Vapor Barrier For Basement Insulation
Basement Insulation and Vapor Barrier
We’ve written several articles about basement insulation and a cost effective approach to basement insulation using foam board and fiberglass insulation. Since writing those articles we’ve received quite a few questions about when to use a vapor barrier and when not to. So we thought it might be a good idea to clear up some of the confusion.
Understanding Vapor Movement
Before we explain where to use vapor barriers it’s a good idea to talk about where the vapor comes from which ultimately makes the discussion easier to understand.
First off you need to think of your concrete (or block) walls as a huge sponge for moisture (water vapor). Over time and throughout seasonal changes in temperature concrete will “dry” out releasing a tremendous amount of water vapor. The adjacent sketch shows an unfinished, un-insulated, un-heated basement wall. We’ve shown arrows that indicate where the water vapor goes as the wall “dries” out.
Depending on the time of year it’s possible that all that humidity in the air will turn around and condensate on the cool concrete surface if the dew point is correct. The point here though is how moisture in the form of water vapor leaves the foundation walls and migrates into the basement space or outside above grade.
Bad Basement Insulation Detail
One of the worst basement insulation details that we see on a regular basis involves framed walls adjacent to concrete foundation walls. The cavities of the framed walls are filled with fiberglass insulation and a vapor barrier is installed between the studs and drywall.
This basement insulation detail is a really bad idea. As the concrete dries and gives off the water vapor it get’s trapped between the concrete wall and the vapor barrier. This causes the fiberglass insulation to become saturated with water. Water laden fiberglass is extremely ineffecient and very prone to mold growth.
As you can imagine once the fiberglass becomes soaked in water it’s nearly impossible for it to dry out. The fiberglass must be completely removed and disposed of. It’s also very likely that mold will have grown on all the studs, drywall and floor joist framing above.
Better Basement Insulation Detail
One of the best ways to insulate basement walls is by using spray-in-place foam insulation. However, spray foam insulation can be VERY expensive for some projects. That’s why we’ve come up with a hybrid insulation detail that uses a combination of rigid foam insulation and fiberglass insulation.
This detail can vary greatly depending on what part of the Country you live in and what R values are required by your local building codes. The idea for this detail is to install a layer of rigid foam board insulation, carefully seal it to create a vapor barrier adjacent to the concrete, then frame a wall and fill the cavities with fiberglass insulation to come up with an R value that meets the design.
- Install a minimum of 1-1/2 inches of rigid foam insulation board. It’s very important that the insulation be installed from the slab all the way up to the top of the wall including the top surface of the exposed concrete wall. If you use a thinner section of foam board you run the risk of it not performing as an effective vapor barrier.
- Carefully seal all the seams in the foam board. You can use a combination of Tyvek Tape (or similar) and spray foam in a can (Great Stuff for instance). This step is very important in order to create an effective vapor barrier.
- Frame a wall directly in front of the foam board. Typically we like to leave an inch gap to allow for air flow around the studs. Be sure to use a pressure treated bottom plate to prevent decay. We also like to install the PT bottom plate on top of a piece of composite decking material to prevent any wicking of moisture into the framing.
- Install fiberglass insulation in the wall cavities to create a final composite R value that meets the energy code requirements.
- VAPOR BARRIER – The real question ends up being whether or not to install a vapor barrier over the fiberglass and behind the drywall. Typically we are not in favor of a vapor barrier if you’ve installed at least 1-1/2″ of rigid foam (approx. R9). It is possible if you install a thinner layer of foam that the surface of the foam could be cool enough to promote condensation if water vapor moves from the conditioned room and hits the surface of the foam board. For that reason we recommend a vapor barrier if you’ve used less than the 1-1/2″ of foam. This is not a perfect situation and it’s one we recommend you try to avoid.
Bottom Line On Vapor Barriers
The bottom line really is to stop and think about where the water vapor wants to move. If you think about where the water is coming from you should be able to devise a plan that works effectively and avoids the dreaded mold growth. You can also read more about vapor barriers here.
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Hi Todd!
Thank you for your service. I was so glad to find your website and get definitive direction on properly insulating a basement.
I live in Michigan and am having my previously semi-finished basement redone. I have had some water leakage (cracks and rod holes) that will be sealed before insulating. Here are my questions:
1. Is spray on foam superior to rigid foam (i.e., is it worth the extra cost)?
2. How important is it to place the frame an inch away from the wall, and does the answer differ if I use spray versus rigid foam?
3. About 2/3 of my basement is already framed tight against the wall. Should I remove the frame (especially since I have had water leakage and the bottom plate is not pressure treated nor placed on composite decking material)?
