How To Insulate Basement Walls
Insulating Basement Walls
Insulating a basement properly is rather challenging in cold climates. In this article I’d like to discuss how to insulate basement walls in cold climates.
Insulating basement walls in cold climates is a great way to keep your home warmer and drier. However, special care should be used when insulating concrete (or block masonry) basement walls. Below grade concrete foundation walls are very cool and damp. If you were to insulate the walls with regular fiberglass batt insulation it is very likely that a mold problem would develop.
Recommended Basement Insulation
Therefore I recommend using a combination of products to insulate basement walls. In order to create a vapor barrier and separation between the concrete walls and wood framing I like to use extruded polystyrene insulation (blue board from DOW).
Next we frame a traditional wood stud wall in front of the polystyrene insulation. We do take one special step in framing the wall. First we install a layer of composite decking between the concrete slab and the pressure treated bottom plate of the wall. Finally we install some type of insulation in the wall cavities.
Step 1 – Install Extrude Polystyrene Insulation Boards
The first step in insulating a basement wall is to install the polystyrene insulation (for more information we recommend reading: R Values Of Foam Board Insulation). We like to use a all purpose adhesive that’s approved for Styrofoam products. After the polystyrene insulation is installed each of the joints is taped with Tyvek tape or some other approved tape that adheres to polystyrene insulation. By taping the joints you are helping keep moisture and cold air from infiltrating into the stud wall cavity.
I recommend you seal the top and bottom of the foam board using spray foam from a can. You can buy products like Great Stuff that will seal those joints very well. Be sure that you install foam board on top of the concrete wall up tight against the wall plate and seal that well.
Step 2 – Frame Basement Wall
Framing the basement walls is very straight forward. Use traditional wall framing techniques with two slight modifications. First I recommend installing a piece of composite decking (Trex, TimberTech, etc.) on the slab (use a Powder Actuated Tool to shoot the decking into the slab) below where the wall bottom plate will rest.
By installing a piece of composite decking (see arrow in photo) you will ensure that water does not “wick” up into the wall should there ever be any water leaks in the basement. Next just frame the wall with a pressure treated bottom plate and stand it up. Nail the top plate into the first floor joists and then nail the bottom plate into the composite decking. Be sure to plumb the wall with a builders level.
Step 3 – Insulate Stud Wall Cavity
Now you’re ready to insulate the stud wall cavity. There are a couple of options here as well. You can insulate the wall with fiberglass insulation, you can use wet sprayed cellulose insulation or you could use spray foam insulation. I insulated my basement walls with fiberglass insulation. For this house we’ll be using wet sprayed cellulose insulation. I’ll be sure to write several posts about the spray applied cellulose insulation when we get to that step of this project.
Rim Joist Insulation
Don’t forget to insulate your rim joists. Rim Joist Insulation is a very important component of the basement insulation process. There are several options including spray foam, foam board and fiberglass insulation. We recommend using rigid foam board insulation if you’re going the DIY route.
Vapor Barriers
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.
Step 4 – Finish Wall Surfaces
The final step is installing some type of finished basement wall surface. We’ll be installing normal drywall in this basement. You could certainly use most any material type that you want.
Basement Wall Insulation Summary
Insulating your basement walls will help keep your home warmer and drier. By insulating them properly and creating an effective vapor barrier you’ll be able to reduce the amount of moisture that enters into your basement from the damp moist concrete walls. Concrete is always full of moisture so it’s very important to keep materials like wood and fiberglass insulation away from the concrete surface. By taking the time to properly insulate your basement you’ll be able to enjoy the extra space for years to come with little maintenance.
Hire A Professional Contractor?
Another option is to hire a contractor. If you’re feeling overwhelmed by all this information then you might want to consider a contractor for your basement remodeling project. We recommend calling your local BBB, chamber of commerce or using a referral service like the following:
Recommended Tools and Reading
I also suggest you check out the following insulation books: Insulate and Weatherize and The Homeowner’s Handbook to Energy Efficiency. If you’ve just recently moved into your home you may be able to take advantage of a Lowes Coupon for new home owners and save some money on your insulation supplies.
Related Article: Basement Wall Insulation Detail
You may want to invest in several tools for a large project like this. I’d recommend you look at the following:
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I am finishing a basement laundry room (10 x 10) of a 1952 house. The walls are painted block and moisture has never been an issue. There is a drain in the cement floor by the washer. There is no finished ceiling. I would like to apply styrofoam sheets to the block for additional insulation. What would you recommend? Metal chanel studs are in place 25” on center.
Thank you
Jill
@ Jill – Depending on where you live I’d recommend a minimum of 1-1/2″ Foam Board. Just be sure you cover that foam board with drywall as it’s a fire hazard if left uncovered.
