanks for the kind words.
Before talking about insulation I’d like to point one thing out. If you do build a basement bedroom be SURE you have proper egress from that bedroom. You mentioned a door from the basement and that’s a good start but unless that door is in the bedroom you will also have to have a way out of the bedroom. Be sure you speak with your local building code official about that.
So for the basement I see one big problem with your plan. 3/4″ thick blue board insulation isn’t sufficient to stop moisture from getting through it and into your stud wall where it will likely cause mold. You need a minimum of 1-1/2″ of closed cell foam to stop the moisture. I recommend you also read the following articles:
Those should answer your questions pretty well.
I have looked over the articles that you mentioned and they make sense. Another question: What is the purpose of pt sleepers on the floor? In some other reading I have been doing, they recommend the insulation foam board with the 3/4″ T&G fastened through the foam board with Tapcon screws into the concrete floor.
You can do it either way. It really comes down to personal preference and ease of installation. Using tapcons to secure the plywood is very time consuming. Some folks like to “shoot” down the PT sleepers with powder shots then screw or nail down the plywood. It’s up to you.
Todd I have been thinking some more, which is usually a dangerous thing. Would it be okay on the walls to install a 6 mill poly vapor barrier, then install the 3/4″ foam board with a 2 x 4 stud wall with fiberglass insulation w/o paper. I realize that we will not have as much R value, but should that take of any moisture issues?
I don’t recommend that approach. The concrete is at around 50 degrees, the plastic will then be that temperature. Now if you have damp air hit the plastic it will condense, now you’ll have water that can move back through the 3/4″ foam and get into the wall framing. I just really don’t think it’s a good idea.
Thanks for the info Todd. I came across this product the other night and wondered if you have had any experience with it? http://www.insofast.com It looks like it incorporates the methods that you recommend.
Looks very cool! Is it available near you? If so it appears to do exactly what you want.
Not sure. I have asked for availability and pricing.
Hello Todd. I am in the process of having a dewatering trench system put in my lower level of my tri-level. bottom section goes 3ft below grade. A vapor barrier with mold inhibitor added to it will be placed on the block foundation wall to force any incoming water from the blocks to go down into the trench system. I will be framing with 2x4s after the system has been put in. The vapor barrier being put in will be thick like a pool liner. Any suggestions on how I should insulate and if another vapor barrier is necessary? Thank you.
Does the vapor barrier extend all the way up the foundation? What are the specs on the vapor barrier for vapor transmission? I would still insulate the wall with closed cell foam before framing the new walls.
The barrier will extend all the way up the block foundation. I am not sure on the specs for the transmission. Any need for vapor barrier between the drywall and the foam?
Only if the current barrier isn’t sufficient.
thank you very much for your response. The barrier is supposedly very thick
What are your thoughts on silicate sealers?
I live in a 40 year old home in MA. with poured concrete walls. I have a french drain and sump.
I was thinking of using the silicant based sealer, than following the rest of your tips for the walls and floors.
Do you think using the sealer is neccessary?
Sealers to me are a good 2nd line of defense much like the old “belt and suspenders” saying. Certainly won’t hurt and not likely to make a huge difference. If you have the time and money I say go for it.
Regarding using polyiso foam board with the foil face, do you suggest using foil tape for the seams? Or is tyvek or similar housewrap tape acceptable?
I just found the answer to this question elsewhere on this site–you said to use foil duct tape.
Another question I have is about basement flooring over concrete:
I have a low ceiling and do not want to sacrifice height by using a subfloor like DriCore. I want to use a self-leveling mix, then install an engineered wood product. Is there an “in between” layer I should consider, or just float the engineered wood right on top of the floor?
If height is of concern then you’ll be stuck installing the floor over the concrete.
I’ve read all of the great posts here, but am still unsure of something.
I’m sure this will sound redundant, but My concern is trapping moisture behind the foam in my block foundation that is mostly underground, thus weakening the concrete over time. I keep hearing that the walls must dry to the inside,but how is that possible when covered with foam? Is it semi-permeable? I taped a square plastic sheet on the wall for a day, and sure enough, there was moisture trapped behind it. Normally there is no visible dampness on the walls. I want to use your method of insulating, but I do not want to trap moisture behind the foam.I have a friend who framed his walls an inch away from the foundation and just placed foam between the studs and it isn’t really sealed tight, so it will breathe.( he was told) He also was told to use a dehumidifier.
I know this is a lot of questions,so I really appreciate any help.
Great website you have,
The answer lies in some advanced understanding of concrete material science. So let me see if I can reassure your concerns. I have a masters degree in structural engineering and my graduate research was concrete material science so this is a topic that I’m very knowledgeable about.
1. Concrete is a building material that’s created by mixing cement (powder) with sand, stone and WATER. Think of concrete like mixing a cake. When you mix water with flour a chemical reaction takes place to create “cake” well concrete is the same way. Water reacts with cement to form the “glue” that holds the sand and stone together.
2. So water is actually GOOD for concrete. Water DOES NOT break down concrete at all. In fact, the strongest concrete is concrete that lives it’s live in water all the time.
3. Trapping moisture behind the foam and against the concrete is not bad. Obviously you don’t want to trap or hide major water leaks but the moisture issue isn’t a problem at all.
Thanks for the reply, and yes it does make sense. That is the best explanation thus far that I’ve heard. As you know,There are varying opinions on the subject,which is why one can be easily confused, i.e. I keep hearing that the old way of covering the block with plastic sheeting is a bad idea because it traps moisture behind it. Another example is that the wall must breathe to allow the block walls to dry to the inside. So you can see how a layman like myself would have concerns about doing this the correct way.
Ok, just one quick question, how about installing plastic sheeting on the warm side of 3/4 in foam board to help keep the moisture away from the studs?
First off the plastic is an issue for another reason.
If you put plastic against the concrete it’s basically against 50 degree temperature. Now if moisture is in the wall framing and hits the 50 degree plastic it will condense. That moisture is from the house side!
If you use at least 1-1/2″ thick closed cell (XPS) foam it actually acts like a vapor barrier (see: http://www.homeconstructionimprovement.com/vapor-barriers-for-basement-insulation/) so no need for plastic.
On the subject of moisture in the concrete, I’ve been told that if there is efflorescence on the block that in time the wall can be weakened. Not true?
Not really true. Efflorescence is leaching of lime out of the concrete and/or mortar. It would take so long for all the lime to leach out and cause structural damage of any significance. What it does do is tell you that water is penetrating the material.
We are currently finishing our basement. It is a traditional basement in that most of it is under ground and we have had no water issues. My contractor and I never discussed insulating the walls as I thought that was just a given. We did discuss the ceiling as we are concerned about the noise from the first floor, so we will have insulation on the ceiling. The other day, I came home and noticed that the drywall was up, but there was no insulation on the walls. I asked him about this and he said that he never insulates basements as he is concerned about mold. He placed the studs about an inch and a half off the concrete block to avoid direct contact with the walls. He said that he has never received a complaint about a cold basement. He also did a neighbor’s house and they say that the basement is not cold. I just worry that in the winter months it will be very cold. I would hate to have him take it down if it really does not matter – the drywall was glued down and screwed, so taking this down will be a big job and a waste of drywall. What are your thoughts?
For starters what state do you live in? Most states with cold climates now have energy codes that require by law new spaces to be insulated including basements. Did he pull a permit?
We live in Ohio and we wanted him to pull a permit, but he said that the code does not require one. He spoke with the city/county and they said that an existing structure does not need a permit. I will check this myself and make sure that this is true.
I’d certainly look into that. Almost ALL remodeling projects require a permit. There are VERY few exceptions for things like windows, and small repairs. I’m almost 100% certain that’s not the case for a major renovation like a basement. I will also tell you that most all northern states now have some sort of energy code.
I will look into the permit on Monday. Let’s assume that the contractor was telling us the truth and we don’t need a permit, if having the walls uninsulated a big deal?
Yes/No. Over time un-insulated basement walls will create enough moisture behind the finished wall surface to cause mold/mildew problems. This won’t happen over night but it will over time.
Probably more important is your energy use. While it might not feel “cold” in your neighbors basement it’s certainly not as energy efficient as it could be. The average temperature of a foundation wall below grade is in the 55 degree F range. So basically you have drywall less than a foot from a constant 55 degree heat sink. I’m not sure about you but heating costs continue to rise every year and I have no desire to heat the outside!
You are in a situation that is probably quite clouded if your contract didn’t specify insulation. However, if a permit is required and your builder ignored the fact and lied about it then I’d say he’s certainly liable for misleading you and not performing his duties appropriately. Permits are a good thing, they not only protect us all from potentially dangerous situations but they ensure that contractors at least follow minimum standards.
This web site is amazing and it is obvious that you know your stuff. I have read several of your articles and blog discussions and changed my entire thought on how to properly finish the basement (more money of course) but have not found answers to some of my questions.
– I live in western NY (Rochester)
– My basement is dry and have never had any water issues
– I do not have a sump pump but do have a draing (I guess they call this a gravity drain)
– I have a floating foundation with a 1” gap along the edge with rock to my drain tile
I am planning on using the following:
– Dow 2″ x 8′ x 4′ Extruded Polystyrene Insulated Sheathing insulated against the concrete block wall
– I will seal the top as you have recommended in your other articles
– I am going to use the DRIcore subfloor (http://www.dricore.com)
– I will most likely use a 2×3 wall or a 2×4 sideways with a PT bottom plate sitting on top of the DRIcore subfloor
– I will use unfaced installation in the wall cavity depending on my local code for R-value
1. Is the Dow 2″ x 8′ x 4′ Extruded Polystyrene the best board to use or should I look into another type of board like Polyisocyanurate, Expanded Polystyrene?
2. The DRIcore recommends a 1″ air space between the subfloor panel and the foundation wall. I will make the 1″ air space between the subfloor and the 2″ insulated board. Is this ok?
3. What about my 1″ foundation drain around my concrete floor? You recommend the insulation board to be snug on the floor and ceiling joists. If I do this I will essentially plug the drain if any water does get into the basement (washer, water heater, etc…). The DRIcore has raised channels to allow for the concrete to breath and keep the water from being trapped. I would want the water from under the DRIcore to be able to flow to the existing drain on the edge of my floating foundation. What is the best option
a. Run the DOW blue board from the floor to ceiling leaving a 1/4″ gap for the drain?
b. Run the DOW blue board tight from the floor to ceiling leaving?
c. Notch the bottom of the DOW blue board so that there is 1/4 gap from the floor and a 1/4 gap from the concrete floor. This means the DOW would reset against the rock in the drain and only be 3/4″ inside the drain channel. This would still allow any water from the floor to drain into the drain channel
4. I am only finishing 2/3 of the basement. Should I install drywall or some other product to the end of the studded wall that is exposed to the unfinished side? If the blue board is flammable and I want to keep the moisture from entering my 1″ air space it seems likely, I would need to close the two exposed ends.
Joe – Thanks for the kind words. I hope you’ll consider signing up for my FREE Weekly Newsletter as well.
Here are my thoughts on your questions.
1. I actually prefer XPS in basements. Polyiso that’s covered in foil isn’t a great choice against concrete so the XPS is the best bang for the buck in my opinion.
2. That’s fine. I think if it were my home I’d put it fairly close to the foam. Foam will move and deform if the floor expands…the floor certainly won’t buckle.
3. I personally don’t think you’ll “plug” off the drain. If water gets in there it can still get behind the foam and into the drain. I wouldn’t worry about it.
4. Foam board needs to be covered with a thermal barrier by all building codes that I know of. So yes, you should drywall over any exposed foam board.
Good luck and let me know if you have other questions.
Thank you for your assistance. I have some follow up questions.
Item 1: I thought that was just the blue board since it didn’t have the foil. Just to be sure it is this STYROFOAM 2″ x 48″ x 96″ Insulated Sheathing board that I want.
Item 2: If I put the DRIcore against the foam wouldn’t that essentially plug the airspace under the floor? I read in some of your articles that a small airspace between the blue board and the wall was good or is that only if I use fiberglass insulation? I assumed the airspace in the floor that DRIcore has would be good to share the same airspace created in the wall.
Polyiso and XPS are not the same thing. XPS is typically sold by DOW (Blue Board) and Owens Corning (Pink Board).
The air space certainly won’t hurt. Frankly even if it’s tight against the blue board I’m not sure the floor itself will be completely air tight.
I am going to use 2” XPS on the interior basement wall. I am digging one side of my house to install an egress and drain. Should I put 2” or thinner XPS on the outside of the block foundation before backfilling?
If you’re already insulating on the inside I’d keep it all on the inside. Doesn’t really matter though.