4. I was not planning to drywall the foundation wall of my utility room and narrow storage room. How do I properly insulate these areas? I assume I should not leave them bare even though the outside walls of these rooms will be insulated?
5. Do I need to treat the walls or floor for any possible mold growth before insulating?
6. If I am installing carpet on the floor, how do I properly insulate the floor?
Sorry for so many questions, but I really appreciate your help!
Richard – Thanks for all the compliments. First off I would suggest fixing the leaks and waiting some time to be sure the repairs were effective especially seeing you want to use carpet.
1. Spray foam is definitely superior, it’s just a matter of cost.
2. The gap isn’t all that important when foam is involved, in fact, you can place the framed wall about a 1/2 from the concrete if you want and spray foam the wall in place.
3. This won’t matter if you choose spray foam. Be sure to check the plate for any rot.
4. If the outside walls are insulated you could leave them un-insulated.
5. You should check for mold before you start. If you find it the surfaces should be cleaned.
6. Check out my article on basement floors, http://www.homeconstructionimprovement.com/how-to-insulate-a-concrete-floor/
Good luck.
I am in the beginning stages of finishing a completely unfinishined/bare cement basement in Southwest Michigan. I read your articles on insulation and appreciate the tips, however I haven’t read anything on floors. Can you give me any suggestions on how or if I should go about insulating my cement floor.
Justin – Check out this: http://www.homeconstructionimprovement.com/how-to-insulate-a-concrete-floor/
I am a little confused. You state to install atleast 1.5 inch foam and seal alll joints to create an effective vapor barrier. How does the concrete wall dry up then?
Tariq – Concrete actually will never fully “dry”. In fact, you really don’t want it to. Concrete is created by a chemical reaction between water, cement (calcium and other chemicals), sand and stone. Water gets trapped in the micro-structure of the concrete and continue to react with cement particles for years. Water inside the concrete is actually a good thing! All we’re trying to do is keep the water away from the wall framing materials. I hope that helps.
Thanks for the great information. i’m starting my basement finishing project and am wondering if the fiberglass insulation you recommend installing in the wall cavity is supposed to be kraft faced or not? thanks for any advice.
Tike – It depends. The easiest answer is to read through the comments for several questions/answers that address that question depending on the insulation that you use.
Question,
You say to use spray foam and tape, is there something wrong with using some kind of caulk in the the tongue and grove as well as where the foam meets the floor?
Thanks,
Fred
Fred – Most caulkings don’t interact well with foam. Spray foam from a can works best.
You have lots of great info, hopefully I will explain my situation good enough so you can tell me what I shoul do.
Just bough a house in NY about 1 hr north of NYC,its got a finished basement no water on floor but, the sheetrock gets wet only at bottom.
Behind sheetrock is a foil like material with paper backing, behind that is plastic sheeting, the moisture is on the plastic and the painted concrete blocks, it runs down plastic and gets absorbed into bottom of sheetrock.
Should I remove plastic? If so I need to remove all sheetrock to get to it. If not where should all that trapped moisture go?
Thanks in advance
Lou
Lou – This is a classic example of what happens when moist air hits a cold foundation wall. The best solution is to remove everything and insulate the walls properly with foam (spray or board). You need both an insulation and vapor barrier to stop this problem. You could remove the lower couple inches of drywall but frankly the water/moisture is going to cause other problems. You’re better off fixing this situation properly.
We’re finishing our basement, and we have a vapor barrier installed against the concrete walls as advised. However, we’re using a foil-based radiant barrier instead of foam.
Does it matter then if we use faced or unfaced insulation between the studs?
Alex – In my opinion you’re asking for trouble and here’s why.
1. That foil faced radiant barrier is only a radiant barrier. It will still transfer the cold temperatures of the concrete.
2. If any moist, damp air passes through the fiberglass and contacts the cold foil it will condensate.
I would not use this method.
Hi,
Very interesting page.
We just moved into a house this winter. Now, with this weather, I start smelling something like water vapor(this what I think).This smell It is only in living and kitchen. This house it is not new. The basement it is finished but in an old stile like ’70 or ’80 with wood panels and carpet and no insulation against the walls. It is also the coldest place in the house.
What shall I do to get rid off this smell in the living and kitchen?
Do you have an idea what the cause could be?
Thank you!
Florin – Not sure what you’re smelling. Water vapor can’t really have a smell. Maybe you’re smelling mildew? Could be old carpets, mold, mildew or a number of things.