I live in the Atlanta, GA area. We are about to start framing our basement. Before I do, I want to insulate the concrete walls. Due to being a high humidity zone, do I need thicker rigid foam, or less? Does it matter?
@ Todd H – Down there I would only use foam, 1-1/2″ to 2″ with NO fiberglass.
This is a great website. Could you please answer my question.
The area of the basement I want to finish has waste pipes, the main water lines, main electrical box and stairs next to the basement walls. There is not enough room to get 1 1/2 (or any size) foam board behind these items. Should I just cut the foam board around those items, and leave them next to the exterior wall?
@ Tom – You do the best you can. Obviously some older homes have things that interfere with a perfect solution. Good luck.
I am finishing a basement in Rochester NY and our basement is a floating slab with a drip edge around the perimeter. What steps should be taken with the DOW board at the bottom near the drip edge?
Thanks
@ Paul – Do you mean a French Drain around the perimeter?
Yes, there is a drainage gap between the floor and the wall.
@ Paul – I would bring the foam down to the top of the stone. Then when you frame the walls, leave them out in front of the French Drain..sitting on solid concrete.
So I am going to use XPS on the walls before framing. Im in KY and I have found recycled XPS 2″ 2×4 sheets for 3each. I think this would do, but adding to the dtud fram would be a 4inch from concrete wall. is 2″ just too much? or is this just the cost of good insulation.
@ Andy – 2″ is perfect in IMHO……
Hi there:
Great article… I live in Toronto … and just started covering concrete wall with Foam board insulation. I have a question …
The foam borad does not flush with the concrete wall… I have tried different Glue type (Premium PL300 to PL500) and push lot of pressure but board just does not flush tight with the lower half of the wall (below grade)…and leave a gap between Concrete wall and board.
Will this gap cause problem? I guess, my concerte walls are not straight…
Can you please advise…
Thanks.
Pradeep – You can fill the gaps with spray foam from a can, products like Great Stuff. The gaps should be filled and the seams taped.
Thanks for your reply.
I am applying Great Stuff around the boam board and between two Foam boards and taping with “Red Tape”.
But how can it fill the gap between back of the foam board and Concrete. Lower part of Board is not flush completely against the concrete wall.
Thanks again!!
Pradeep – The back doesn’t matter. What you want is for the “plane” of foam to be sealed. If you need to seal the bottom as well. You’re just trying to create an insulation layer that has no holes in it. Make sense?
Perfect !!
Yes I am sealing around (including bottom) the plane of the foam.
Thanks again for your prompt reply!!
Have a nice day.
I removed the existing drywall and fiberglas insulation so I could check for cracks in my below ground cinder block basement wall. Now that I’ve seen the wall, I wonder if I should paint the block wall with a water proofing paint before installing the “blue board” insulation. What do you recommend I do? Our basement has always been very cool and damp. So damp that we have to use a dehumidifier to make it comfortable. How do you recommend I finish the wall?
Burnselk – If you’re going to install foam I don’t think the paint is necessary.
NEW CONSTRUCTION. BASEMENT WALL, FOAM BOARD THICKNESS (ALSO TYPE BOARD)IN NASHVILLE, TN.
Walker – It depends on your local/state energy code. Typically we like to see a minimum of 1-1/2 inch thickness which should be sufficient down there. Check your local code to be sure. I prefer a closed cell blue board (or pink). You can also use foil faced polyiso.
I have a framed out basement in North Dakota the walls are all in place with no foam board behind them. Can I just put foam board up in between the studs? Do I use an 1.5 inch and then put fiberglass in front of that or should I use a 3 in product to get close to the front of the stud
Aaron – Ideally you’d move the walls and put it behind. If you can’t/don’t do that then there are a couple options. One is to have the cavities spray foamed and the 2nd option is to install the foam between stud bays, however that makes it VERY difficult to seal properly and keep moisture out. Put in as much foam as you can afford.
Todd,
I have started hanging up the blue board, and I was wondering if you would answer a couple of questions. I assume I leave the main electrical panel attached to the concrete wall. So how close do I put the blue board to the panel? If there is a gap, do I fill it, if so, with what?
My second question is, how close to the HVAC ducts do I put the blue board? Do I just leave a gap, or do I fill it the “Great stuff”, maybe the high temp stuff.
Thanks for all of your help on my project.
Tom – There are practical limits to this approach obviously without disconnecting all the utilities. Having said that I think you’re on the right track. I would install the blue board up as close as possible to the electrical panel so long as you don’t hinder it’s safe access. As far as the duct work it really shouldn’t be an issue unless it’s a flu pipe.