I live in southern Ontario, Canada. I have a 23 year old basement with some walls fully insulated with tar paper, fiberglass, 6 mil vapor barier. I know your preference for XPS. The basement seems very dry with little ‘blacking’ of fiberglass. Will the tarpaper insulate the fiberglass from any wall moisture?
Some walls are half insulated in this manner since they are below ground.
Do you support this method of insulation (tarpaper, fiberglass, vapor barrier)? If so, can I build another wall right in front of it and just drywall it to avoid the half-wall insulation hastle? Should I add some blue board under the half insulated below ground wall so it’s covered completely?
Or… should I rip it all out and glue blue board to concrete, then build frame with drywall?
Bill – Your situation is not that uncommon. Older homes have been insulated fairly successfully that way in the past. However, It’s not an approach that I would suggest. Obviously if there are no signs of mold then you probably can keep the existing in place. If it were my home I’d remove it and use foam. It’s really up to you, for me once I’ve got an old wall opened up I like to upgrade it with the best approach available.
Todd, I have a new er construction we just studded the basement wall and held the studs out 3/4″ from the poured wall. For most of the basement we have 4 foot poured walls (Daylight basement)- the rest is 9 foot poured wall. Above the 4 foot walls is sheeted and has stone on the outside, these are 2×6 construction and filled with spray foam, then the stud wall is infront of that and 3/4″ from the concrete below. My intention was to put fiberglass insulation in the 2×4 wall and then drywall, hoping that the 3/4″ space would allow the concrete to dry to the inside (fiberglass would not touch concrete). Here are my options–
1) Fiberglass batt
2) spray foam the concrete lower wall and space between concrete and studs
3)spray foam a 1.5 inch layer on concrete(flash foam?) then put up batts
I dont want to put spray foam on the upper portion and seal in the 2×6’s and I’m not sure you can do foam over foam. The basement is very dry (built above grade and tons of $$ spent on fill), any shrinkage cracks were sealed this year (3 of them).
Hope this makes sense, I will spend the money on the right system, just don’t want a disaster.
By the way, I live in Michigan- great site!!
I would highly recommend spray foam. You can either “flash” foam (thick enough with closed cell to stop moisture) then use fiberglass or you can go all foam. I would NOT recommend option #1.
Thanks for the fantastic information.
I have some questions that the “professionals” at my local HD and Lowe’s just muddy:
1. On exterior basement walls that just need furring strips, will the furring strips (2×2) be able to hold drywall in place AND keep the XPS in place?
2. On exterior basement walls that will have 2×4 wood walls built, should I use 1/2″ or 1″ XPS, then the wall, then the R-13 fiberglass batts, or is that overkill? I am seriously having a tough time understanding why 2″ XPS alone (R10) is better than a 2×4 wall with R-13 batts in it (with a 1/2″ to 1″ gap between the wall and block, of course). Then again, I wonder why both are necessary.
3. How do you hold the XPS in place if you have a 2×4 wall in front of it with a gap?
Forgot to mention: I live in Pittsburgh, with a one year-old, Energy Star certified home. I am just finishing my basement, ripped off the white backed fiberglass stuff they throw on the walls, and am looking at concrete blocks.
Thanks for the kind words.
Before going any further you’ll want to read a few of my other articles so you really understand the situation with regard to insulating foundation walls.
So before we start you need to understand that in your part of the Country you’ll need a minimum of 1-1/2″ of closed cell foam board like a DOW or Owens Corning board. This is to create a vapor barrier between the concrete and insulation. After the 1-1/2″ you can then add more R value with either more foam board or fiberglass insulation.
If you decide to use strapping then I’d use 1×3 boards. This will require fasteners long enough to penetrate the strapping, the foam and then sufficient embedment in the concrete.
If you do the studded wall (preferred method) the foam board can be cut and set against the wall (small amount of adhesive if necessary) and then the framed wall keeps the foam in place.
I hope this helps. Good luck.
I read all three articles you posted. I just have three more follow-up questions:
1. I’ve read that the XPS panels form a vapor barrier, but do allow water to slowly get through to evaporate. If that is the case, wouldn’t the fiberglass eventually get wet anyhow? If that isn’t true, how would the water get out of the wall?
2. Gotcha’ with the 1x3s. How do you attach drywall to 1x3s, though?
3. If I put the stud wall right up against the foam, that would hold the foam in place. You recommend leaving a gap between the stud wall and the foam panels, though. Why? If I left a 1/2″ gap between the foam and the stud wall, what would happen if the XPS adhesive eventually let go?
Thanks again! Great help!
1. The best information available right now is that 1-1/2″ of XPS forms a vapor barrier. This is not to say that a small amount of water vapor won’t get through it but it’s very minimal. Water doesn’t necessarily have to get out of the wall. Water is always present in concrete. However, most of the time the water vapor will dry to the outside (above grade).
2. Drywall screws.
3. Leaving a space just helps let things breath which is a good thing! Even if the foam comes off the wall in a few places it’s not going to be in full contact.
Hi again Todd,
I followed your advice and installed XPS- used globs of PL 300 -I also had to use mechanical fasteners until glue dried – is there a preferred way to attach it to poured concrete wall?
Not really. Frankly the foam board doesn’t need to be super tight and perfectly touching the wall. The real key is sealing the seams and keeping out moisture from the stud wall. The stud wall will keep it from falling down. The key is framing the wall laying down….set up the foam…..seal it….then tip the wall up. Being ready for all of it at once makes the difference.
Situation: Finishing basement, 2 inch XPS fastened to concrete wall with insulated 3-1/2 inch stud wall next to it. Please go over (again probably?) what to do when you can’t install the XPS boards on the concrete when you have an obstruction or pipe running parallel and lengthwise along the top of the concrete foundation wall. I will obviously need to build a soffit around it when buildiing my inner stud wall, but how do you create your continuous insulating barrier before putting up the insulated interior stud wall? I can still fit 2 inch xps to insulate the rim joist and top surface of concrete wall where the main plate of the house rests. Thanks.
In construction nothing ends up perfect and this is one of those situations. I’d run the foam up tight to the drain pipe, seal to the pipe, then start again on the other side. Obviously this isn’t perfect but it’s far better than re-running your drain pipes.
Is the foam board sealed at the bottome to the concrete floor? Why/why not?
Also, are all foam boards created equal when it comes to air quality in the living space? Do some give off harmful toxins over time?
Whether or not you seal the bottom is up to tons of debate. It really depends: if you have a really “dry” basement I’d seal th bottom. If you have a basement prone to water leaks, high humidity, etc then I’d leave the bottom unsealed.
Each type is different in both materials and how it’s made. You’ll have to research each type.
we have an old basement in the mid west (1925), no signs of current moisture problems in there, I taped foil to the floor and walls and there was no condensation on it after a two weeks.
However there was/is old paint on the wall, that I have been removing. Some of the concrete was crumbling in areas, nothing major. The plan is to scrape all loose paint off and repair crumbling areas with hydraulic cement.
I then plan on doing the following:
dryloc on external walls
1 inch foam on floor
2 inch foam board glued to external walls
tape and use great stuff to fill in gaps and top and bottom
then add 2 X 4’s to frame wall and put up drywall
1)Is dryloc a waste of time in this case?
2)If I use dryloc, what should I use to glue foam to painted walls?
3) The walls are a little uneven, will this matter behind the foam, that is if it has small air pockets where the wall is not smooth?
Thank you for taking the time to answer all these questions, really appreciate it.
Might actually use 1.5 inch foam on walls to save money, if it is good.
Dryloc has it’s place. however, it’s not going to stop water from pushing through cracks. It acts as a vapor barrier to some extent until it cracks. I tell people go ahead and use it if you like a belt and suspenders approach :)
Great stuff foam works great.
Uneven doesn’t matter at all.
Thank you Todd,
I do not mind the expense of the dryloc, it is just the extra time it will take. If it does not really add any value to the process I will most likely skip it.
Good news about the uneven walls not making a difference. Should I use Lepage PL 300 or great stuff to stick the foam to the walls? I will be using great stuff at the top and bottom and around any pipes etc.
Do you have a preference on foam? I was looking at foamular f150/f250 from Dow. None of it is cheap but I did not want to pick a product if there is a better one around the same price.
Seems like 2 inches on the walls will be worth the additional expense, based on your comments in here.
If you need to decide between 1-1/2″ of foam and dryloc the answer is simple…more foam!
In most situations you don’t need much adhesive especially if you cut the sheets to fit tight top and bottom. I prefer the foam.
No preference on the foam other than for basements the best bang for buck is XPS foam, blue or pink.
last question, I have read in your posts above and other articles that closed cell spray foam is even better still than foam boards.
I was looking at the spray foam kit here for about $650, it would be enough to put 1.5 inches on the walls.
Are these DIY kits all they are cracked up to be?
To do the same area in 2 inch foam and drylok will cost about $450, so it is not a huge jump up to do spray foam directly on the scraped and prepped concrete wall.
Jack – There are VERY few of those kits even sold now. In most cases it’s against the law to sell them to home owners direct. But more importantly they are hard to use and unless you’ve been trained it’s likely that there will be tons of waste and frustration. So….either go with foam board or pay a spray foam guy.
Foam it is then.
What do you know about CertainTeed DryRight insulation for using in basement wall studs? Is using XPS against the concrete still necessary if we go with that?
I would HIGHLY recommend you still use foam board. That product would be great as additional insulation in the cavity after a proper insulation/vapor barrier.
I live in the Kansas City area–so cold winters, hot summers. We live in a 12 year old house with an unfinished basement–it is heated. We are thinking of finishing part of the basement. When we purchased the house 2 years ago, the half of the basement had already been framed out on the exterior walls. There is a thick plastic vapor barrier between the concrete and the wood studs. We have no water issues aside from humidity. In the summer I have noticed condensation on the vapor barrier–the side that faces the concrete. I bought a dehumidifier and that seemed to help. My question for you is how can we best insulate our basement for finishing if the framing is already up? Will the thick plastic barrier be sufficient since it is between the concrete and the wood framing or would we be better off to tear out the framing and start over with the foam board? Any suggestion you have are greatly appreciated.
I would highly recommend using foam. However, all is not lost. It shouldn’t be very hard to slide the framing forward (or lay it down), install the foam, then move the walls back. Forgive me if you already know this but: Walls are typically framed on the floor. All the studs are nailed to the top and bottom plates, then the wall is stood up. At that point nails and/or screws are driven into the concrete on the bottom. Additional nails are installed at the top. This hold the wall in place. To move the wall you simply cut those fasteners and reverse what was previously done.
If the walls are already wired than it becomes a bit more cumbersome.
Thank you for your response. Yes, it does make sense and I had been wondering if that were the way to go, but having never done a project like this I wasn’t sure how to about it and it is good to get a professional’s opinion. Makes my planning process so much easier!
I live in southern Ontario and am looking at finishing a currently unfinished area of my basement. I looked at spray foam but was instantly put off by the costs. I also will be doing it in small stages as space, time & finances permit.
Anyway, so I was planning on creating a hybrid system and have been doing a lot of research on various levels of vapour retardation.
If I use 1/2″ pink foam, then technically this is only a Class 2 vapour retarder (1.1 perms). I was hoping that with this against the blocks, some 3 1/2″ roxul against that in a 2×4 framed wall and vapour barrier on top of that, that I will NOT then have the ‘double vapour barrier’ situation.
I can see how you would not want to do this if you were using 1-2″ foam, but surely it’s OK with the 1/2 stuff.
The reason I want to use the foam as it seems to me to be the nicest way to keep the roxul & studs away from the block wall in my basement. I could use building paper, but with the foam, I get a little extra R so it seems like a good idea to me.
As for signs of water damage, it’s hard to say. There *is* a line along the bottom edge of the blocks, but it looks like it may have come from the inside. Maybe there was a basement flood. Could be worth running some concrete sealant along the seam between the block wall and the floor?
Tom – I’m not sure I agree with your approach. You’re missing a key component here. If you use 1/2″ or 3/4″ foam, it’s certainly going to be a semi-permeable vapor barrier, which will allow moisture from the block wall to enter the wall cavity. The trouble is that if that happens, the thin layer of foam has such a low R value that it’s surface temperature will likely be close to the dew point, which means the vapor will condensate. Now you’ll have liquid water trapped in the wall cavity.
The only way that I recommend doing this is with at least 1-1/2″ (2″ preferably). Anything short of that I don’t recommend.
So I have been going back and forth on this for a few days now. It seems that the industry itself is not even on the same page when it comes to this.
One of the problems I was having is that I am only doing 1 12ft wall right now and we did not budget for any basement reno just yet so I was trying to keep costs down. In the end, I am only doing a 12ft wall, so I decided it’s a good ‘practice’ wall for me.