Hello, I am in need of some help. My wife & I live in a new home(2yrs) in Spruce Pine, NC. Because of the moisture problems of building a below grade basement, we built an above grade block basement with 8 inch hollow block walls, so our house actually looks like a split level. I have sealed the walls with a water-proofer & have no problems with water intrusion, the only problem we had this winter was that if the basement temperature was allowed to drop to 55 degrees the walls would start sweating and was actually causing some problems. The reason I have come to gather that it is sweating is because after we applied heat to this space, about 62 degrees the sweating stopped & the walls were dry. So now my question is what can I use to insulate these walls to make a living space & then not have to worry about the walls sweating?
Thanks for your help & God Bless
Jason – I would suggest insulating the walls with 1-1/2″ to 2″ of closed cell foam board, seams taped, then framing a wall in front of them. We have several articles on this site about that topic. The insulation will keep the walls from getting cold on the inside which is why you’re having sweating problems. The nice thing about foam board is it’s a very friendly DIY product. Good luck.
I am about to start my basement and have been going back and forth on whether to use a 4mil – 6mil vapor barrier plastic on the walls or use something from HD that is a 3/8″ pink insulation board from Dow. I was then going to frame and insulate the walls with faced insulation.
As a follow up to my posting, I read on the Building Science site that using 3/4 xps on the foundation walls along with 3.5″ fiberglass batt insulation would work well as long as humidity was kept low during the summer.
Does that include faced and unfaced batts?
Would 3/8 xps work?
Can I leave a gap between the xps and the studed wall?
Julius – Thanks for visiting the site. 3/8 xps really won’t cut it and frankly 3/4″ is marginal in my opinion. The foam board serves two purposes, first off if it’s thick enough (and most people argue about this but i say 1-1/2″ minimum) it will act as a vapor barrier so long as it’s truly closed cell foam, secondly the thickness creates a thermal break so that any damp air from the “warm” side that migrates towards the concrete will not hit a cold surface and condensate. Make sense?
I live in Pgh and purchased a home with a finished basement. Noticed some leaks under baseboard and had interior french drains installed. The basement is heated by furnace vent and gas stove and Air Conditioned along with dehumidifier. I want to reinstall the wood paneling that was removed to have french drains installed. Do I need to do the vapor barrier and insulation? I’d like to eliminate those to avoid future moisture problems. Thanks.
Carolyn – Where abouts do you live? Where is Pgh? If you live in a cold climate I would highly recommend that you properly insulate the walls. I suggest you consider using foam board (which we’ve written about quite a bit on this site) as an alternative that’s DIY friendly.
Thanks Todd – Pgh is Pittsburgh PA. I was leaning towards using the foam board, until the interior french drains were installed and the drain contractor said to attach 6 mil plastic to hang over concrete block wall 1/2″ into drain “trench.” My thinking was that since the basement is heated and cooled that condensation would be eliminated and the air flow between the block wall (without vapor barrier) and wood paneling (or drywall) would be beneficial.
Carolyn – The problem is this. If you heat the space in the winter, you have warm, damp air floating around. That air will get behind the paneling and come in contact with a very cold foundation wall. That wall will be well enough below the dew point for the warm damp air to condensate. I would highly recommend installing a minimum of 1-1/2 inches of closed cell foam on those walls prior to installing the paneling.
Thanks Todd. That does make sense. After much review, my question now is which is better, extruded polystyrene or polyisocyanurate for basement walls?
Julius – I like using xps, it’s cheaper and does what it needs to do. Polyiso is better suited for exterior wall applications where you can also take advantage of the foil faced covering for radiant.
Hello Todd, Just wondering if it makes any difference if the concrete foundation walls are exposed to the air outside or if the foundation is below grade. My house in Missouri has both, with the back wall of the house being a walkout basement with framed walls but the other three walls being concrete. The side walls have the grade traveling up the wall until it is fully below grade, and the front wall being completely below grade. I want to use foam board but am not sure if the exposure makes any difference as to the thickness of foam required. Thank You.
Mark – It doesn’t really matter that much. I always size the foam based on worst case, the exposed concrete. I say pick a thickness and use it everywhere.
Todd,
Thanks for the basement insulation info. I am in the planning stages of finishing my basement and wanted to know your thoughts on the following:
I originally had planned to put a 3 mil vapor barrier directly against the basement walls (painted concrete) and then frame while leaving a gap between the frame and basement wall and insulate with fiberglass batts. The rationale was the 3 mil vapor barrier would serve to protect the framing from any water intrusion (i.e such as the vapor given off from the walls as in your first figure). After reading your website I realized that this approach isn’t the best.
It seems that the key here is to separate the “interior” (i.e. the conditioned air in the finished basement) from the “exterior” (i.e. the basement walls and their associated moisture vapor) as much as possible.
That separation needs to be done both physically (vapor barrier) and thermally, something the vapor barrier alone doesn’t do. Without a thermal barrier there will be condensation on the interior side of the vapor barrier which can lead to problems.