Hi Todd:
I have built the frame in front of Foam board. But there is little time before I put the Batt insulation and drywall. Can you suggest how to keep wood frame (& 2x4s) from NOT bending? I heard if I do not put the drywall quick enough … wood stud will start to bend.
Thanks for your help!!
Pradeep – How long? Stud framing will be ok for several months or more if you’ve purchased fairly dry lumber.
Hi Todd:
It is 2x4s from Homedopet (I have to find out if they are dry lumber… I assume so). I have to give a break from now until mid-June. I hope hot season is not going to ruin my hard work.
Thanks,
Pradeep Puri.
Pradeep – You should be fine, keep the sun off them and things will be ok.
Todd,
Thanks again for the great site. I have the foamboard up on the basement wall and have taped all the seams.
We are currently not planning to anything with the floor at this time. There are slight gaps on the flooring side should I spray foam the gaps to completely seal this in before I butt the walls up to the foam.
Thanks,
Drake
Drake – I would while you can still reach the gaps.
Todd,
I have a house with brick on the first floor and siding on the second. I have started the basement but I have a problem. Yesterday it was very winding and rained all day, very heavy. I have some water coming in the basement. It is coming in above the glass block window. I do not see anything wrong with the mortar around the window and I have checked the siding. I read that if there is driving rain the rain could be absorbed thru the brick, and that I should put on water repelent on the brick. Have you heard of this. If so do you know of a product? I do not want to contiue the basement until I stop the water. I would appreciate any help. Thanks
Tom – That can happen but more than likely you’ve got a bad flashing detail. How is the siding flashed above the brick?
There is a 1×6 piece of wood that goes the length of the house. Half of it is above, half is below the top of the brick. On top of the 1×6 is a piece of aluminum trim, that bends over the top of it. It is 1″ above the 1×6, bends the thickness of the 1×6 and then extends down 1/2″. Above that is the siding starter strip. That is it. Can you tell me how it should be.
Tom – I emailed you, can you send me a photo?
Todd,
Great site, thanks for answering all of the questions that are posed – it really adds a great deal of depth to the discussion.
I have a question regarding adhesives used for bonding the XPS to the foundation wall. It appears that the “industry standard” adhesive is PL300 (specifically for foamboard) but after reading the warning statements on the tube it raises some concerns. I am interested in using products in my basement finish that offer as low as possible VOC levels; I found a Loctite PowerGrab product that is also specifically formulated for foamboard (and is much friendlier) but haven’t found anyone who has used it in practice. Has anyone here used the Loctite product?
Thanks,
Chad
Chad – We’ve never used that product. Frankly most adhesives do a poor job with the foam. We’ve been using Great Stuff Pro lately with very good success. Having said that it’s an aerosol product which speaks poorly for VOC’s most likely. The one thing is the adhesive goes between the foam board and concrete which most likely seals it out.
I used the Loctite adhesive with good results.
The Loctite “Foamboard” specific adhesive is more for ceiling tiles I think. It doesn’t say it bonds to concrete. So I used the Loctite “General Purpose” which it says is ok for foamboard.
I haven’t used the Great Stuff Pro so I can’t compare, but I can say it worked better than PL300 and it doesn’t smell much at all. I did have to use some bracing along some of the top and bottom edges where the board and/or walls were a bit crooked, what I used for that is plate weights on the bottom and scrap wood screwed into the trusses on the top.
I’m no expert by any means but that seems to have worked for me.
Phil – That’s a good solution as well. Lately we’ve been using the Great Stuff Pro and it works beautifully, by far the best thing we’ve tried. Thanks for sharing your experience with Loctite
I have a concrete block foundation in which someone drilled holes at the bottom of the blocks for drainage (although i’ve never seen water). I’m guessing this is so that the water that may seep into the center of the blocks from the outside has a place to go instead of seeping through the walls. There is about an inch gap between the wall and floor with crushed stone in it for any potential water to drain into. I intended to put rigid foam board on the wall, but what should I do at the bottom of the wall where these holes exist? Should I just run the foam down to right above the holes then then frame the walls a few inches away or is there a better plan of attack?
Zach – Here’s what I would do if it were my house or one of my projects.
1. Adhere the foam board to the wall and stop just short of the holes. Tape all the seams well.
2. Install a small piece of foam along the bottom so that it over laps the first layer and leaves a gap at the concrete. The gap will be the size of the thickness of the first layer. Seal that small piece well to the first layer.
3. Frame your wall in front of the second layer. This allows the weep holes to still work properly if you do get water.
Excellent, that makes the job easier. Using your suggestion, and assuming i’m using 1.5 inch foam board, i’ll be 3 inches away from the concrete wall, and can frame my walls there. I assume there is no need for a vapor barrier with this configuration and then use standard fiberglass insulation and then drywall, correct?