As such, I just got back from Lowes with 7 pieces of 2″ thick Styrofoam :)
Thanks for your advice!
Tom – You’re not wrong. A vast majority of contractors still struggle with this issue. The real authority out there is BuildingScience.com
They have spent lots of money doing research and testing. Those guys have it figured out. Good luck!
So I have my pieces cut and resting in place for now. It’s already warmed up the space. Anyway, I need to do my rim joist before gluing the panels in place as I am low on space. I have 2 questions about the general rim joist area. I have read your rim joist article, but thought I should ask in here so you have my context.
1. Can I use my 2″ Styrofoam offcuts? I realise that will only give me R10, so maybe I can use 2 layers, or put Roxul in front. The PolyISO with foil covering only come in 4’x8′ sheets which will not fit in my car so that’s not an easy option for me right now.
2. My sill plate(Not sure if that’s the right word, it’s the one sitting on the block wall with the rim joist on top of it) has a sheet of poly between it and the block foundation wall. I am not sure how/where to tie in the poly. I will assume I should cap the top of the basement wall with foam board (Only 1.5″ will fit under the blocking) and cap around the sill plate with foam too. Should the poly sheet be against the wood as far as possible, or sealed to the concrete wall or just sealed between any foam-foam join? This really needs a picture to explain well, so let me know if you need one.
1. Yes by all means. You can use multiple layers or you can use some Roxul.
2. The poly doesn’t really matter. Just be sure you create a continuous layer of foam from the wall, up over the top and tied into the rim joist insulation. Seal all the joints really well.
Do i have to use the thick foamboard. Can’t i use the 3\4 thick foamboard and then put up 2×4 walls or do i need the thickness for the barrier.
Yes…you need the thicker foam. I’d suggest reading the following:
Thanks Todd…your articles are by far the most informative, well-reasoned, and easiest to understand that I’ve found in my research on my little basement insulation project.
Is there a trick for taping the inside corners? I just can’t seem to get the tape tucked in far enough to make a nice sharp corner. I suppose I could caulk instead but the picture up near the top shows that you’ve taped your corner.
one more question…when putting up stud walls, putting them up where the wall runs perpendicular to the floor joists above is easy. What do you do when the joists run parallel and there’s no joist lined up to attach the top plate of the wall to?
Matt – I’m glad you find the site useful. I hope you’ll consider signing up for our FREE Weekly Newsletter for more great home improvement tips, product reviews and even giveaways.
There’s no trick……frankly I end up using two pieces typically. It’s just a hard spot to get to so don’t feel bad.
You need to install backers. Basically small blacks get nailed between joists (in the bay the wall goes in). The blocks are usually spaced 16″ to 24″ on center. They should be flush to the bottom of the adjacent joists.
Thanks for all your good advice which I have used plus the links to Building Science Corporation articles. I still have a few issues to clarify in my basement project which is a retrofit on a 1945 home. Basement is dry, no sign of water or flaking cement on floor or walls.
Walls: I will use the 1″ XPS installing as detailed, frame with 2×4 then use 3.5in glass batts finishing with gypsum. I have seen a vapor barrier (plastic sheet) between the batts and gypsum in some literature. I suspect that is not a good idea as one wants a free flow of air, right?
Floor: This is where I am really confused. I follow the steps from first laying the XPS down and sealing with tape/foam as with the walls. Then Building Science articles mention a sleeper floor made of 2×4 or similar that “floats” followed by a plywood subfloor. Does this just float or is it fixed to the concrete. If it is attached to the concrete, it seems like it just breaks the seal one just made with all the taping, foaming
Tom – 1″ is not sufficient. The minimum that is recommended is 1-1/2″ of foam and 2″ preferably.
This is the best website on basement insulation I have come across!
I have a question based on some info I have read across some other websites (a little information is a dangerous thing). I live in Southern Ontario in a ~60 year old house with block foundation. There are signs of efflorescence on the walls and floor and we have an active sump pump (usually drains 3-4 times per day). However there are no signs of water likage – just moisture. Last year we had closed cell foam insulation (BASF Walltite) profesionally sprayed on the inside walls to a thickness of ~4 inches followed up by a layer of fire retardant material (code) on all exposed walls (about 2/3 of the basement – the other 1/3 is covered up). The basement is now drier and much warmer. My dumb question:
Q: I have read on certain websites that spray foam insulation is a BAD thing. Supposedly capillary action can draw moisture from the wall and get the wooden supports that anchor the house to the block foundation wet. Since the wood is now incased in foam, the wood rots and the house becomes structurally damaged. They even have nasty photos showing rotting wood beams in the foundation. This feels very much like a scare tactic to get you to buy their product and services (basement insulation / water proofing / etc). How much truth is in their analysis? Will spray foaming the walls up to the support beams, structurally damage the house?
Just to clarify, the efflorescence is no longer visible under all the foam. However the floor still shows signs of it – but no signs of any water likage.
Edward – Thanks for your kind words. I hope you’ll consider signing up for our FREE Weekly Newsletter or sharing our site with family and friends.
There’s no doubt that in certain situations spray foam can cause problems. A vast majority of the problems that I’ve either witnessed or read about come from the use of OPEN CELL spray foam. Open cell spray foam can/will absorb water like a sponge and create all kinds of nasty problems.
There are some shady contractors out there pushing open cell foam because it’s MUCH cheaper and they go out there saying we can beat anyones spray foam price. What they don’t tell homeowners is they are using open cell foam.
From my point of view, if the basement is dry (no flooding) and closed cell foam is used, it’s unlikely that enough moisture can wick into the wood to cause problems at least not in a short amount of time. 50 years down the road? Not sure….10 years no way.
Hi, and thanks for an interesting article. We are renovating the partially finished basement in our 1950 ranch in eastern Massachusetts. It was originally finished by the previous owner in (I think) the 1970s.
I removed the wood paneling and found that the walls were framed with 2×3 studs which are offset ~1″ away from the foundation. There is a thick, foil-like paper against the interior of the foundation. I think it is Reflectix(?) There is no other insulation.
The space, which is heated, seems to stay fairly comfortable, though we have not used it as living space yet. We sporadically run a dehumidifier in the summer to keep things relatively dry.
Should we put in additional insulation before putting up drywall, or is the Reflectix sufficient to keep things comfortable? If more, what kind?
Marc – Glad you found the article interesting. First of all if it’s going to be living space then it’s almost certainly required to be insulated by current energy codes. I’m almost certain that MA follows a similar code to what we have here in NH. Having said that you should read the following articles to get a feel for what I’d recommend.
Thanks, Todd. I’m going to start reading those articles ASAP.
Should I leave the reflectix? I can’t seem to figure out what it’s actually for, but when I touch the wall (on a very cold day) it feels fairly warm.
Marc – I’d get rid of it for sure.
We recently purchased a home in Northern VA. The house is 6 years old. The inside basement walls are wrapped with an insultion. Like a big blanket. How do we finish the basement?
Stacy – I highly recommend the method that I’ve shown in the following article. http://www.homeconstructionimprovement.com/how-to-insulate-basement-walls/
So, leave the wrap up and just build the stud wall in front of it?
No. You need to remove the old insulation and start over. Be sure to follow the method outlined in the article.
I am finishing my basement, I used sanitred on the floor and 2ft up the walls the rest of wallhas 3 coats of drylock. I live in northern MI. I then glued 2in. xps directly to block walls and then 1.5in over the 2in. then a 3.5in stud wall. My question is the upper 4ft of my basement is out of the ground, being I have 3.5in. of foam at r17.5 is this enough being its all sealed with tyvek tape and spray foam?I also wonder if leaving my stud space empty would cause condensation or other problems behind the drywall?
Sounds really good Cory. I’d think it’s plenty unless your local code requires more. No condensation issues that I’m aware of.
thank you todd. You seem very knowlegdeable on all of this I have talked to alot of builders and nobody seemes to have a real answer on a dead air space, but to me it would seem to be a conditioned space and with the 3.5in.of foam there should be no transfer of hot to cold. what are your thoughts on this?
The real deal is that the 3.5″ of foam will PREVENT moisture from the house from entering that dead space. Without the moisture from that side, and if the exterior has a decent siding on it, then it really can’t be a problem.
Good luck. Glad I could help.
We are building a new house in kentucky. It will have a partial basement, no walk out. We want to construct it the best that we can since this is the last house we will build! Which do you think is better- ICF or poured walls then insulate with closed cell spray foam? Thanks alot! Love this website!
Heather – Both solutions will work quite well. I would start with getting estimates. It’s likely the ICF will be slightly cheaper. However, ICF won’t be as easy for running electrical and other things. Kind of a toss up in my mind.
Hey Todd, I was about to start my basement project a year ago but it got kicked back. I had done a lot of research but, now I don’t remember it all. I have the xps 3″ board, I live in Greensboro nc, I have 3 walls (front and both sides) that are underground. My plan is to do the rigid foam on the concrete blocked walls, frame the walls and then use the paperbacked insulation between the studs. My question is about the floor. I don’t want to lose ceiling height. What is the best way to approach this? Do I need to do a vapor barrier/retarder on the concrete floor? If so what do you advise for my area?
John – It depends on what you want to use for flooring.
Great sight, it is very helpful! This is Probably a stupid question but, after i install the 2 inches of blue board and build my 2 x 4 walls do i have to use fiberglass insulation? I live in Connecticut and the basement will have baseboard heat. Thanks for the website!
Not at all, it really depends on two things.
– What does your local energy code require?
– What level of comfort do you want? Fuel will likely never get cheaper.
This was a great article. I just moved to a new house and the basement is unfinished. Unfortunately, I don’t know if they had any water issues in the basement other than a downspout being off and a small amount of infiltration. With that s…aid, would the foam board/insulation still be an option? If I do have a slight water issue, what should I install. I know it will definitely help with heating if I can insulate now before winter hits. Its chilly down there now.
The foam board is a good approach. I’d say make sure all your down spouts are in place, be sure the grading around the house is sufficient to push water away from the house and be sure any sump pump or other drainage is working properly.
Hey Todd, I live in Ohio. Basement walls were drylocked and then framed with wood studs 1/2″ from wall. I cannot get 2″ EPS behind studs since ceiling is finished with dry wall in place. Could I put 1/2″EPS behind studs to seperate studs from foundation wall, then put faced fiberglass insulation between studs, and then 1/2″ drywall? Or am I better of to cut 2″ EPS and fit it between the studs and then cover with drywall? Any suggestions would greatly be appreciated. Thanks.
Todd, Or would I be better of sliding the 1/2 inch XPS behind the studs then cut 1.5 or 2 inch XPS to fit between studs and seal any gaps for plumbing or electrical with great stuff? Thanks again for any advise.
This option is best…not great…but better than the others.
The 1/2″ XPS has a visqueen like sheathing on both sides. Do I need to remove this sheathing before applying it to the foundation? Also, could a better option be putting the ½” XPS in the 1/2 inch space behind the studs, tape the seams, and then have dense-packed cellulose blown in to the wall cavity? Or would I still be better off with cutting the 1.5″ XPS and puting it between the studs? Thanks again for the advise. Your site and input you offer is great and much appreciated. Thanks.
We’ve done dense pack cellulose before, but only after installing 1-1/2″ foam board first. You need that insulated vapor barrier first otherwise you’re going to completely saturate the cellulose.
Learned a lot here. I have a new house with walkout basement in SD. On the walkout side the walls are floor to ceiling 2×6, the opposite side they’re all concrete, the sides transition from full height concrete, down to a 4 foot concrete, then down to full height wood framing (2×6). All wood framing has 6 inch insulation and vapor barrier. The wood framing sits on the concrete and there’s a 2 inch ledge of concrete showing in the basement. The concrete is bare.
I have to energy code requirements that I’m aware of. I’ll put 2 inch pink foam board on the concrete. Then build a 2×4 wall with a 1/2 inch gap to the foam board. I figure on unfaced 3.5″ fiberglass in the 2×4 wall, then drywall. Does this sound good or should I change something? I’d like to make this as energy efficient as possible.
that will work fine. Good luck.
Just finished putting the 1/2 inch XPS behind the studs I could not move. Then cut 1.5 inch XPS to fit between studs and sealed any gaps for plumbing or electrical with great stuff. Would you recommend an additional vapor barrier with plastic attached to studs before installing the dry wall? Or would that be a waist of time? I plan on running a dehumidifier on a regular basis. Thanks again for the advise.
Don’t use a vapor barrier…..that would potentially lock moisture into the stud cavity. Good luck.