Would it be advisable to put a vapor barrier directly against the basement wall, then put foam board against the vapor barrier and then frame and insulate as described? Would a second vapor barrier over the fiberglass and behind the sheetrock be a good idea?
Thanks in advance for your thoughts!
Jason N
S.E. Michigan
Jason – Glad you read it and understand it!! It’s a complicated thing to wrap your head around and you seem to have picked up on it.
Frankly the vapor barrier on the wall prior to foam won’t buy you much unless you’re going to use a layer of foam less than 1-1/2″. If you properly install a layer of at least 1-1/2″ of closed cell foam, tape all the seems, seal the edges with foam (spray foam in a can) then you’ve created a thermal and vapor barrier.
I prefer to then use a kraft faced insulation as it tends to breath better than poly. Also, if you can afford it, you can just use 2″ or more of foam and skip the fiberglass all together.
Todd i have a basement with cinder block walls. these walls are damp about 2 to 3 blocks up in some places. Also in the corners it goes higher in 2 corners 3 to 5 blocks high.Also i think i have an old french drain system.I say this because i found clay pipe about 4in below the footing.This was found while digging around the sewer outlet pipe. This house was built in 1956.I am going to have a new french drain installed with 2 new sump pumps.I am then going to finish the basement.My question is can i install riged foam about 2in from the block walls then frame it out?I figure this will let some air movement between the block and foam and keep it somewhat dry and stop mold.
Mark – Water won’t hurt the cinder blocks a bit. All concrete (which is the main material in blocks) contains water in the pore structure for years so the water is not a problem. Water also won’t cause a problem with the foam. That’s why we attach the foam to concrete and block walls. The foam will keep moisture from entering the finished space and keep it against the block/concrete wall.
It’s pretty hard to come up with a way to attach the foam 2 inches off the wall and keep it in place.
Todd,
I have a trench around the perimeter of my basement leading to a sump pump. I’ve lived in the house for two years and have never had any water problems thus far. How should I go about insulating around the trench?
Matt – I would just bring the insulation down to the top of it. Are you going to frame a wall?
Todd- Yes I plan on framing the walls. So with 1.5 inches of foam should I space the wall an additional 1.5 inches away from the foam? I believe the trench is approximately 3 inches wide.
Matt – Couple of options. You could do just that or you could install a piece of composite decking down first over the 1-1/2 of remaining drain, then frame on top of that.
Hi
I live in a colder northern part of Canada and I have a cinder block foundation that had many leaks and problems. To this point I have done the following. Dug up the outside of the whole foundation and repaired damages areas. Applied 2 coats of sealer around the footing and up 36 inches. I than installed Delta MS house wrap around the whole foundation right down to the footing. On the inside I have installed 1″ foam board and taped with tuck tape. I have the 2×4 framing up around 1″ from the foam board. My quetion is should I also put 6ML vapour barrier or just install the fiberglass insulation, than drywall.
Also any comments on what I have done to this point, weather it should be enough etc. would be appreciated.
Carl – Sounds like you’ve done a great job so far! Congratulations for fixing the outside problems first!
1″ of foam isn’t really enough to act as an effective vapor barrier on the concrete side. What that means is during the warm summer months moisture in the foundation could possibly permeate through the foam and enter the stud bay with the fiberglass. Because of that you’re going to want to use a vapor retarder and not a vapor barrier. 6 MIL poly will not let it breath enough. There are quite a few vapor retarder products on the market now that should work well. Ask you local building supplier.
The rest of it looks great.
Good luck.
I am currently in the middle of refinishing a finished basement. The house was built in the 50′s. It is located in central new jersey so summers are hot and winters are cold. The problem I have is that the wall studs were built up on the cement basement wall. I can not get behind the studs. First thing I was going to do is drylock the walls, then buy R-15 (used in this area), and install the insulation backwards, so the plastic vapor barrier is touching the drylocked cement wall. Then finishing with Drywall.
There is no signs of water damage. I do not think water is getting into the basement, however the floor does sweat when the air difference is great between the basement and the rest of the house. I am trying to for the cheapest solution to sweating floors.
Thank you in advance for your answer.
Ryan
Ryan – Your solution only solves half the problem. It will keep moisture from the concrete wall from getting to the insulation. However, it won’t stop moisture from the finished basement from entering the stud cavity, passing through the insulation, hitting the cold plastic and condensating inside the wall. What you need is both a vapor barrier and insulation at the concrete interface to stop both processes.
Todd,
Thank you…one last question (maybe)
I am not worried about the temperature of the basement. It is cool in the summers and warm in the winters. I am only worried about stopping the tile sweating. I would like to put a carpet down. Will insulating the walls help with the floor sweating. If not, I might just make the it easier and not insulate at all. Will Drylocking the walls, and running a dehumidifier, with the AC/Heat on upstairs stop the sweating enough that I do not need to worry about mold going under the carpet?