Zack – You are correct. Just be sure to seal all the joints VERY WELL. Good luck.
HELP!!!
I’m finishing my basement and it’s already partialy framed with 2×3 studs.. I figured I’d get some rigid foam pink board and cut the boards to fill the studs.
Now I see these boards are a fire hazzard… Even if I cover them with Sheetrock, won’t the backsides be exposed and flamable – heck there will be electrical wires runnig behind and thru the foam board – will a spark set these things off?
Should I just use regular roll out batt insulation – maybe put a section of foam on the bottom for moisture reasons?
Steve – Foam board is only an issue for flame spread. Putting it inside the wall and covering with a layer of sheetrock is perfectly acceptable. DO not put plain fiberglass, it will mold!!
BUT.. the inside of the wall (facing the cinderblock wall) won’t have sheetrock covering the foam board… Only my interior (finished portion) will be covered…
Sounds to me like that’s still a fire issue?
tks
Steve – Not at all, it’s done every day. You would have to have a flame source to even consider it an issue.
OK… I guess I just don’t get it.
You do realize I have a 1 ft gap in between the framing /insulation and the Cinderblock, right? Maybe that wasn’t clear – it was framed that way already with wood paneling that I took down.
In my opinion, you would “Need a flame source” for anything to be a fire issue.
Your prompt feedback is AWESOME btw…
Can you come help me? ;)
tks
Steve – If it makes you feel any better you could use a foil faced foam. Foil faced foam has a really good flame spread resistance.
quick question
1.5″ Foamboard on basement walls. Do I frame right against the foamboard or should I leave a space.
thanks.
Trez – I’m not sure it really matters much to be honest, which ever is easier.
I have a finished basement and noticed the walls over the concrete area are finished in this order concrete wall vapor barrier (plastic) foam insulation and wall frame then vapor barrier plastic and wall board. They seem fine.
Do I need to re-do these walls
thanks
Mark – I think you’re ok. The fact that you’ve got plastic, then foam, then framing should keep any moisture out.
great thanks Todd
Our basement is a concrete poured basement (Wisconsin) and the builder installed a rough in walls w/ fiberglass insulation. We’re now in the process of finishing off the basement, and I want to add foam board insulation to the floor sills then spray foam it in place. There is already fiberglass insulation there, can I keep it there along with the foam board?
Thanks
Brent – Not sure I completely understand your situation. Are you saying that you want to use foam board to insulate the rim joist? Where is the existing fiberglass? Fiberglass does VERY poorly by itself in the basement and will most likely mold at some point if it’s touching the concrete directly without some type of foam insulation as a buffer.
I am currently building a new home. My plan was to put 4 mil plastic against the concrete wall, than 2×4 studs with fiberglass batts in between studs, followed by another layer of 4 mil plastic over that. Is this a good method? Where do you see this being a problem?
David – I wouldn’t recommend that method at all. It’s the classic wrong way to insulate a basement wall.
First off you’re creating two vapor barriers with in a sandwich with fiberglass, a classic recipe for mold. Secondly the first layer of plastic will always be cold/cool, any water vapor in the cavity will condense on that cool/cold surface.
thats kind of what i was thinking. what about an inch foam board adhered to the concrete than 2×4 stud wall with bat fiberglass and 4 mil over that. is this a good way to do it? i guess just tell me what would be the best method and that is what i will do. i can insulate the outside side if that is the best way. thank you for any and all help.
David, if you read through the article you’ll see that I recommend a minimum of 1-1/2 inches of foam. You need that much to create a vapor barrier.
I live in Northern IL and my basement is cool and damp. I will be installing the 1.5 inch foam board as suggested. My brother-in-law used a plastic sheet material on the wall first, then nailed studs to the cement wall. Then, used fiberglass insulation in the stud wall. What do you think?
John – I think it will have problems. Unless that plastic material carries a strong R value it will get cold/cool and any moisture inside the wall cavity will condense on that surface, get the fiberglass wet and cause mold/mildew.
Great Article, I am finishing my basement and I have completed all the stud work. Due to the number of sewage pipes in the basement my framed walls are located between 12 and 20 inches away from the poured concrete foundation.
I live in New England and I am unsure how to insulate the walls. Do you recommend me using foam board on the concrete walls and then fiberglass insulation inbetweeen the studs with a vapor barrior towards the drywall?
Thanks
Steve O – Well you have a unique situation with the walls being so far from the concrete. Typically we install foam board and use the walls to keep the foam in place. You don’t have that luxury with your situation. So….I guess if I were you I’d install foam on the concrete and use a few pieces of strapping to hold it in place. If you use at least 2″ you really won’t need fiberglass unless your local energy code requires more.