I live near Philadelphia, Pennsylvania and I would like to finish my basement. I do not believe that there are any moisture concerns; however, I have read your article and others stating that just because I don’t see them that doesn’t mean they dont exist. Every contractor that I have spoken to does not use the foam and most don’t even know what it is. They all recommend a gap from the wall and then faced R13 insulation in the wall. Against their recommendations, I have obtained a permit from my municipality and I would like to do this once and right. By code, I have to install fire blocking behind the new wall and under the floor joist, but also vertically every 10′.
One of my concerns is that the vertical fire stop eliminates the air flow that the contractors rely on in the gap behind the wall to reduce the chance of moisture on the new wall and insulation. Have you written any articles on this topic or are you aware of any?
I have heard that there are fire / flamible / vapor concerns with the spray foam. Do these concerns exist with the rigid foam?
Would you install pressure treated 2x4s every 10′ as the firstop and then 2″ of rigid foam in between?
Thank you for your time and for maintaining the website to spread the word of the right way to prevent moisture.
Chris – That method that so many contractors still use is why I see so many mold problems out there. It’s a horrible detail and one that will hopefully be eliminated eventually through education and better building code inspections.
Firestopping is an issue that is seldom understood correctly. It’s true that the code requires fire blocking every 10′ vertically. In 95% of the situations the 1st floor deck framing creates the fire stop except at penetrations. So my understanding (which I’ve used for years successfully) is there really are no fire stops required other than at penetrations. Now the foam MUST be protected thermally usually by drywall or an approved thermal barrier.
Most foam actually has a fairly high flame spread rating, which means you’ve got to really get it going with a hot fire to have an issue. Most of the “concerns” about foam insulation are “urban” legend.
I’m really not sure where this idea of 10′ horizontally comes from. If that is required for your location then I would do as you suggest, install PT 2x or 1x as a fire stop.
Good luck..hope that helps and I didn’t rant too much.
Hi! Wonderful tips and advice indeed. I made the silly error of finishing off part of my basement last summer with fibreglass (rockwool) insulation direct on the concrete walls, in between the studs then plasterboard (you call it drywall in the US) – with the some PVC Panels. I guess it’s only a year, so hopefully the mould won’t be too bad when I dismantle it in the summer. If only I had found this site before I did the work. Luckily it was only on 2 of the 4 walls – 2 were concrete – the other 2 walls were added as separators – and the room is only 4m x 3m ! I now know what to do for the rest of the basement! I live in France, and the boards I think that are comparable are XPS (PANNEAU POLYSTYRÈNE EXTRUDÉ) – I will use 40mm (around 1.6 inches thick) – which should be enough – as I’m not make it a 100% living space, just a better workshop area. So, you still recommend fibreglass insulation after the rigid foam? I know you black-eyed it – but as long as the foam boards are first against the concrete – the fibreglass can be used right? But, to avoid all fibreglass insulation could I simply double the foam boards to come level with the framing, so there is no fibreglass at all? Do the first layer directly on the wall to cover entire surface with 20mm rigid boards, then do the framing as 20mm wood, then inbetween cut some 20mm rigid boards to fit ? Then I can put the plaster board (drywall) onto that? It sounds like an ok idea to me? I would appreciate your thoughts?
Alessandro – Welcome to our site. Rockwool is not bad by itself. However, it can still hold moisture which will lead to mold on the framing and any finishes. So you really should replace it.
You can certainly use two layers of foam board. Its’ really a matter of cost.
Thanks for the advice. Given that the space won’t be heated, but more so to clean it up and reduce the cold air coming in and going up to the rooms above (my basement ceiling is concrete blocks, not wooden joists like you guys in the USA) (http://campobassi.com/images/photo3.jpg).
Are those foam panels suitable for the basement ceiling as well? the basement is only 216cm high (floor to ceiling) so having some insulation at 20mm, plus dry wall 13mm – means only 3.3cm or approx. would be lost in height of the basement. Is that ok?
For the walls – if I were to use treated studs – could I fix them to the walls, and glue the foam panels in between them ? I wouldn’t need double the panels that way and I could put the dry wall over that ? I know it would only be 20mm of insulation, but it would still be better than bare concrete walls or should I go with the 2 x 20mm thick ? Insulation is only as good as your weakest point, right? I mean if there are other parts not well insulated, the cold air will find a way in despite the other areas insulated….?
Thanks again! Best resource on the net for basement work, and most consistent too.
Sorry to hassle you Todd, but if I use the foam boards for the insulation, I assume that I don’t need the special drywall for humid rooms (like the dry wall they use when finishing bathrooms etc) ? The foam boards should be enough to keep moisture from getting to the drywall right? it’s around double the price here in France compared to standard dry wall. So I could make a lot of savings there. Thanks again, Alessandro.
You are correct.
Hi Todd- Again thanks for the info on the rim joists!
Our problem now is that we have a half wall that we are insulating and framing in but we need to know how to attach the frame to the wall. We don’t want to go over the top of the foam with the framing because it would interfer with the windows we have now and those we plan to add in the future so we need to figure out some way to attach the frame to the concrete wall. My husband’s idea is to use L-brackets on the frame and go through the foam and into the concrete with Tap-con or some such screws. Is this a good idea or would it be a moisture conduit to the wood and fiberglass insulation? Or is there a better way to do this? Our plan for the top of the wall is to have a shelf running the length of the wall.
Does the half wall run the entire length? is there another framed wall behind it? If you frame the half walk just above the top of the concrete, you can then put framing back to the rear wall after the foam is in. This will only make the half wall a tad higher. L-brackets are ok…but as you pointed out…a passage for moisture. Good luck.
Todd – just another question. Might sound a bit silly. If a basement is fully insulated, i.e. all the walls and ceilings done, does it not perhaps cause some humidity issues therein itself? Sorry, if I sound vague – but, what I mean is – if the basement is currently not insulated and the floor of the basement a simple concrete slab, moisture is coming up from the slab and in from the sides. Is there any issue with sealing off any chance of the normal small amounts of humidity in underground areas causing problems elsewhere? Would the moisture try and creep up inside the basement walls into the ground floor? In reading this back – it sounds kind of silly – but I haven’t really explained it well. I’m just worried that if the basement becomes fully insulated, where does the natural moisture in that basement then go? In the warmer months we can open the basements windows and get air flow, but in the winter? Is some kind of air extractor needed? I’m just worried to be breathing ‘underground’ air with no natural moisture in the air etc? Sorry for the vagueness of the questions! Thanks again! Alessandro.
That’s a good question. First off moisture/water vapor can still dry to the outside. The outside faces of the foundation can pass moisture to the atmosphere. In addition, 95% of finished basements really need some sort of humidity control. Without it you’ll always fight the humidity.
I am renovating an 1890’s house in Southern Ontario which has a fieldstone foundation. The basement was previously finished with wood stud, batt and drywall and has mildew problems due to water infiltration and presumably condensation issues as well.
I intend to replace downspouts with PVC c/w 90 elbows and drain pipes below grade to remove water at least 10′ from building. I also intend to install a ‘ground roof’ of 40mil EPDM as a 10′ ‘skirt’ installed around the perimeter of the building on a 10 degree slope and backfilled with crushed stone finished with artificial turf.
On the inside, I intend to prep the surface and fill any obvious holes with mortar and then spray 1″ of medium density closed cell foam directly on the stone. I will then use reclaimed EPS sheets between steel studs leaving a small cavity between spray foam and EPS (because spray foam will be uneven). On the floor I will use XPS over 6 mil poly placed directly over concrete with 3/4″ OSB subfloor floating on top.
My questions are as follows:
1) Would you recommend using an interior coating like Hydro-Seal 75 on the field stone substrate before spray foam or does that seem redundant?
2) Does the closed cell foam, properly adhered and installed, provide a high degree of resistance against water infiltration on its own? Are there specs on this? Would the two products, hydro-seal and the foam, work in tandem with one another?
3) Do you think the EPDM ground roof approach is sensible….obviously a substitute for excavating and damp proofing but much easier and cheaper…have you seen it done? Soil conditions seem to allow decent perviousness/not too much clay content.
4) Does the vapor barrier on the floor seem redundant given the xps foam? Is there any need for vapor barrier anywhere given the layer of spray foam on the walls and XPS on the floor?
Thanks alot for your input
1. Seems redundant and may cause issues for the spray foam.
2. It’s been well documented that 1-1/2″ to 2″ of closed cell foam work very well at stopping water vapor. I’m not sure it will work well stopping water especially if it’s under any type of hydraulic pressure.
3. I’ve not seen this approach before but it certainly seems like it has the potential to help reduce water issues.
4. I think it’s worth using both. On your walls, regular latex paint will prevent indoor moisture from penetrating the walls.
I need an expert to confirm (or deny) my concerns about the dampness (not wetness) in my basement and my proposed approach to sealing/insulating it. I own an 80 yr old home where I plan to finish the basement (it has never been finished in 80 years). What I have seen are the occasional damp spots in several areas of the basement (where the floor meets the wall. I have also done tests on the floor to see what happens when a section is coverred in plastic. Moisture forms on the underside of the plastic; thus water issue vs. condensation. From all the research I have done…so far my approach for this project would be to install a 2-in-1 vapour barrier/insulation product on the floor (such as DMX)that has dimples to allow for air flow. Then for the walls, I was thinking that a 1.5 to 2″ foam (XPS) afixed directly to the wall with adhesive would provide enough of an air gap between the insulation and the wall so that I wouldn’t be trapping the inevitable moisture that is constantly in the concrete. My feeling is that an interior french drain would be overkill in my situation. I also am not willing to entertain the ultimate solution which is to excavate around the perimeter of the exterior, seal the walls and install weeping tiles, etc. My overall concern is; even though I would be completely sealing off the moisture from the living space, am I going to be doing damage to the foundation over time by trapping the moisture between the wall and the insulation? Am I overly paranoid and that what I have described above is normal?
Any guidance is appreciated.
Your approach is very sound. There’s no damage in trapping water within the foundation. Concrete needs water in order to achieve strength, the longer water is present the stronger concrete can be.
Todd, would you then agree that from what I’ve described, my basement shouldn’t be considered one that is wet? Rather, my issue is more about controlling the passage of air between 2 extremes (conditioned side vs. outside) through the concrete?
I’d say you have a normal basement that needs to be concerned about vapor transmission :)
Thanks for the great article, this is just what I have been searching for. However I have a few questions. My ranch’s basement is completely unfinished and due to height limits and no walkout, I don’t foresee ever hanging drywall down there. The laundry and utilities are setup in the basement and we plan to otherwise use it for storage, possibly building shelving along the wall.
My basement is on a floating slab, all the way around are some weep holes about 3″ above the slab. I started hanging foam board about 2″ off the slab. Is this correct or should the foam be flush with the slab?
With shelving I was considering after the foam board is up building framework up against it. Where would I want to secure the framework to?
I have been using 3″ plastic plates to secure each foam board. I wonder about using an adhesive instead.
Glad you found the article useful.
If you don’t put drywall or some sort of wall covering over the foam you run the risk of a serious fire hazard. Most codes require a 20 min rated covering.
Mechanical fastening is much better than adhesive.
For shelving you’ll need to attach to the concrete or wood that’s been attached to the concrete. Fasteners like Tapcon’s work very well.
Thanks for putting this together, very useful. I’m getting ready to finish my basement here in Fairfax County, VA. The building codes here regarding insulation and fire blocking are going to make using foam boards difficult. Our county requires a min of R-11 battened insulation with the vapor barrier on the warm side of the wall. No biggie here, but like you said most references say to eliminate the vapor barrier. The hard part is that the county requires horizontal fire blocking required every 10′, where the fire blocking material extends back to the concrete. My thoughts would be that this would work against the vapor barrier the foam provides, and make it extremely difficult to frame my walls. Perhaps I could use a fire-resistant foam like Dow Thermax (if approved), but it is much more expensive than regular XPS. Thoughts…
Well I for one think that there’s a bit of confusion here. Typical fire blocking (10′ interval) is a vertical distance. So if you’re basement has a ceiling less than 10′, it’s likely you will need no fire blocking. Secondly, fire blocking in every situation I’ve dealt with is for open spaces and not spaces filled with some type of material, like insulation. How old is your home? modern framing by nature had “blocking” at the first floor deck level. Even if that argument doesn’t fly, you could certainly install a row of blocking at the top of the foundation wall, parallel to the sill plate. This type of issue comes up often, but it’s typically a miss-understanding of the code.
Todd thanks for the quick reply, I appreciate your insight. I definitely plan to speak with the inspector prior to going forward with my work. According to my county’s publication on basements based on the 09 IRC, it states horizontal blocking every 10′. It provides a basic diagram of this looking from the top down (figure 7, page 6). I’m gonna put my plans together and try to speak with an inspector during my permit application.