Ryan – It’s very hard to say. Sweating is based on dew point. While you might not be concerned about the walls what I’m pointing out is likely to happen. Warm in the winter inside the finished space can and will cause a problem as you described. Warm, moist air from the finished space, passes past the insulation and hits the cool/cold plastic that sits against the concrete causing condensation.
In the summer the opposite happens.
Insulating the walls will make a drastic change in your basement. Are you sure the floor is sweating or is moisture coming up through the slab? If a vapor barrier was not used that is a real possibility.
Drylocking walls won’t necessarily stop the situation on your floor. Dehumidifiers will help some but i’d be very cautious about carpet in a basement that clearly has damp/moist air problems.
Todd,
I’m planning on using 2″ square edge Dow foam (T&G was twice the price) insulation adhered to the block wall with adhesive and taped along the seams. I plan to leave at least a 1-1/2″ gap between the foam and the wall. I have extensive 3″ DWV pipe running along the walls.
1. Should I cut a channel for the 3″ DWV and fill that channel with spray foam?
2.
a. Since I’ll be paying only $18 a panel for the 2″ foam, should I place the rigid foam in the 3.5″ stud cavity, i.e.,
block wall–> 2″ XPS foam–> 1-1/2″+ gap–> 2×4 framing (rigid foam within the stud cavity with the foam pushed against the drywall, and a 1-1/2″ gap to the rear edge of the framing)?
b. Instead of placing rigid foam in the stud cavity, could I glue some 2″ and 1″ together against the wall with the seams staggered?
Thanks,
Ben
Allow me to clarify 2b
Can I glue 2″ and 1″ rigid foam panels together on the block wall and leave, leaving the stud cavity empty?
Ben – Square edge is fine, the T&G just makes it a bit easier during installation but it’s not necessary.
1. Not sure exactly what you mean. Are the pipes tight against the block wall? and therefore you’ve planning on leaving a gap at pipes and sealing along the edges?
2a. Depending on where you live 2″ of XPS is a pretty good insulation layer and you may not need additional insulation for the stud cavity. Having said that using another layer in the stud cavity is sure better than using fiberglass in there.
2b. This method would also work nicely and create an even better vapor barrier.
Thanks, Todd!
The pipes in some places are pretty tight to the wall. I was going to use a hot knife or something to shape the panels to fit around the pipes. What do you think about this technique? Should I spray some expanding foam behind them?
I still have to run 3″ drain pipe along one wall, but I would really like to keep that pipe pretty close to the wall. Would cutting the panels and sealing the gap between the block and the pipe with foam significantly compromise the barrier properties of the foam panels?
One more question: how critical is the gap between the foam and the framing members in the wall?
Thanks again!
Ben – If you can fit the foam tight and foam the edges that should work pretty well to lock out water vapor. The gap isn’t critical if you don’t plan on using fiberglass.
Todd do i have to use blue foam on my damp basement walls,or can i use another type then stud up aginst it and then use batt with a vapor barrer.I have seen pink,green,foil backed both sides and white,and the type they put behind siding,but no blue.Also i was told i could tar the inside wall then put plastic on the tar?.
Mark – The color doesn’t matter, pink and blue are typically XPS, closed cell foam. Foil faced is typically polyiso insulation, that works too. Be careful of white as it’s typically open cell foam board. I’d avoid putting tar inside your home. Good luck.
Todd,
Thanks for providing a wealth of info here. Back in March for the first time, I had some water seep into my basement which I refinished 15 years ago when I purchased the house in Queens, NY. The water entered at the bottom of the poured concrete wall where it meets the slab in a small area after a 3″ rain storm. I removed the drywall, 6 mil vapor barrier and insulation from between the studs which does not have any mold. However, some of the paint / moisture proofer applied to the concrete has spalled off in some areas near the bottom of the wall and the bottom plate, which I removed had some mold. The studs are about 1/4″ from the concrete which were in place with wood paneling from previous owner. I was going to cut 1′ off the bottom of the studs, install trex deck as you recommended, install pressure treated plate on top of it and splice in studs. Is it necessary to remove everything and install 1.5″ foam unto concrete wall and reframe and insulate? Do you have any recommendations to treat the seam to prevent water from penetrating again?
Thanks, Peter
Peter – Sounds like you’ve been quite fortunate with a relatively dry basement. 15 years is a good run!
First off the water issue. Sounds like this was caused by a very rare large rain event. I’d say the best approach is to evaluate the outside of the house, gutters working? grade sloped away? any cracks in the foundation? settling? etc. Deal with any issues you may have outside.