Our basement has the insulation, blue board, installed on the outside of the foundation walls, full height. Should I or do I need to install it on the inside also. I was told installing it in bothe sides will give the concrete moisture no where to go and one side needs to be able to breathe. It was also suggested that if I want insulation on the inside for sound issues that I use a white styrofoam board.
Josh – without knowing how much you have on the outside and what details were used (does the foam run up onto the house in a continuous manner?) it’s hard to say if you need more. Frankly concrete will ALWAYS be full of water for it’s entire life, it’s a result of the internal chemical composition. So, saying that concrete needs to breath is a big off base. Concrete does like to go through drying and wetting stages and it’s important to not allow the water vapor to enter places that it’s unwanted, like a wall cavity. If you exterior foam runs from the foundation, up over the wall framing up to the roof then I would agree in part with what you’re saying. However, if it stops at grade or slightly below then I would argue that the water vapor can dry to the outside above the foam. In that case I would still insulate the basement with closed cell foam.
Whom ever suggested the white foam is suggesting you use an open cell foam. The problem with that is it can actually let water vapor pass through it, into the framing and likely the back side of drywall or plaster. I wouldn’t recommend allowing that to happen. Make sense?
I have 1″ blue styrofoam on the outside, full height of the foundation walls which comes a few inches above grade. Above that I have vinyl siding over the same 1″ blue styrofoam over the wall framing. Any different thoughts with that info?
Josh – Does the lower blue board run continuous or is there at break at grade level? If there is a break then the foundation can dry to the outside, above grade, which means I would still insulate inside with additional foam. 1″ on the exterior isn’t sufficient to meet most energy codes.
Todd,
I have the foam boards and 2×4 up. I have a couple of questions if you could please answer them. Thanks
I am install four new circuit breakers and I want to extend the HVAC ducts down to the floor. How do I route the wires around the duct in the wall. The duct is an oval 3″x8″. Do I just drill holes thru the 2×4, route the wires though the holes, and then route the wires behind the HVAC duct?
Thanks for answering my question
Tom – Sounds like a plan to me. Often times wire are routed along side duct work.
Todd, I thought I remember reading that you wanted to leave a 1″ gap between studs and foam for air circ.? Above you mentioned that you can put the studs against the foam.
Also, if I use 2″ foam and it meets the code it is okay to not use fiberglass? Basement is pretty warm in the winter with nothing right now. I think the foam is enough.?
Roger – If you can get 2″ and it meets code then it’s absolutely better not to use the fiberglass. The 1″ air space is a nice thing to have if you’re using the fiberglass but not 100% necessary.
Thanks for the quick response. I thought of two more minor things.
On my one wall I mounted my piping with uni-strut on the wall. I was going to bring the foam to it from underneath and down from the top. Then spray foam the gap in between (behind the PVC drain)
Second… On the two walls that will face the unfinished washroom and storage areas I was thinking of using that thin foil bubble type insulation stapled to the back of the studs.
They claim a pretty good R value with sufficient space. I would have 3 1/2″ on the one side and 8′ or so on the other. Plus it would be clean when walking near it. What do you think?
BTW excellent site, I was glad to have found it. Thanks.
Roger – Sounds like a decent plan. Most basements have issues with utilities and it’s always a battle between cost and best approach. I’d stick with your plan.
Todd -
A few questions about a new basement remodel. We are in a new house in Colorado. The basement is poured foundation walls. Some exterior walls are full height- Some are not and or completed with 2×6 framing on the tops.
When considering using foam board for the first layer of insulation applied to the concrete before the studs, how do I deal with the upper section that has been framed in with 2×6′s and insulated with regular open faced insulation and covered with a plastic vapor barrier ?
The basement slab has a 1/4-1/2″ gap around the exterior. Is there anything special I need to do to be sure that any type of radon gas is able to escape ? They have installed a radon powered vent into our slab, but the edges are not sealed and some of the expansion joints have cracked a little. Can I fill them with flexable driveway cauck ? Thanks Walt
Walter – You can either stop the foam at the transition or run it over the framing.
As far as the cracks I’d fill them with a flexible concrete crack filler.
Todd,
We are finishing our basement in a Townhouse…do we use the foam board on concrete walls that are adjoining other basements as well.
Deona – Foam board or spray foam is the only material that I recommend in direct contact with concrete or block wall.
Great resource! Thank you.
Living in Southern California, I have a basement that has a crawl space: Part stem wall that’s poured concrete and part masonry … roughly 40′(l) x 2′(w) x 2′(h) or 160 cubic ft. The basement has an exterior french drain, but hydrostatic pressure pushed moisture thru the control joints and random spots on the plate.
This space contains mechanical, electrical and plumbing of sorts that services the upstairs and finished basement. Do you recommend venting this space with exhaust fans? One on each end of the space?