Well Fairfax County sure does have some “over the top” regulations in my opinion. With details like that, the horizontal blocking will make it hard to stop moisture issues. If I had to follow those requirements, I’d see if I could use 3/4″ pressure treated plywood as the horizontal (10′) blocking, from the wall back to the concrete. Then I’d install the foam in between that. Good luck! Glad I’m not a builder in VA!
Ok, talked with my inspector today and he said he’s not worried about the horizontal fireblock going all the way to the concrete. He said with any horizontal fireblocking it will be better than 90% of the basements in the county. Two other questions…
1. Building science says to avoid vapor barriers. Is kraft-faced batt considered a vapor barrier? Looks like you used it in your Hybrid approach.
2. Building science also wants a capillary break between the slab and bottom plate. Is this to prevent wood from contacting concrete because of vapor? Unless I’m misunderstanding the purpose, this seems a little excessive. I have pressure treated lumber on my deck that gets rained on all year long.
Glad to see your inspector is using good common sense! There’s still hope for our industry after all. :)
1. The vapor barrier issue is an important one. From my perspective, it’s a case by case situation. If you have a ‘relatively dry’ basement then using it isn’t as big of a deal and the paper is my preference over plastic. The paper will actually breath a bit if it’s not all taped and sealed. The paper makes for easier installation and if you’re worried about trapping vapor then I cut some small holes in it. If you’ve got a basement that has a history of water, I’d completely avoid it.
2. In many situations I also use a capillary break. http://www.homeconstructionimprovement.com/how-to-insulate-basement-walls/ We use a piece of composite decking to do that. The reason is to prevent water from wicking up through the wood plate into the wall assembly. Kind of like belt and suspenders.
Yeah the building code for basement here are pretty excessive, including mandatory egress regardless of a bedroom. What is more concerning is that the insulation code conflicts with the findings of building science corporation. So the homeowner is left with the dilemma, do it right or do it to pass inspection. If I get any more insight when I apply for my permit I’ll be sure to post it. Thanks for sharing your insight and creating these articles.
Good luck, you’re right that in many situations building code provisions conflict with other sections.
Like so many have said before, this is a great article/website and the best I have “googled”. I am about to finish my basement and all the concrete walls have been studded floor to ceiling with fiberglass insulation between the studs and vapour barrier covering it all. There is a black paper-like covering on the concrete walls that the studs are up tight to. Is this sufficient or should I be pulling the studs away and installing a rigid insulation?
But in addition to that, I have an unusual situation. I had to have a new well installed (long story involving a septic system that went bad after 5 years). The original well was in the backyard and the pipe came into the house under the poured basement floor. The new well is in the front yard and the pipe comes into the house about 30 inches up from the floor and is attached to the stud wall, on the inside of the room. It runs along the entire wall. Before I found your website, I had planned on just installing another wall in front of the pipe, but had concerns about it sweating in the summer humidity. Now I may have to somewhat remove the stud walls to properly finish them. There is enough play in the pipe that that won’t be an issue. I don’t really want to completely dismantle all the stud walls though. Do you have any suggestions what I should do with this pipe? It is 1.5 inch OD. Is the “extra wall” a silly idea? (It’s a large basement and losing 3 inches isn’t that bad). And about the sweating, is it as simple as wrapping it with a foam or rubber tubing?
I look forward to your expert advice!
First of all thank you for the kind words.
I highly recommend removing the fiberglass and tossing it out. I’d also recommend moving the walls out away from the concrete and insulating behind them with at least 1-1/2″ of foam, 2 would be even better. Along the wall that has the pipe, Ideally you’d remove that wall and temp support the pipe, insulate the concrete, frame a new wall and secure the pipe to the stud wall. I’d also wrap that pipe with pipe insulation.
Thanks for the quick reply!
I should also scrap the vapour barrier too, right? Should I remove the black paper from the concrete wall, or install the foam over top of it? And should I try to get the pipe inside the stud wall or between the stud wall and concrete?
You can likely keep the black paper, although if it comes off easily I’d probably remove it. As far as the pipe, I’d just try to get it in a place that you can get to later, and a place that’s not in direct contact with the concrete.
Hi Todd –
We have been reading your advice and are preparing to insulate our concrete wall basement with the hybrid foam board/fiberglass method. (bilevel – 8 ft deep at rear of house, only 4 feet into ground at front) We are planning to use rigid foam board glued to concrete, taped, sealed with great stuff where needed, with 2×4 framed walls and R-13 insulation. *** Would 1 inch foam board be sufficient as a vapor barrier, rather than the 2 inch board we keep reading as suggested? We can only get 1 inch tongue and groove locally, and are hoping it would be enough in this case, and figure we will have plenty of R value with the fiberglass.
Also, one room will have (Ultratouch) denim insulation rather than fiberglass – and we are trying to determine if it needs a permeable barrier as well, or not, before adding drywall.
We would greatly appreciate your input and advice! Thank you!
One inch doesn’t meet the criteria to prevent water vapor from migrating through the foam. You need at least 1-1/2″ of foam. You can certainly install 2 layers of foam. The key is the foam regardless of insulation type; fiberglass/denim.
Great thread on this. One question I had was the scenario where you are running 2″ of rigid insulation with 1×3 strapping to hold in place and gives you something to attach your drywall to. How do you rough-in for outlets and run water/waste pipes? Do you notch out the insulation or is strapping not recommended for this application?
Also, I have a small leak between the floor and masonry wall in my basement. Addressing it from the outside is not economically feasble becasue beyond the wall is a slab on grad for the garage. Can you recommend a product that will address this from the inside?
Jason – For situations with mechanical items in the wall I do not recommend the strapping approach, it’s best to frame a regular wall. It’s nearly impossible to stop a leak from the inside. The hydraulic pressure is typically too great to stop. Having said that, you can try hydraulic cement. Good luck.
Hey there Todd,
Since foam board is a fire hazard when exposed. How would you insulate a crawlspace or portions of the basement left unfinished? Dont think I could drywall the crawlspace.
Foam board is flammable. However, the code has different requirements about protecting it depending on the location. A crawl space is not considered a habitable space typically and doesn’t always need to have foam covered. A basement is typically a different story. It’s best to meet with your local building code official to discuss.
I have a sump pump in my basement, can I put the blue board right up against the wall and insulate after framing or will this stop the sump pump from working correctly?
Sump pumps typically drain a “well” that collects water from a perimeter drain. So I’m not sure what your question is, are you confusing the perimeter drain with the actual sump pump?
I am getting ready to renovate a basement room. The block walls had a plastic vapor barrier, covered with 1/2″ foil faced polyiso, then 2×4 and 1/2″ gypsum. We just bought the house. The basement was never fully finished so no heat source ever placed there. Not knowing how it was done I ended up gutting the room. When I removed the vapor barrier most of the wall was dark gray, barely damp to touch. Wall dried out in a few days and after heavy rains only showed a few small wet spots. Never saw water running from the wall.
I live in central NC and am trying to figure out the best way to insulate the walls with foam board. Was thinking of vapor barrier, extruded polystyrene, possible 2×4 walls with R13 then mold resistant gypsum. What would you recommend?
I plan on using the room as a kids playroom and for storage. I’m also planning on a baseboard heater and dehumidifier.
Do I need a vapor barrier? Should I put a vapor barrier or foam board in direct contact with block? Should I leave a gap between block wall and vapor barrier so the moisture can escape? The other half of the basement is a 2-car garage so only 3 walls are up against cinder blocks, the other is framed out facing the garage so air can circulate around the 3 walls. What thickness of foam board would you use? Is it a mistake to use R13 in the walls?
Thank you in advance for your help.
Chris – You’re on the right track. This is what I’d recommend:
– 1-1/2″ to 2″ of XPS Foam, joints taped.
– Fiberglass (unfaced or kraft faced with holes cut in paper), R value as needed.
You can learn more here: http://www.homeconstructionimprovement.com/how-to-insulate-basement-walls/
Hi Todd, I starting the process of finishing my basement. I currently have a vapor barrier with wood studs, and fiberglass insulation. I do have a drain tiled system, but the walls are visibly wet on the bottom 1/4 of the wall. There is also moisture on the inside of the vapor barrier on the bottom 1/4 of the wall. Am I going to be ok with the foam boards if I glue them directly to the concrete (Brick) wall, then put the studs back and put the fiberglass insulation back in, or do i have to much water?
Eric – Certainly sounds like you have a wet situation. Too wet? Not sure, do you ever have standing water? If not, then you probably can make this work. I’d recommend removing what you have, installing 2 inches of XPS foam board, sealing it well, then your framing. I’d be careful about the fiberglass if you have this much moisture. Either add more foam between studs or go with something like Roxul.
Hi, great stuff here. Quick question: I am using the rigid foam (2″) method. On top of the block, there is a black plastic sheet that sits under the sill plate, and there is enough left over to fold it down about an inch over the concrete wall (or the foam when it is applied). Is it safe to just fold it over and tuck tape it onto the foam board to seal the top? Thanks!
Can’t hurt…good luck.
Todd, for framing, what kind of studs do you recommend? is the the Kiln-Dried Douglas-Fir the way to go? Thanks!
Locally available framing lumber, species really doesn’t matter. Good luck.
Hi Todd, thanks for the great article. I was wondering if you have any opinion on using Roxul ComfortBoard in place of XPS foam board. They suggest installing it similar to your hybrid approach with 1.5″ of ComfortBoard insulation behind a frame wall filled with batts. It seems a bit more environmentally friendly than the foam board but there’s not much info out there beyond their marketing materials.
James – Interesting new product from Roxul. They make some great stuff but I’m a bit skeptical of this application. This material can let moisture vapor pass through it. That gives me great concern as the moisture can get to the framing and/or drywall. Also, any moisture in the wall cavity can condensate on the plastic sheathing against the concrete (concrete wall is typically about 50F and that is too cool for warmer, damp air).
I’d pass until there’s some concrete (no pun intended) evidence that this system can perform as well.
I have read a few different comments here about using Tyvek tape to tape the seams of rigid foam such as XPS. I have also read the same recommendations on various forums giving people what I feel is incorrect and possibly costly advise.
Tyvek themselves states that their tape should not be used in this application. The reason Tyvek advises against the use is that they state that the Tyvek tape is made strictly for use with their wrap and will not expand and contract at the same rate as XPS foam and in time, it will crinkle and begin to peel off.
I actually removed the Tyvek tape the day after I installed it after finding out this fact. I was very surprised to see how easy the Tyvek peeled off the foam and how weak the glue bond actually was to that surface…
Owens Corning Foamular JointSealR tape is expensive (about 3 times the cost) and somewhat difficult to find (buyinsulationproducts.com), but it is specifically designed for use with their XPS products and when actually using it after using other tapes such as the Tyvek and various Construction tapes, I personally felt the moment I installed the first few inches that it is probably a superior product for the task.
As I am finishing my exposed walkout basement, one corner of the basement that I will be framing has 13 courses of cinderblock. Most of the other walls have been step framed, drywall and insulated.
For the wall with mostly cinderblock, I have held the framing off of the wall 1″. If I do no’t insulate and just put drywall up, will I have mold issues?
Justin – Almost definitely. That block wall is FULL of water. The drywall will likely trap that moisture…and moisture and drywall are a perfect recipe for mold.
Todd, does it matter if I am not putting a ceiling up? This way, the cinderblock can breath. Thoughts?
Doesn’t matter. You won’t be able to get enough airflow behind the wall to matter.
Looking for recommendation on insulating walkout basement exterior wall (3/4) of basement have poured concrete and walkout side(exposed) is normal wood construction. Considering putting 1″ rigid foam between studs with un-faced fiberglass batts filling remainder of 6″ wall void, vapor barrier and drywall.
Home is newer construction and in SW Wisconsin; Recommendations?
Thanks and great articles and tips.
Are you just asking about the wood framed wall? If so, that’s a decent plan.
I am wanting to finish a small area of my basement. The house is 5 years old and has insulation halfway down the poured concrete wall with a silver cover. I lifted several areas up and it had yellow insulation and I could see no signs of mold or moisture. I am planning to frame away from the concrete wall by several inches and was wondering if using fiberglass was still ok since there would be air space between. If so should it be faced or unfaced? If not is roxul an option? And would there need to be some kind of barrier? Thanks!
Jessica – For basement walls I only recommend two approaches; spray foam or foam insulation board like I’ve shown in the article. Probably not the answer you want to hear, but it’s the only approach that I know works, all of the other approaches, air spaces, fiberglass, fiberglass in blankets, none of it belongs in a basement. Best of luck.