Sealing inside at the base of the wall is probably a futile attempt but I suppose it doesn’t hurt to try and seal that joint with a good quality masonry patching materials.
Why are you removing the lower foot? Rot? Mold? Damage? If there’s no damage that may not be necessary. The bigger issue is whether to leave the walls in their current position or to move them. You could install foam board between studs in lieu of fiberglass. This won’t create a tight barrier but it seems as though your moisture problem isn’t too bad.
Or you could cut free the walls, move them forward, add a layer of foam and go from there.
Hope this helps.
Todd what is the thinest rigid foam i can use over my damp walls since i am also going to add batt.
Mark – you really need a minimum of 1-1/2 to create a vapor barrier.
Todd,
Thank you for the great articles. I’ve read both this and your basement floor finishing article in preparation for my basement repair (in southern New England.)
We have had a few intrusions of runoff during bad storms recently, and as a result I need to repair/replace at least the bottom, below grade half of our finished basement walls. I would like to avoid whole-wall replacement due to lots of soffits and trim up at our basement ceiling which would add complication/cost to the project. We have never seen runoff damage or condensation higher than about 12″ off the floor, and I’ve verified there is no seepage on the bottom 4′ of wall with the aluminum foil test.
The interior walls are currently just drywall attached to 1×3 firring strips, which are attached directly to the concrete block wall. No insulation, no vapor barrier. Since I am not planning on replacing the whole wall, what I’m hoping to do is take down the bottom course of drywall, and cut the damaged firring strips off. When replacing these parts, I was thinking of first adhering a 6 mil poly barrier to the concrete, then installing new pressure-treated 1×3 firring strips, and putting 3/4″ XPS foam in between these strips.
I realize this is not really insulating the wall since I’m leaving the top half, which is all above-grade, untouched, but I figure something is better than the current nothing. Also, the poly against the concrete, covered by foam, will make me feel better about condensation, which we had on the bottom 2 courses of block in the summertime before we started using a dehumidifier. Maybe help that dehumidifier not run so much? :)
Could I have your thoughts on this approach?
Thanks,
Mike
Mike – Thanks for the compliments. Honestly I think that you’d be wasting the foam board. Frankly having 3/4″ part way up the wall and fit between firing will likely do little good and it may just trap water against the strapping and drywall. I think you’d be better off trying to stop the water problems especially if they can be corrected outside. Can you remove the wall from the soffit down? That’s maybe a better idea and get a good layer of insulation in there?
Thanks, Todd. Yes, I probably should have mentioned that we are taking big exterior measures, including regrading the soil near the foundation, adding a French drain to divert some runoff that comes down a hill towards the home, and tying our downspouts into a drywell/pipe system that should pull the water past the house rather than allowing it to pile up alongside. I’d also like to dig around and waterproof the exterior, but the budget won’t allow it – it would involve getting under a crawlspace to the true foundation footer in one area of water intrusion and moving a small back deck in another area.
Is just insulating from the soffit down that much better than my original halfway-up idea? I’d still be leaving an area of the above-grade wall uninsulated. I guess I see that it would be a smaller portion and the extra “stuff” around the soffit would give me some more insulation, but if it’s not significantly more than my first idea I’d probably rather save the materials cost and time.
I would love to just pull everything down and start from clean concrete, doing the 1 1/2″ of foam, etc. but in addition to the soffits and stuff at the top there’s also an 8 foot baseboard heater I’d have to work around. And I don’t feel like draining the system to move the pipes out from the wall. Unless I could recess the heater slightly in the finished wall? I’ve never seen that done – do you think that’s possible?
Mike – Sounds like it would be a rather big job to take it all down and start over. Then again, might be worth it. What if you do the exterior repairs and wait a year, see if water comes back. If water doesn’t come back it might be worth spending the time and money to move it all. Having a plumber come in and move that 8′ section wouldn’t cost too much.
Thanks again, Todd. Sound advice. I very much appreciate you taking the time to respond to all your readers’ comments.
-Mike
Todd:
I have a new constructed home with poured concrete basement walls. Basement is rough plumbed for one full bath. I will be insulating the full basement including walls and floor. I plan to insulate the floor with 1″ thick T&G foam panels with OSB overlay (like you have explained in another article, or maybe even use the “Barricade” brand subfloor system). Then I will insulate the walls in a method you have recommended with 2″ thick foam board. My only real question is related to the basement wall surface: It was formed with a faux “brick” surface. Will this be a problem with the obvious gaps between the faux “brick” concrete wall and the foam board?
Jay – Was it just a form liner? Meaning is it concrete that just has a surface that looks like brick? If so that’s fine, I’d only worry if it was a material that might mold.