Do you let fresh air? or does this promote moisture?
Slightly confused. Any info is greatly appreciated!
Than
Justin – Crawl spaces are a real challenge regardless of where you live. There are now two different approaches used for crawl spaces today. In the past ventilated crawl spaces were considered the best approach. Today more and more building officials are embracing the unventilated crawl space. Each of these methods are drastically different with pros and cons.
With unventilated crawl spaces the idea is to insulate the walls and not the floor above. However, if you have mechanical equipment in there you’ll need to be sure it’s properly ventilated. You would also install a vapor diffusion retarder on the sub-grade to control moisture. The idea here is that the space gets dried out, then additional moisture is locked out.
With a ventilated crawl space you insulate the floor system and install a vapor barrier on the bottom of the insulation. This can be difficult to do properly if there are lots of electrical and mechanical penetrations. Running ventilation can be expensive and prone to periodic maintenance.
Considering your water problem I’d suggest trying to get the water away from the house and then deciding if you want to insulate the floor or the walls. Once you determine that it’s pretty clear whether to ventilate or not.
Make sense?
Todd-
Lots of great information here, wish I would have found this a few months ago when I started my basement project. After reading the site I am trying to determine my best course of action. Here is what I have already done:
- I am only partially framed with wood studs (about half way around a small basement ~ 700 sq ft)
- Bottom plate is PT and is fastened directly to slab with powder actuated fasteners.
- Back of studs are about 3/4″ from painted block foundation walls.
- For what its worth, my basement was created by deepening the crawl-space of the original house, probably in the early 80′s according to seller. This creates a shelf all the way around the basement with the original concrete crawl-space wall at the top 3 feet and a newer masonry foundation wall about 1′-6″ inside of that for the bottom 5 feet.
My issues. I want to do this properly (I thought I was), but I also don’t want to rip everything out that I have already done (money and time).
1) I could probably get a vapor barrier and 3/4″ foam in behind the studs. Would this be okay with batts placed between studs?
2) I noticed some dark spots appeared on the slab below and around the bottom plate (in isolated areas). This is obviously moisture, and it makes me concerned that there will be a problem when I close the walls in and lay carpet. Should I replace the bottom plate with the composite plate as you recommend and if so, any idea how this could be done simply?
3) Along the same line, I am also thinking that I should put in a prefab panel floor system like dri-core as vapor barrier/insulator. Raising the door frames shouldn’t be too bad. I don’t really need any comments here, unless you think this will not be adequate to address the minor moisture issues we are having with the slab.
I appreciate your time,
Nate (Rockville, MD)
Nate, Thanks for the compliment and stopping by the site.
1. Being from MD certainly helps your situation a bit as your really cold weather probably isn’t as severe is it can be up here in New England. 3/4″ foam plus a vapor barrier can stop moisture from the foundation side from entering the framing. However, if moisture from the finished side, enters the wall cavity, and hits the 3/4″ foam (which may or may not be cold (at least at the dew point) it could possibly condensate on a really cold day. The 1-1/2″ minimum requirement tends to exist for two reasons: First because 1-1/2″ will generally provide a good vapor barrier and secondly 1-1/2″ which generally be enough R value to keep the surface from reaching the dew point.
Having said all that I suppose you’re in a gray area. If you could afford it I would recommend spray foaming the stud cavities in place with closed cell foam, that way you’d get good R value, good vapor barrier and a super product. Short of that you could install the 3/4″, seal it very well, then install another 3/4″ between studs and seal that to each stud with spray foam in a can.
2 & 3. Anytime you place a material down on a concrete slab you’re going to see that. If you tape plastic to the slab you’d see this happen. At this point I would worry about the composite decking unless you take down the framing to install foam (which is another option for #1. You might be able to cut free your walls, lay them down, install foam, stand the walls back up).
I hope this helps.
Good luck.
This is one reason I try to steer folks away from carpet unless you install some type of sub-floor. Dri-core is a great product and likely to take care of any problems from moisture.
Thanks for the advise. I am going to go with taking down the walls and putting up the 1.5″ foamboard insulation. I will be installing dricore (or similar system) on the slab, thus I will just set my walls directly on this sub floor. This should keep moisture off of the bottom plate (which will also be PT).
Dricore recommends a 1″ gap from the concrete wall, I would think in my case that would be from the insulation (I assume this is for air flow). Since our basement is small I may turn the studs to save space. Another option would be to replace everything with thinner metal studs and run the electrical behind the studs (in the 1 inch “cavity”). Does this seem reasonable?
Thanks again, I am very happy I am finding this out now so I don’t end up with a much bigger problem later.
Nate
Nate – All sounds pretty good. Why not just frame the walls with 2×3′s?