Todd, thanks for providing this site. I have visited and “liked” your Facebook site as well. I live in the Chicago suburbs. I have installed 2 inch thick XPS Foamular 150 rigid foam directly to the concrete wall. I taped the seams using Tyvek tape and then added 2×4 framing. I then used Great Stuff spray foam to seal the rigid foam to the studs and plates. I plan to insert R13 unfaced insulation between the studs. My question is regarding the drywall. Should I use regular drywall or moisture resistant greenboard? Thanks again.
Roman – In most cases regular drywall is fine. If there’s a risk of high humidity in there then consider the moisture resistant. Good luck!
I am planning on partially finishing my below grade townhouse basement in South Eastern PA. My approach is to latex Drylok the cinder blocks at the front of the house and then paint over the Drylok. At the back of the house I was going to Drylok, then 2″ XPS, then 2×4 frame, batt insulate the cavity, then drywall. I was planning on framing tight to the XPS. Most of the rest of the party walls will be painted cinder block. Do you see any benefit to Dryloking the party walls as opposed to just priming and paint?
By Dryloking the back exterior wall and then applying XPS directly to it, am I creating a problem where moisture could be trapped there?
Also, my joists are the open webbed truss variety. Unfortunately, they run parallel to the back of the house, which means my rim joist there is a mess of 2×4 triangle shaped cavities with no insulation. Should I take the time to cut the triangles of XPS or Polyscio and spray foam any gaps? Or, can I leave the cavity uninsulated and use rigid foam against the face of the webbed truss? The latter is preferred as it is easier to do but I’m concerned if moisture would be trapped in the cavity with the lumber of the 2×4 truss. In either case should I be using XPS or Polyscio, with its vapor barrier quality? Thanks!
Paul – Lots of info in your post so forgive me if I miss something. Using Drylok is fine, and it’s not a problem of “trapping moisture” as that means nothing to concrete. Concrete always has water in it and it’s perfectly fine to be saturated. Not sure what you mean by party walls that are block, do they have air on both sides? conditioned space on both sides? As far as the rim joist goes I think you’ve hit the head on the nail….the trick is sealing it off without burying that joist behind the foam. Cut and patch would be my suggestion.
Thanks again Todd
Michael in upstate NY
Thanks for your input Todd!
The party walls are the common cinder block walls in the basement that I share with my neighbor. Since it’s a townhouse I’m presuming that the other side of the cinderblock walls are my neighbors basement. A rep at Drylok suggested I do not need to Drylok these walls- only cinder block walls that are exterior on the other side. The only benefit to Dryloking these walls would be if my neighbor had a flood.
Paul – The Drylok rep sounds like he/she doesn’t really understand masonry. Sure..there’s no standing water on the other side, but all masonry block contains water and over time wants to release it during drying processes. When not insulating this type of wall I think a product like DryLok or some other good quality coating is a good idea to stop / slow down the release of moisture. Good luck.
hi todd .
I noticed radiant barrier insulation at the box stores
comes in 4 ‘ x 12’ rolls . how to install. it says between roof rafters for one way.. will that really help
lessen the summer heat in the attic. I have a dark roof
Michael – There are studies that show some radiant barriers can be effective at helping reduce heat in attics during the Summer. I’ve not personally used it as we typically try to insulate attics at a very high level which ultimately would reduce the need for this type of product. I’d try to find some reviews and feedback from users. Good luck.
Thanks, I will do that
Awesome posts and answers.
We’re building a new house with 9′ basement walls in Iowa. In fact we’re forming walls today.
I was planning on installing 2″ rigid extruded foam on the interior basement walls with 2×4 framing and batt to follow (no vapor barrier).
1. Should we install the foam prior to slab pour or after?
2.Also any recommended way to install 8′ board on a 9′ wall, as I can’t seem to track down 9’x4’x2″ foam?
Jonathan – If you can install a short 2′ piece lengthwise along footing before the slab that would be great. Then you can add on top of that after. That would be the easiest way to deal with the tall walls.
Thanks for the reply. I take it that bringing the foam down to the top of footing pre-slab is preferred over setting it on top of the slab? Any specific benefits?
BTW is there a real benefit of installing interior rigid foam on basement walls prior to the slab pour or is installing on the finished slab just as good?
It provides a thermal break so exterior cold doesn’t transmit to the slab.
Great site! I have two questions:
1. My poured basement walls are of the “brick form” variety (looks like brick but poured concrete). Does this change anything from a adhesive standpoint (product or method) for the XPS?
2. I am planning on doing some carpeting (with double sided water resistant pad) and some ceramic tile for flooring. I have heard they make membrane systems that go between the flooring and the concrete to allow airflow in case of water/dampness. What are your thoughts on these products?
I am in Illinois and thanks for your feedback.
1. That shouldn’t matter at all.
2. I’ve not heard of this product, I’d inquire about it with your flooring supplier.
This is great information. Thank you for sharing this info with everyone on the site. We are in the planning phases of finishing our basement. I have a question regarding Dryloked walls. I applied Drylok to the front wall (completely underground) because we have noted moisture seeping in during rain storms. We are also planning to regrade the front yard. I am planning on using XPS foam insulation but after reading a few of your articles, I want to ensure that I can apply it directly against the Dryloked wall. I have currently only painted the front wall (below grade). Would you recommend not dryloking the rest of the basement? Thanks again for sharing your knowledge!!!
Bob – Drylock is fine, I’m just not sold on it completely. Trying to seal the inside from water pressure seams like a fruitless battle. It works fine as a “damp proofing”, but it won’t stop a water leak, and it won’t stop the need to insulate as I’ve detailed. Think of it as belt and suspenders I guess :)
Also – If it is OK to apply the foam board directly against the Dryloked walls, can I still use spray foam as an adhesive? or would this have a reaction with the Drylok paint? Thanks again!
I think it’s fine to use the spray foam. In many situations we’ll just use a tight fit on the foam if we’re installing the wall soon after. The wall framing keeps everything in place.
Sorry, used wrong email on previous post.
We had water flowing into our crawl space for many years. We were told that there is a high water table in our area, but we discovered that when it rained, water was simply coming up and over the foundation wall right into the crawl space. It looked like Niagara. We have a vinyl stucco exterior. A contractor recommended digging down about a foot and a half around the problem areas(which was around most of the home) and pouring cement and attaching it to the foundation wall with rebar and thick screws etc. We went ahead with this and it solved the problem. I’m just starting to think now, could all that extra cement that was added to the foundation wall cause too much stress on the foundation?
Mark – Without seeing the detail it’s hard to completely understand the situation. However, based on what I’m thinking he did, I don’t see a problem.
Wow I wish I would have found this information a couple of years ago. I’m hoping to begin finishing my basement. The builder framed interior and exterior walls and used fiberglass between the studs against the concrete exterior walls. The basement is dry but we do live in a cold climate. The space between the studs and the concrete varies from 1/4″ to 1″ so there is a bit of space there. I thought about pulling out the fiberglass. I do not have space for the rigid foam behind the studs. Would it work to use spray foam against the concrete wall and try to get that behind the studs as much as possible. Then I would install or spray in cellulose or fiberglass on top of the foam. Would I have problems with those areas behind the studs which would have a very thin layer of spray foam.
I guess bottom line is this going to work or am I going to have to go to work to cut all exterior framed walls out and move them which will also require me to reframe most the interior walls. Thanks for the help in advance.
Troy – I get this question weekly, sometimes daily so you’re not alone. If it were my house, or one of my clients, I’d move those walls out and get sufficient foam behind the studs. A better solution than the fiberglass would be foaming behind those studs with can foam, then installing foam between studs, and many people do choose this option. WIll you have problems? I can’t say for sure how this will perform, but it’s certainly better than plain fiberglass. Good luck.
I’m having my basement finished and wanted to run by a quick scenario for you and see what you think. I’m in NY, I don’t have a water issue but there is some moisture concerns. My dehumidifier keeps the entire space below 45% in the summer.
One section of my basement will stay unfinished (mechanical area) and then one adjacent walkway/wall that will be left unfinished the full length of the basement so workers can get to my electrical panel, plumbing pipes and main line etc (about 3 feet wide.
The exterior walls in this section I was recommended to have R15 Roxul comfortabatts pinned directly to the concrete and left as is without any type of barriers or anything. For the opposite interior walls that will be up against the conditioned space i’m told to use Roxul safe and sound for where the furnace and mechanical s are. Is this suitable for the unfinished space that won’t have any drywall? This area will not be cooled or heated, but is where my dehumidifier is located. Is this acceptable or should I pin Rigid Foam board (R15 equivalent, seal the seams, and leave it as is in the unfinished sections?
The other exterior walls will be up against the interior conditioned space . This is where I get into so many conflicting views..
One guy suggests that I use the same 3″ roxul (R15) AFB and pin it against the concrete, then have my contractor frame the wall up against the roxul, (don’t back the areas between the frames because i’m doing R15 against the wall, then just drywall and a primer that will act as the vapor barrier.
This doesn’t seem to be advised at all here, is this proposed idea acceptable to avoid moisture and mold issues?
Or should I use R15 rigid foam against the concrete, then have the frame built, then either add roxul insulation or left empty, then drywall with primer and paint?
So basically roxul where the unfinished section is going to be ,and then roxul behind the finished space, or should I do rigid foam in both sections? I’m totally confused here
Jason – Roxul is a great product, unfortunately it doesn’t stop moisture problems. My only recommendation against concrete is closed cell foam. Roxul is great after that layer to add extra insulation value, but I would not use Roxul only against concrete. Also, with all of these products, be sure you’re not creating a fire code violation, many of them need protection from fire (Roxul is fine, but if it has a flammable covering that may be an issue)
Todd, I have a question. I live in Manitoba. I have 1 1/2 ” Styrofoam (R7.5) on the exterior foundation of my 22 year old bungalow (poured concrete). I’m going to finish the basement interior. Should I use 2×4’s and insulate (batt 3 1/2″) the cavity? I understand no vapour barrier is not required. I have no moisture concerns presently.
Ron – even though you have foam on the exterior, you’ll need it on the inside, otherwise you’ll have a serious moisture problem.
Hi Todd, I live in MN. Currently I have a block wall with drylock applied. I am finishing the basement. I would like to ask you 2 situations.
1) Is there any benefit or harm of using a layered approach for me to achieve an R-20 rating. I have some 3″ XPS, and was thinking of doing an additional 1″ layer staggering the seams, and using JointSealR owens corning tape on both layers. My concern is if the top layer of 1″, is not a water vapor barrier, could I get water entrapment between the 3″ and 1″ layers? The alternative would be just do 2 layers of 2″ (Both have low enough perms to suffice as a vapor barrier). With this much insulation I plan on leaving the stud cavity open.
2). My other option is to only do the 3″ layer, then once famed up, install XPS (maybe 1.5-2″) between the stud cavities sealing the edges to the studs with great stuff foam. This would only be done where the option of block is above grade. (approximately 3-4 courses).
Given that info, also note that I believe I have some type of foam board running from the ground level to the footings. 1.5 or 2″ its yellow. Not sure if it is unfaced polyiso, XPS, or EPS. House was built in 2000. Would this change your recommendations at all?
Jeremy – Happy to give some advice.
1. Layers are fine, I would probably do just as you describe, the thick layer against the concrete, thin layer last. Just seal real good and you should be fine.
2. This option would work as well.
Good luck! Should be a huge impact on the insulation value in your home.
Thank you fro this blog.
I’m in the planing stages for finishing my basement.
I previously finished the basement in my last home with, what was to me very satisfying results.
That house had a fieldstone foundation that was plastered on the inside. It was always dry.
I painted those walls, and the floor, with Drylok paint.
I then used R-25 between the joists with a fire stop underneath of that where I mounted my top plate.
I then built a 2×4 wall keeping the studs at least an inch away.
When I was finished the atmosphere of the room felt like the rest of the house; not like some other basements I have been in.
I have seen use of rigid insulation being used in basements in different publications and I am beginning to question my previous decision although I was completely happy with it.
What do you think?
Jay – In my opinion…you were LUCKY :)
We do lots of renovation work…basements insulated with fiberglass adjacent to the foundation wall most often result in serious mold problems. Having either foam board or spray foam as the first line of defense is crucial in my professional opinion.
Hi Todd ,
I have a classic split level in upstate NY . Four foot cinder block foundation then wood framed the rest of the way up. When they finished the basement originally the top part of the wall had faced fiber glass installed. The bottom part the cinder block nothing but furring strips were installed and sheet rock essentially installed right to the block. So no question I have a mold issue with the sheet rock. I have removed the sheet rock , sealed the block. Now thanks to your site I’m installing 2″ foam to the block and will frame after the foam. My question is can I leave the faced fiber glass on the top part ( wood framed portion of the wall) ,and install the foam right to it . So I would be coving everything with the foam ?