Todd,
I am finishing my basement in Iowa. The builder had framed the entire basement leaving less then a 1/4″ gap against the concrete. They then put r13 unfaced fiberglass between the studs and covered with a vapor barrier. Everyone (including the wife) says to just leave that and put up the drywall. Spray foam for the entire basement would be pretty expensive. I was thinking about putting 2″ xps between the studs and spraying the gaps. That would give me R10. That would also leave a 1.5″ gap between the xps and drywall. The hardest part will be behind the tub surround which is already installed and has the fiberglass/vapor behind it. Should I rip that out if I can get to it?
Jeff – If you leave the fiberglass I can pretty much guarantee you will get mold. It’s been well documented and proven time and time again.
I would say you have 3 options.
1. Remove fiberglass and poly. Have the stud cavities spray foamed. (Most expensive but best solution)
2. Remove fiberglass and poly, cut loose walls, move walls forward a couple inches, install 1-1/2 to 2 inches of foam board, seal, re-attach walls, then sheet rock.
3. Do as you’ve indicated above, fill stud bays with foam. This solution will work fairly well but it doesn’t create the best vapor barrier. It’s not an approach I would do in my home but it’s certainly better than fiberglass.
Good luck.
Todd, thanks for all the great articles. My mind is swimming with a problem I am trying to fix…I have a 20 year old home. The basement is 3/4 finished with 6 mil plastic vapor barrier, then studs with fiberglass insulation. We have been here 7 years with no moisture or mold problems. Last year I finished a room. I did not use plastic (I was told that was a bad idea). I coated the exterior cement block walls with DryLock. I have the studs up against the walls (some spots there is a 1/4″ gap but that is due to irregular shape of the cement block walls). I put R-13 faced insulation between with the face on the exterior wall (oops!!!). I started smelling mold/mildew and pulled off some drywall. There was evidence of condensation and some mold. I pulled off ALL the drywall and insulation and cleaned the walls (spotty mold, not too bad). Now here is my dilemma. How can I FIX this with what I have in place? Can I put 1″ foam board between the studs and use great stuff or caulk in the cracks (I can squeeze it in the gaps behind the studs too), then use R-11 or R-13 (I’ll only have 2.5″ left after the foam)? If so, should I use faced or unfaced (with the paper toward the warm side!?!)? I have also already finished the ceiling with drywall and don’t want to pull it down. I used fiberglass in the rim joist gaps (I should have read your stuff first). Can I have holes drilled and foam or cellulose blown into the gaps?? Ultimately, we have had no problems at all and it was all done the “old way” with the plastic and fiberglass…was my main problem that I put the facing the wrong way?? I have seen no reason inside or out that the walls in questions should be any different than the rest of the basement. Please help!
Tom – Sorry to hear about your problem. In order to fully answer this it would be very helpful to know what part of the Country you live in. I’m a bit surprised that the old method worked as well as it did, but that may in part be due to where you live. Let me know where you live and I can better assist you.
As a followup, I still have 1/4 of the basement unfinished. It is untreated, unpainted cement block. Do I need to put anything on that or can I insulate the interior walls that separate it from the rest of the basement and just write that off as a ‘colder” space (it is just storage)?
I live in SE Michigan, so I too am surprised we have had no problems with what had been done already…
My question now is, since the condensation was on the “cold” side of the facing paper (against the cement wall), then if I put foam up against that, or plastic, won’t I have the same problem, just mold between the wall and the foam??? Will it be different if I put the facing toward the inside (to prevent warm moist air from coming into the wall space)?
Tom – Based on how your existing walls are insulated I would conclude that most of the moisture is coming from the cool/damp concrete wall. So, having said that, if you were to install foam (I’d recommend 1-1/2 to 2 inches) board in the stud cavities, foam/seal the edges, you should be able to stop the moisture from getting to the back side of the drywall and causing mold. In your situation I’d skip the fiberglass and just install a good layer of foam board.
Good luck.
Thanks. I talked with an installer today and he suggested the even foam board would allow a small air cavity where mold could grow. Is that true? If so, I will spend the $$ and have spray foam (closed cell) applied as that will adhere to the walls, fill gaps behind studs, and prevent any moisture. I was told that TigerFoam sells a kit online for about $300 that will allow me to do it myself if I want…Any advice?
Tom – First off mold needs food. If you seal the foam then there’s basically concrete (mold can’t eat concrete), foam (mold can’t eat foam) and then small strips of the back of studs. Mold could eat that. The reality is if you install a thick enough layer of foam it will stop moisture from getting into the stud cavity where the mold would be a problem on the back of drywall, etc.