All right, I think I am on the right track. Last couple questions then I will leave you alone.
Could I use non treated wood as bottom plate since it will be on top of the subfloor? (hard to find PT 2×3)
Also, as mentioned in my original post, this basement was an afterthought add-on to the original house. This brings up a few items I am unsure about.
1) The original crawlspace walls have a tar-like substance with some form of yellow fiberglass insulation adhered to the wall. My plan has been to leave this in place, but seems like I should remove that to apply foamboard insulation. I assume I should not install the foam over top of the this stuff?
2) The shelf around the perimeter of the basement is being preserved. Would it be okay to just use the subfloor material up there, or should I lay the insulation flat on that level about 5′-0″ up? I figure I can maintain that air void from low slab to top of wall if I use the subfloor panels. Thoughts?
3) There are several areas where walls are uneven and bumpy. I should just do my best with the foamboard and tape then fill with ‘great-stuff’?
Thanks again, Nate
Nate – No worries on questions….
1. Not sure what that insulation is. Does it make any sense to frame your walls from the floor up past the “shelf” and make that disappear? If so you could run the foam up to the ceiling, seal it and forget about all that back there. Just a thought..hard to say without seeing a photo.
2. I’d put foam on top of the shelf.
3. You are correct.
Good luck!
Todd,
Thanks for all the great advice. I live in the Buffalo area and just purchased a bungalow that’s 4 years old and want to finish the basement for my kids.
My plan from the wall out: 2 inch foam board glued and taped, composite 2×4 fastened with concrete screws around perimeter where PT bottom plate of frame will be fastened, fibreglass insulation in wall cavity and drywall.
What the best type of foam board? Do I need fiberglass insulation?
Should I spray foam the floor joist space?
I’ve read to put foam board on the concreate floor and put two layers of 1/2 inch plywood on top, is this necessary? What’s the best flooring option? What’s adequate?
Finally, what’s the best ventilation set-up for the basement.
Should I set-up a return air in the basement?
Thanks,
Steve
Steven
With 2″ of foam board it’s a toss up. It partly depends on if you have a State Energy code requirement and what R value is required. 2 inches provides a pretty nice insulation for basements.
Just be sure to use XPS foam (closed cell), either the blue stuff or pink stuff is fine.
I like to cut foam board to fit between joists, see: http://www.homeconstructionimprovement.com/rim-joist-insulation-insulating-options/
Insulating the floor is an option. Just depends really on how warm you want it and the type of flooring. I prefer doing tile in basements here in New England then doing area rugs if you want some warmth. If you go with carpet then insulating and sub-floor is a good bet. However, ONLY do that if you’re certain that flooding isn’t going to be a problem. See: http://www.homeconstructionimprovement.com/how-to-insulate-a-concrete-floor/ Note: there are many versions of this approach that work well.
Ventilation is a tough one. For starters be sure you ventilate any bathrooms. Also be sure that if you close off the furnace room that you tell your heating people so they are sure plenty of make up air is available. Finally installing a simple exhaust fan with a timer wouldn’t be a bad idea, especially if you run it occasionally to help more the air.
Todd,
Does it help to paint concrete walls and floors first?
There are products that contain dimples that act as subfloors, do you have any experience with these products?
Finally, if I use laminate or cork flooring do I need to go through the process of creating a subfloor?
I basically want to achieve as much height as possible in the basement and have a warm floor.
Thanks,
Steve
Steven,
Most of these questions can be answered if you follow the questions from other readers. However, here are some short answers.
1. I’m not convinced that “paint” or “sealing” the interior of concrete walls will pay off that much. I say this assuming that you may install a good layer of foam to help seal things up.
2. The products you’re referring to do work very well. We’ve used them several times and never had a problem.
3. Really depends on if you have any moisture problems and how cold that floor is. Many commercial buildings install laminate directly on the slab without any issues. It’s really a decision based on cost, available height and if you think there’s any potential for water.
I have insulated the exterior walls of my walk out basement with
kraft faced insulation ( which I found out is not code)
The building inspector advised me that there is a product that is fire rated that I can put over this to meet code, he did not know the name of the product. I am not ready to put up the drywall yet do you know the name of the product
Bruce – Depending on where you live the issue is flame spread. What you need to do is find out which building code you have, and what the flame spread criteria is. It’s possible that some kraft faced products actually meet that criteria if installed properly. If not I’d seriously consider drywall as the product itself is rather inexpensive if you can install it yourself.
Todd,
What do you think about using PVC trim board in lieu of the composite decking? It’s half the cost of composite decking, however I’m not sure if will crack when I nail it. Also, I can’t find deck boards in widths less than 6 inches. Do you rip the boards when you install them or just let the sheet rock take up the last 1/2 inch (1.5 inches foam + 3.5 inches pressure treated)?