Pete – Glad you found this information useful. I would peal back some of that fiberglass in several locations and check for mold, if it doesn’t exist, you can leave it in place. I would not cover it with foam, if you want more R value at the framed level add more fiberglass.
I am beginning a new build with block foundation walls. I would like to insulate the outside of the wall as well as inside. 2″ rigid foam outside. My question is where do I put the dimple board. I will use blueskin against the wall. Is the dimple between the foam and blueskin or on the outside of the foam? thanks for your help!
Tony – Can you tell me more specially what products you’re using?…ie dimple board.
basically a platon membrane
The platon membrane should go against the concrete prior to the insulation. It acts as a drain plane.
Would you recommend this way of insulating a basement? I am thinking I can keep the basement mass warmer this way, but certainly open to suggestions!
For me, the best approach is insulating the interior with a minimum of 2″ foam board. For the exterior, I like sealing the foundation and installing a drainage plane to help get water to the perimeter drain.
Great site! I live in central Wisconsin and our home was built in 1922. The foundation is block and does have some moisture issues. Nothing too severe. I am planning on sealing the blocks with drylok or some form of sealant and then applying insulation and framing. From what I have read, it sounds like I should apply 2 inches of insulation and make sure to seal it. Would this be correct? Then apply the framing? Should I worry about fiberglass insulation between the beams? Thanks Todd!
Tyler – That plan will work, just be sure you’ve done all you can outside to redirect water away from the house/foundation. Not sure I understand “between” the beams. Do you mean studs or floor joists?
Nate from Colorado here (Zone 5) I am limited on space and money so having a 1-1/2 inch or 2 inch XPS followed by a 2×4 studded wall cuts into both of those. What do you recommend for someone who is tight on space?
I was originally going to go 1/2 XPS followed by 2×4 wall, but then I found your article.
1. Could I go 1/2 XPS, followed by Roxul (r15), followed by vapor barrier (recommended by Roxul); then sheetrock? Or would the XPS in this situation be wasted effort?
2. I know you mentioned that an Air Space is not feasible when there are other, better, options available, but with limited space, how much air space should I leave?
3. Would you recommend an air vapor barrier (such as Tyvek paper) against the concrete? Roxul showed this in one of their How to Videos.
Nathan – Basements are tough….if you can’t afford to do it the right way I’d wait…this isn’t something you want to mess up (I mean this with all do respect).
1. No….you have to have an insulated vapor barrier…otherwise water will condense on the cold surface on the other side of the 1/2″ foam. The absolute minimum is 1-1/2″….2″ preferably.
2. The air space is “nice to have” but not necessary.
3. No…I wouldn’t do this as it won’t stop moisture from condensing on the cool concrete surface.
Thanks for the good article. We’re in a new home with a walkout basement in NW Florida. The basement was partially finished and has its own HVAC system separate from the rest of the house. The walls are poured concrete and the “soil” around here is all sand. We’ve had no issues with visible moisture.
Where the basement walls are concrete there is currently no insulation. The exterior walls that are framing (as the lot slopes down and the walls become above ground) the 2×6 framing has R-19 Kraft Faced batts.
Here’s my question. We’re in a very temperate area. The basement is always comfortable and the HVAC runs far less than in the upstairs. Some of the concrete walls have 2x4s ramset into the concrete on the flat acting as furring strips. Do you have any recommendations for what to do with these? I’m thinking cutting 3/4″ EPS to fit between the furring strips and flying it to the concrete. I know this is minimal R value, but I’m mainly concerned about water/vapor/mold issues. Any concern with this approach? It would leave a 3/4″ air gap between the foam and the drywall.
Tim – I wouldn’t recommend it. In order to stop moisture from leaving the foundation and penetration into the finished wall you need a continuous layer of closed cell foam, at least 1-1/2″ thick, preferably 2″. Anything short of that will likely result in water vapor getting trapped in the drywall.
Todd, I am wondering where I would find my local energy codes to determine what thickness of foam board to use in Virginia. Thanks!
Your local building code office should have that information. Here in NH, it’s regulated by the Public Utilities Commission.
Hi Todd- I have a unfinished basement which I am in middle of finishing with contractor help. The concrete wall was painted with water proofing paint. The stud wall (2×4) is already installed. Can I still install insulation board on the stud wall or it has to be done on concrete wall. Please advice. I am planning on using RMatte Plus 3 (closed cell polyisocyanurate foam with alluminium foils on both side)
The foam really needs to be against the concrete, otherwise moisture will cause issues with the framing lumber. Sliding the wall forward isn’t that difficult, it does NOT require rebuilding it.
Ok. I read through a lot of comments with very good information but I have a brick wall in Chicago (3 bricks thick and 120 yrs old). Does the same approach apply? And this brick wall extends above the crawslpace for most of the height of the first floor. Could I use rigid foam on the brick in a similar fashion? I am about to insulate the first floor now. And I think the carpenter may have already put up 2X4 studs along this brick wall. Could I cut the 2″ rigid foam board into strips between the studs and seal it and fill in the gaps with something like great stuff? It sounds like fiberglass is not a good idea.
Dan – Foam board will work. It REALLY should be continuous though, not just in the stud bays.
Hi Todd–I want to create a room approx. 1/2 of basement in my CONDO. Poured concrete walls all around. Front and back walls are “exterior,” side walls merely separate my basement from my neighbours. Basemt Systems (we won’t go with them too xpensive) guy told me the side walls don’t have to insulated. T/F? If True, we would insulate front and back walls, and finishing front and sides with cedar paneling, leaving back wall with just the insulation since it’s only for storage /workshop/laundry. There won’t be a solid wall between for now, just a drapery curtain (so taxes don’t go up by us “creating a new room”). Also, Basemnt Guy said we could remove the fiberglass insulation from between the ceiling beams and paint for an industrial look(we don’t want a dropped ceiling) and it wouldn’t really affect the basement or the upper floors. T/F?
Jasmine – Lots of “issues” that I can see:
– Even the concrete walls that are not exterior, would benefit from being properly insulated. Concrete walls are kind of like a huge sponge, they are full of water, and will be forever. So using a foam insulation helps prevent that moisture from getting into whatever wall finish you install.
– Insulation must be covered to protect against fire in most building codes. I highly recommend checking with your local building code official.
– You can remove it, but if that space isn’t heated, then the result will likely be a colder floor above.
Thank you, Todd. This is what I thought. When I touch the dividing walls they are always not quite as cool as outside walls, but still…and our LR with W/W carpeting is above the part of the basement we want to do, but I still am not sure I want the increased Electric heat bill from conductive heat going to the basemnt. Isn’t it odd that the “best” basement company in Southern New England would give this advice? Well, since their estimate was way too high this will be a DIY project, short budget, sort of pay-as-we-go, SO, yes, a lot to think about.
Thank you, Todd. That is what I thought, too. Odd that the “best rated” basement company in Southern New England would give that advice. Since their estimate was way too high, this is going to be a DIY, pay-as-we-can project with my not too handy Other. But, I AM handy, and he’s great at taking directions, bless his heart. (1) What is your opinion of the InSoFast UX2 panel system? (2)If we use them and leave the ceiling joists exposed should I cut small pieces to go up in between the joists to the above floor? (3)The Code Compliance Report on their website indicates they should be installed with adhesive AND screws, but none of the videos show that, so I guess I need to check with my town BI. (4) I don’t want drywall, I wanted vertical 1/2x6x7 cedar tongue and groove, but their vertical integrated studs won’t allow that? I’d do anything to avoid drywall. I would settle for Drywall with a 4′ cedar wainscotting, I guess. Sorry to ask so many questions, but I see on your “about” you are from New Hampshire (where my sister lives) so I think you must know your stuff and I need to pick the right brains before we start this project the wrong way. Thank you again.
Jasmine – I’ve never heard of the InSoFast UX2. Is there a website/link to that?
What ever you do, the wall should be sealed from floor up to the floor above. This means all of the rim joist, top of foundation wall, etc.
Most codes are looking for a flame spread retardant, and in some places, wood paneling will count towards that. If the system uses vertical studs, then you might need to strap the wall first before the vertical panels.
I stumbled on this site while looking for insulation ideas for my basement. Great read!
My basement was framed (wood) before i moved in. What are my options?
Hi Todd. Tried to read through enough comments to find my specific question but thought I should just ask it. I have a 2 year old walk out basement in MN with an exterior vapor barrier and rigid insulation panels up to the wood walls which have spray foam and standard insulating and plastic vapor barrier. Do I need EPS between the studs on the walls against the block walls?
Matt – Regardless of what’s on the exterior, I recommend at least 2 inches of foam between studs and concrete/masonry walls. Good luck.
First of all your site and references, as well as the thread here is extremely helpful. I only wish local code enforcement would be more educated in these areas. If only you could be cloned…
I have a complex problem I am trying to solve working with Owens Corning and my city code/inspection folks, and was hoping you might have some helpful insights or suggestions.
I’m in a new construction home in Colorado completed in late 2016. 9″ thick concrete foundation walls with a “coating” on the outside, vinyl covered fiberglass blanket on the inside. Sump pump installed, floating slab with 1/2″ fiberboard spaced from sides of slab to foundation wall. I want to remove the blanket and use 1.5″ or 2″ of XPS foam prior to finishing the ~1,000 sq. ft. space, and use R-13 unfaced fiberglass in cavities. I also plan to use DRIcore w/ the foam backing (1″ thick), with bottom plate attached to DriCore.
The challenge I have is with floating walls required and the slab movement (code requires minimum 1.5″ gap for float, up to 3″).
Challenge 1: Owens Corning suggested to install 1/2″ XPS floor to ceiling matching the fiber board gap, layering 1″ or 1.5″ on top of that, and leaving the bottom portion open with a gap so the slab could move. The city is fine with this, but I am concerned that the bottom 1.5-2″ will have condensation issues due to only 1/2″ XPS at the bottom portion. If I install floor to ceiling where the slab floats, even if the foam did “crush” without wanting to pop off the wall if/when the slab moves back down there would be a gap there.
Challenge 2: How can I install a thermal barrier (eg. 1/2″ drywall) to cover all foam into interior space with the floating framed wall? Usually, you stop the drywall at the upper floating bottom plate, and cover the lower portion with molding attached to the lower bottom plate. If the lower bottom plate moves upwards, the molding moves up(attached to the slab). No drywall cracking/buckling. This leaves a gap that is not covered by ).
I have some SketchUp drawings of the proposed solution if that helps explain. Please let me know if I can send those to you.
Thank you for your assistance.
Joel – Thanks for the kind words. Feel free to email me: todd “at” frontstepsmedia “dot” com
After gutting my basement earlier last year from flood damage. I have discovered the stud walls are right up against the concrete foundation with no insulation. Is there a way to properly insulate those stud walls without tearing them out and away from the concrete? Doing so would result in me having to disconnect and re-rout electrical wiring. I look forward to your response. Thanks!
Dan – If it were my house…the walls would be moved. You may be able to free the top and bottom and slide the walls away from the foundation without removing the wires completely.
I have 1 ½ inch wool insulation over our concrete basement walls. How to I frame over this type of insulation and do I need a vapor barrier?
I wouldn’t frame over that…I’d remove it and start over.
I have an unfinished basement that already has framed walls up. What should I do about instalation? I don’t want to take the walls out.
Jeremy – In 90% of cases I tell clients to bite the bullet and move the walls. Unless there is a space between the studs and foundation, spray foam won’t even be an option. Wish I had better advice, it’s just not worth risking in my opinion.
Hi, I recently bought a house and the basement was finished, I am redoing a few room so I ripped the drywall off and noticed the framing by the block foundation is butted up against it, they didn’t put that foam insulation up first before framing, I wasn’t planning on ripping everything out. Do I have other options? The block foundation is painted so maybe that helps with the moisture, but I don’t know. Have any ideas for me? Short of ripping everything out
Jesse – Unfortunately we see this far too often. Were the stud bays insulated with fiberglass? The paint isn’t going to stop water vapor from condensating on the surface of the block. The only way to definitely prevent a bad mold situation is insulated the block wall properly. Wish I had better news.
Todd, we are renovating the basement of our home. It was built in 1953 and is in Louisville, Ky. The walls are 7 foot tall and were all covered by 3/4 inch thick knotty pine. I removed the knotty pine (I hope to resell it to keep out of landfill) and behind the knotty pine was not much. There were three furring strips attached to the concrete wall. One strip on the floor, one about 2 feet up and one about 5 feet up.
I do not notice any moisture leaks.