Just to clarify, the mold was on the facing paper that touched the concrete, not the back of the drywall. Does that change anything?
Doesn’t really change things.
I read a post about my question, just didn’t see answer.I have a poured wall with a brick coarse on the inside,(very Rough) how do you apply foam to that surface? I understand that by using 2″ foam i will not need to add any plastic as a vapor barrier is that correct? Thanks
Todd – You can use Great Stuff Pro – It’s a spray foam adhesive in a can and it works very well. You are correct…no need for poly. Just seal all the joints very well.
I live in minnesota I have 2 and on half inches of spray foam on the outside of some of my foundation it looks ugly can I scrap it off and put something over it and if I can what?
Mike – Now that’s a first, haven’t really seen that before. Is it insulation or some type of water proofing membrane?
it is insulation you can tell it was sprayed on but has bulges all over so it looks ugly only on the heated basement walls any sugestions the house next door has the same thing
Mike – I suppose you could try and scrape it off and install foam board. If you do that it can be stuccoed over for a nicer appearance.
thanks one more question do I need that on the out side wall.
the basement is finished and heated ?
thanks for you quick response
Mike – Really depends on how much insulation is on the inside and if you’re willing to give up such a good layer of insulation that currently contributes to a warmer basement.
Todd,
I have a walkout basement made from block. Currently there is a window and door on the back of the basement (walk out) area that was framed out in 2x4s with batt and plastic over top. I plan on putting 1.5 inch foam around the perimeter of the basement with a 2inch gap between the foam and the wall frame, then insulate with faced batt, possibly R13. I was wondering what to do around the window and door that already have the insulation and vapor barrier? Do I not put the foam over top of this, but then have the R13 in the frame? Would this be a double vapor barrier? Thank you.
Derek – You have a couple options.
1. Leave it as is.
2. Remove the poly, install the foam over the fiberglass, install extension jambs on the doors and windows.
I’d go for option 2.
Todd,
I live in SE Michigan have a 7 year old home with a poured concrete basement and no moisture problems… yet. I am beginning to finish it. I was planning on leaving the ceiling open and spraying it black. I’ve studded the walls, but not secured them in place yet. Based on your comments and articles, it’s clear to me I need to use 1.5″ to 2″ of foam insulation against the concrete and no additional vapor barriers. Then leave a 1″ gap to the back of the studs. My questions:
1) Where the wall meets the ceiling do I need to do anything special to seal the backside of the wall from the rest of the room? Or is it advantageous to have the additional airflow allowed by not closing it with a ceiling? (seems like the latter would be true)
2) I have a storage closet (8sqft) & a storage room (200sqft) that I didn’t plan on insulating. Is there a good way to transition from the insulated concrete to the uninsulated concrete?
3) The walls are already studded w/o a composite board on the bottom to stop wicking. Would a strip of 4mil plastic beneath the PT2x4 provide the same benefit? Or not necessary?
4) Is polyiso worth the extra money for its higher R value per thickness? If I went with polyiso could I use a thinner layer attached to the concrete and achieve the same thermal/vapor barrier? Or is the 1.5″ thickness a guide for vapor permeability mainly, and polyiso maybe isn’t better at vapor, only thermal??
5) To keep from a 7″ thick wall (2″foam+1″gap+3.5″stud+.5″DW) I was considering only putting foam insulation between the studs, sealing between the wood/foam gaps and adhering the foam to the concrete. However, does having an open top to the wall (unfinished ceiling) expose the backside of the studs to mold? Or are the studs less susceptible than the drywall?
Sorry for so many questions, and thank you for your clear answers.
Ben. Thanks for stopping by and posting your questions. Below are a few thoughts on your situation.
1. I would be sure you insulation up the entire height of the concrete wall, over the top edge of concrete and then up the entire height of your rim joist. What you want to avoid is gaps in your insulation/vapor barrier. It’s important that all seems are sealed well.
2. Again the idea is to create a good continuous barrier around the entire basement as best you can. Sometimes this is difficult to do at utilities and stairs but the more you do the better. If you’re going to skip those rooms I would be sure that the interior walls of them adjacent to insulated concrete walls are also insulated.
3. The composite board “trick” is a nice thing to do but not 100% necessary. You could also use a role of Vycor (or similar) and adhere that to the slab under the bottom plate.
4. In some situations polyiso is actually cheaper. I would go with which ever you can afford but stick with the minimum 1-1/2 inches.
5. The 1 inch gap is 100% necessary, again, it’s a nice to have feature. You can install the framing up tight to the foam especially if you’re not going to insulate the cavities with fiberglass. I prefer not to do the “between” studs method unless it’s the only solution available, just not as good of a system and prone to air leaks and vapor movement.
Good luck!