Thanks,
Matt
Matt – PVC trim would work just as well, great idea! We rip it down to width.
Todd,
Great article. We live in North NJ and we had a sump pump fail in our finished basement (done about 1974) and as a result everything had to be ripped out. We are getting ready to rebuild but unsure what to do with the walls. When we ripped the paneling off the walls we noticed the walls were framed out with cedar planks which were offset off the wall by furring strips. The cinderblock walls have a black/aluminum finished ( I think tar and aluminum ) with this cedar on top.
Basement has a new sump pump with a back up , so no issue there and it was dug up at some point and French drain piping was put around the perimeter.
my question is, do I leave the cedar and XPS then sheet rock over that? or should I tear that down as well and install the XPS/2×5 frame method you suggest in the article.
thanks
Mike
Mike – I would tear it all out and here’s why. What you want to do is eliminate any materials that would mold (wood) from between the block and foam.
Hello Todd,
I’ve been following your site (and your patience) for some time now. My semi unfinished daylight basement in the Pacific NW has some stud walls we need to leave. This negates the ability to build walls on a subfloor. So far I have glued pink 2″ xps all around the 3 concrete walls. We are then framing with 2x4s up against that. I meticulously taped all seams, and put either 2 or 1 inch pink on top of the concrete horizontal portions, spray foaming or taping them to the vertical boards for a vapor barrier.
Questions: in one small space tearing out the studs that are close to the concrete would be a nightmare. Any issues using great stuff behind the studs, then xps between them, using the spray stuff as a sealant?
Also, here in Oregon code says R15 for basement walls, 2″ pink says R10. You seem fairly clear to avoid fiberglass, but any issues with it in the studs up against the foam board?
Finally, this basement, for being 25 years old, was done really well. No leaks at all, plenty of headroom, etc. We planned on tile in the kitchen area, bath, and maybe the resilient tile for storage and craft areas, using like a low pile commercial carpet for the family area. With no water issues, what would you do with the floor considering the walls are in place first? My main helper-a contractor, is wanting to tile to the slab, rubber pad over slab for the carpet. I’m thinking you won’t like that approach.. :) Subfloor or not, if so, ideas?
Thank you,
Dave in Oregon
Dave – Thanks for following the site and the compliment.
So far you’re doing a great job!!
Everyone has a small area that is impossible to do much with, old wall framed in front of stairs, etc. In these cases you do the best you can and your approach is exactly what I would do.
You can actually install fiberglass in that stud bay as you’ve described. I’ve done this in my own basement and it works fine so long as things have been sealed properly. The real question then is weather to use a faced or unfaced insulation. I prefer using a kraft faced insulation or no vapor barrier at all. I would avoid poly at all costs.
I think your flooring approach is fine. Just be sure to discuss the fact that this is on a slab with your flooring people. You need to be sure and use a pad/carpet that is acceptable for on slab applications. Sounds like your basement is plenty dry.
Good luck!! Thanks for sharing your project with us.
Thank you as well….I will update as I go!
Kind regards,
Dave
I am in the very early stages of finishing my basement(planing and pricing), and am going to be following your and building science advise, xps on concrete then insulated 2×4 wall. My problem/question is that 2 of the 4 concrete block exterior walls are thicker at the bottom then the top. So basically about 4 feet up the wall their is a four inch angled shelf. What I was thinking is run the xps up the wall, cut the xps at the shelf, put xps ontop of the shelf then continue up the wall. Now when I build the wall in front it will be up against the xps at the lower half of the wall and a 4″ gap at the top half. Is this 4″ gap a problem, any other solutions?
Thanks for the informative site and any advise you can give me.
Im sure I’ll be back. as this is only the beginning.
Mark
Mark – Your plan is fine. No issues at all. Good luck and hope to see you back. Be sure to sign up for the new forums and post questions there as well.
Hi!
I have two questions. My parents bought a house in Birmingham, Alabama which had a very bad mold/mildew problem in the basement. We located where the moisture was coming in treated the mold, painted the cement block walls with a sealer, framed the walls, added fiberglass insulation and a plastic vapor barrier, then sheetrock. Did we do too much? After reading your articles and comments I’m afraid the over kill will kill us. I plan on following the same steps in my Queens, NY basement. What would you recommend we do? I really hope we did the right thing.
Zillah
Zillah – It’s likely that the basement will develop mold over time. Installing fiberglass against concrete block (even though it’s painted) is a bad idea. Concrete block is typically cooler than the surrounding air, full of moisture and likely to cause condensation to develop. Once that happens the fiberglass will absorb that moisture and things will start to mold.
So…I wouldn’t recommend you do that in Queens and I wouldn’t be surprised if the AL house develops mold.