The furring strips they used are 2/4’s nailed horizontally to the wall. This gives me 1 and 3/4 inches wide giving me room to place 1 1/2 foam boards. Or I can increase the thickness and use 2 inch foam boards. I really don’t want to use anything thicker since it is a small basement and I don’t want to loose footage.
Would DOW Foamular 250 work if I put it directly against the wall and then seal with canned foam and then put drywall next to it. There would not be any batt insulation at all. I have read that the Foamular 250 is closed-cell so that should prevent mold, I think.
I know this is not idea, since it is not 4 inches thick, but I was wondering how much better it is than none at all.
Thank you for your input!
John – If you can install 2″ of foam, sealed well, then that will be a huge plus. The tough part is how do you install the drywall directly over 2″ of continuous foam (I’m assuming the firing strips are removed). What you really need is some new firing over the foam board (3/4″ thick), attached through the foam into the concrete. Then you can install drywall. A better wall material would be some sort of wood paneling, or wood tongue and groove, as it’s more resistant to getting damp on occasion. Good luck.
hello just framed in walls on basement have exactly 4 inches from cement to front of 2×4 , was thinking 3 inches of dow foam or xps board would work between studs how do you keep the foam sheets in place. push them against concrete or keep them flush with 2x 4 wall? do you spray foam where electrical boxes are ? or should I buy 2 2inch sheets and double them up in wall and have 4 inches…. THx
You need “CONTINUOUS” foam board, not cut between studs. The foam should go behind the walls.
Hello…wondering if i can get some help on the issue i am having. I have a woodstove in my basement to heat the level above with plenums and ducting however what doesnt make to the upstair is expelled into the unfinished basement. The woodstove is located on one end and i am noticing condensation and mold building up in the far corners. I have no plans to ever finish my basement but would like to take care of this problem as cheaply as possible. Would 3/4″ foam board be enough to prevent moisture and mold from growing?
I doubt 3/4″ is enough R value to keep the surface above the dew point. Most likely need a couple inches.
Recently moved and am looking at houses in WI. Two that I’m interested in have finished basements. However, in one case the owner did not use any insulation/vapor retarder. The wood framed/sheetrock wall has 6″ standoff from the wood to the painted cinder block, and the upper two feet of the cinder block has 1″ styrofoam over it. At first I thought, ‘no way’. Then thought, the dead air space will provide some insulation AND allow for drying of the void. Opinion? The other one has 6 mil over the cinderblock, then a finished wall (can tell if it’s insulated). I’m afraid of mold forming between the poly and the cider block. Opinion?
Both give me serious heartburn. We’ve torn so many of those type of installs apart to find mold. Both are really similar and certainly not how I would do it. It really becomes a part of your decision and negotiation…at least you’re going in to it with the information, now it’s up to you to weigh the pros and cons. Good luck.
I Leave in Toronto Canada, I am finishing my basement and I am using the foam/insulation method on my walls. My basement is dry with no water issues.
Can i use the foam board directly on my concrete floors as well, or is that something that you wouldn’t recommend? Would I be better to install a dimpled underlayment first then foam board on top? Would I need another layer of OSB on top of the foam? I will be installing a floating laminate floor. I don’t expect to lay tile anywhere in the basement.
I’d install foam, then plywood, then the flooring.
First off, thanks for the awesome information on this site! I’m attempting to fix the basement of the house I recently purchased and want to make sure I’m insulating it properly. The basement has a concrete foundation running halfway up then the rest of the basement height is wood studs sitting on top of the foundation. The previous owner installed studs over the top of the concrete wall and then ran them down to the floor (so it makes a shelf). At the bottom of the concrete wall is copper pipes for baseboard heat. I would like to install 2″ of foam insulation over the concrete (there was no insulation there previously). I’m just not sure how to seal it at the floor and at the top of the “shelf”. Should I run the foam down the wall and stop just a little bit above the copper pipe (I do not have enough room behind the pipe to get the foam all the way to the floor)? And at the top I’m thinking maybe use the spray foam? Will that give me a good enough seal? I’m in the North East, btw, and the basement is pretty dry. Thanks in advance!
That will work…might try to get a thinner piece of foam behind the copper if you can.
I live in an area that is extremely humid in the summer and very cold for only a few months in the winter. 1970 raised ranch with walk out basement. Current there is cinder block in basement and I can see a few areas with white crystal residue on the interior side of the blocks. Would spray foam or foam board be best?
Either product will work, just know that it must be covered after to protect against fire issues.
First of all let me thank you for giving out such a nice information.
I am planning on finishing my basement (not fancy) by myself by:
1] Just placing the drywalls on the open area where the studs are and
2] Paint the cement wall.
Reason for this is because my house is already 3 floors with lot of room and spending too much money basement finish is not going add any value to the house.
The ceiling was left open with the fiberglass and I just covered it completely with think plastic.
So, my question is what kind of insulation shall I use for step#1? I am thinking of R-13 pink faced.
Or do you have any suggestions?
Thank you Todd.
Are you talking about stud walls adjacent to concrete or block walls? If so you need foam board.
I’m just beginning my basement finishing project. I live in Northern Illinois. I’m planning on 1 1/2 or 2 inch foam board on the inside walls, and then the usual stud wall. What I’m curious about is the house was built with 1 1/2 pink foam board on the outside of the foundation already. Do I need another 1 1/2 layer inside? And, will having insulation on both the inside and outside foundation walls cause any issues with the concrete drying to the outside as you explained earlier?
Heff – While the insulation is nice outside, it doesn’t really help when “finishing” the basement. Having it on both sides isn’t an issue at all. Good luck!
I have a home in Western Colorado (high desert) with a mostly finished basement that is partially insulated (all but a closet and a large game room) on the inside. I have no idea of the R rating or quality of the work. Temperatures get chilly during the winter with floors and some walls in direct contact with the ground and/or air. I have been considering adding EXTERIOR insulation board to the walls that I can access. Due to sidewalks, garage, and windows with egress, this only amounts to about 25% of the walls or about 40% of the two “longer” walls. If done, I plan on doing the digging and placement of the board myself (fairly easy access to foundation with no impediments). So cost is not a great factor (just the boards, maybe $200?). My question is whether it is worth the effort at all with relatively little coverage? And if it would be pointless to do so if it is not done all the way from the base of the basement up to the bottom of the siding? If this last is the case, what is the recommended way to hide it? Finally, would it be ideal to allow the concrete block to dry out for a couple of weeks before placing foam board?
Adam – Will it help? Sure it can’t hurt. Will it be super effective? Probably not. Insulation on the inside when done properly stops moisture from getting into the finished space. Insulation on the outside simply can’t do that, “drying” out the blocks won’t work, they will always have moisture in them. Your time and investment would be much better spent on the inside in my opinion. Good luck.
Thanks for a great site. I am finishing a basement and my house is semi-detached but includes a garage. My questions are 1) do I need to insulate the wall shared with the neighbor’s house and 2) do I need to do foam board + batt insulation in the walls shared with the garage?
Thanks a million!
I’d insulate ALL concrete walls, the more the better.
I just went through a huge mold remediation with a 1991 walkout basement in zone 4 Minneapolis where the original builder installed the framing against block foundation, unfaced batts touching said walls and then vapor barrier for a photo finish. 27 years of bad building practices led to a huge mold issue in the fiberglass! We demoed out all the framing but left the 2×4 treated bottom plate that is nailed to the floor at the base of the walls.
Solution will be 3″ closed cell spray foam on the rim joist and the framed/Bildrite walkout wall and then 2″ closed cell boards on the block walls, frame in front, no poly, then rock etc.
1) Would you recommend those bottom plates be removed so that the 2″ foam boards are installed to the floor or would it be okay to install on top of the 2×4 plates? Would install another 2×4 bottom plate in front of that and build up from there.
2) Also, I work for a large builder (Lennar) that uses the foil backed thermax on the unfinished below ground poured wall foundations. They use .5″-2″ on the interior depending on the situation but they also are installing foam boards on the exterior foundation which is standard practice these days. I likely don’t have that exterior insulation and was planning on using the 2″ double foiled thermax boards from DOW so I wouldn’t have to frame and rock right away.
However, Building Solutions doesn’t recommend foil backed boards and you only show the standard boards as well in your discussions and photos. Would it be best to stay away from foil backing? Is there an issue with the foil backing touching the block walls? It will likely be a year or so before finishing the basement. Since they are double foil backed from DOW it is what it is…
I have learned so much in a short amount of time reading your articles, I would certainly appreciate any insight you have about the two questions I posed. I don’t cut corners and I don’t do band-aids. I want to do it right the first time and be able to sleep at night.
Christian – You’re not alone, many basements have been built like yours.
1. I’d leave those plates in place, no harm.
2. We never put foil faced polyiso in contact with concrete, the foil will deteriorate, and the insulation will absorb water. We only use 2″ blue board or pink.
Hi Todd, we have a flat basement house that is built in 2017, the exterior wall is insulated with R-10, when we finished the basement, how should we insulate the wall?
Hi, we are insulating all basement interior walls with 2” closed cell spray foam and then framing/sheet rocking. Where do septic/plumbing/gas pipes get placed? Do we need to make sure they are between insulation and framing? How do we access them for clean out and in case if leakage?
I like to bury them in the wall/insulation. But it’s important to have access to the cleanouts.
I am putting 2” closed cell on the basement walls then framing and sheet rocking. Should I make sure the waste lines are kept off the wall so that spray foam can fully get behind those pipes? Thank you
If you can that’s better.
I just had closed cell spray foam insulation done on my basement walls to help with moisture. The foam was placed directly on the cinder block walls with a fire barrier paint.The foam has an uneven and somewhat unsightly appearance.Can I build a wall over the foam? Will the foam last if left exposed?
I would definitely frame a wall in front of it. Sure it will last, but if you want to finish the space then framing a wall is the way to go.
I have an old house that has river rock foundation. I want to finish my basement but do not know what to do about insulation. My husband is an ex contractor that is now completely disabled and therefore I have to figure this out on my own. Please help me.
I’d spray foam the walls…then build walls in front of the stone/insulation.
I am redoing my basement. I have the wooden frames on the wall itself, I plan to cut the foam boards and put in between the frames. Can the insulation boards be thicker than the frames. Say for example the frames are 1 inch thick and i want to use a 1 and half inch thick foam board in between. Can I do that and not have a problem when putting up the Sheetrock? Or will that small gap become an issue with moisture. Should everything by lined up and no spaces whatsoever? Also I had a mold expert tell me to install the foam boards on top of the wooden frames and leave a small space for air flow to let the house breathe. I dont know if that is a good idea.
That won’t work…..plus you really need continuous insulation not just between the “frames” as you call them.
Hi Todd, I’m finishing my basement I don’t have any water issues. I was wondering what to use far as insulation on my concrete block walls, I used drylock 1st. Do I need a vapor barrier? Can u use fiberglass insulation with facing ?
Same as a concrete wall…zero difference
Todd, what about work-arounds for basement utility areas containing water softener and pressure tank, or utility room with water heater, washer/dryer and utility sink? We’re laying DriCore R7; is it advisable to raise these service items so they are on top of the subfloor and foam insulating those rooms, or is it reasonable to leave those areas as unfinished basement? Disconnected, adjusting and reconnecting is no small job!
Also, I was not aware that foil faced polyiso would be a problem for interior basement walls as it is closed cell, then one of your responses indicated the foil would deteriorate with the insulation absorbing water. I’ve got a garage full of polyiso (and a possible problem). Did I misread your response, or is there a way to isolate the foil so the problem does not occur?
Thank you for your reams of great information!
yes, foil faced polyiso is not recommended to be in direct contact with foundation walls. It’s REALLY only a product to be used above grade.
Hi.. I did not read “every” post..
But.. in your presentation you didn’t mention.. Outside of the basement wall insulation..
From one of the commenters.. they had the perfect chance to do that – when they re-waterproofed their wall.
This location can address most if not all of thwe “usual” basement problems.
Saying that.. Reto-fitting insulation on the outside of the wall..
“could” lead to problems in “really-cold” climates..
Best time to insulate.. is during building. Some of which is outside wall surface alignment.
BUT – With me saying all that.. I liked how you handled a compilcated subject. Thanks
Paul – I didn’t mention exterior insulation because I’m not a fan. It doesn’t really solve all the problems and still ends up requiring something inside. Concrete is a huge sponge, full of water for life. It’s also a huge heat sync. So for me, the best application is on the inside, taking care of vapor issues and getting insulation closer to the finish surfaces. If you insulate the outside, it’s still fair to say the interior concrete will be at or below 50 degrees most of the time. And it has a lot of mass…so that 50 degree temp (while great in the summer), stinks in the winter. Just not an approach I think is worth much.