Can you install foam boards directly against basement walls?

Ted Gorab
Updated: Feb. 26, 2021

I read in one of your articles when putting up rigid foam to Install two inches of rigid insulation board directly against concrete.  A mold expert told me to put the board on top of the framing, not directly onto the concrete wall so the house can breath.  What is the right course of action? Thank you

Responses (26)

Ted Gorab 4 years ago

The mold guy said houses are meant to breath, I'm not sure which way to go. There is no signs of water, just humidity.

Mike Reynolds 4 years ago

Wow, if understand correctly that is a terrible idea. Cinderblocks are porous, so they will continue to absorb moisture from the ground unless they are well-protected, and I mean with at membrane and not just the bitumous spray. Since you likely don't have such a membrane on the exterior, the blocks will always have a certain amount of moisture content. And due to the laws of physics,  moisture will always want to go where it is dry, which in this case will be your studs, at least until they are completely saturated to the point of equilibrium and the wood just stays as wet as the concrete wall.

If there is dirt against the outside of the blocks (which I assume there is) then it can't dry to the exterior, and by covering the stud wall with XPS insulation boards (which are a vapor barrier) you would be ensuring that the wall cannot dry to the interior either, so what you would have is an ideal mold incubation chamber. 

If you are still able, I would move the studs away from the wall, and see if you can get a 6 mil poly vapor barrier behind it, if not, I would make sure to use breathable materials. The best insulation for a basement stud wall in your case would be mineral wool, Rockwool and then drywall overtop. a 6 mil poly barrier before drywall will prevent your wall from drying, hopefully your municipal building inspector will let that pass if you're dealing with one.  This way the moisture in the cinderblocks can pass through to the interior of the basement and be managed by ventilation equipment or even  a dehumidifier. 

Read those links above, and I would say start with the one about why basements are moldy, and I think all this will make more sense. I would for get yourself a dehumidifier down there and see if you can keep it at 50% RH or lower. 


Rocky Ford
Rocky Ford 3 years ago

I have built a block masonry house I want to put 2-inch foam on the inside for now does the block need to be painted before I put the foam against the block are waterproof and what with.

Mike Reynolds 3 years ago

Hey Rocky,

At 2 inches thick EPS and XPS insulation both work as a vapor barrier, so no need to paint them first. 

Robert J. Pierson 3 years ago

Here's a project done recently in BC - panels stuck directly to concrete walls with low expansion pro Sikka foam as per manufacturers recommendations ( Airboard ), then they were taped to really seal them up - before and after readings on the humidity were like night and day, and the temperature with zero additional heat in the basement.

Insulating bare concrete basement walls with rigid foam panels
Insulating basement walls with foam bare concrete high humidity low temperature
Insulating basement walls with foam bare concrete foil faced
Insulating basement walls with foam bare concrete lowered humidity raised temperature


Scott Frazier
Scott Frazier 2 years ago
I have an old pourous foundation that has some infiltration during heavy rains. Do you think those panels would be ok to put on walls like that? I have a drainage system under the walls.
Mike Ortiz
Mike Ortiz 3 years ago

How about painting the cinder block walls with Drylok then placing your rigid foam board insulation do your framing throw in some batting for further insulation then finish off with your green board basement sheet-rock and h y our good to go. 

Mike Reynolds 3 years ago
Hi Mike, Painting it first won't do any harm, but rigid insulation panels will be doing the job of stopping vapor diffusion from the foundation wall anyway, so painting it is not really necessary.
Mary Franklin
Mary Franklin 3 years ago

Can rigid foam go on cement walls showing some moisture and lime stains.  Exteriors not sealed.  Daylight basement gutted due to mold in drywall.  Working with blank slate sort of speak.  Seattle homeowner DYI cause single parent.  Wise advice appreciated.  

Mike Reynolds 3 years ago
Raye Mac
Raye Mac 2 years ago
Ronald Tetrault 1 year ago

That's exactly what I have put, 2" EPS glued on inside foundation walls.  It is so much warmer than before.  One local contractor who came to inspect my property, tells me that he often removes cheap white foam boards all the time, being useless with only R1 and that he sees mushrooms on the concrete walls after the foam removal. He proclaims that would be where my musty smell comes from.  What's your take on it?  I know the R1 is not true!

Dan Ellement
Dan Ellement 3 years ago

You must be as useless as your comment to go attacking a single mom on a foam board Q&A forum. 

Mike Reynolds 3 years ago
Comment removed Dan, Thanks for stepping in before we saw it.
R Mark
R Mark 3 years ago

Is half inch rigid enough? I already have framing in their but enough to squeeze in half inch against a rock foundaiton (built late 1800's). Trying to decide if I water proof paint wall AND rigid. Nor rigid w/ paint and roxul. Or no paint, rigid and batt. Apprecaite feedback as I can;t decide what's best as I'm trying to retrofit my basement.


Mike Reynolds 3 years ago

Yours is a bit of tricky one R Mark, you have to be careful with stone foundations. With no insulatuion the stones have likely stayed warm, so insulating from the inside could subject the foundation to much colder temperatures than it is used to and that can put the mortar at risk of freezing and cracking. Before you do that it's worth a look to make sure the mortar is in good shape. In a perfect world you would insulate it from the exterior if possible, that way you keep your heat but you also keep your foundation warm. 

Bridgett Phillips
Bridgett Phillips 1 year ago
Steve Werner 1 year ago
Mike Reynolds 1 year ago

The EPS and tar on the outside together will provide you will pretty good moisture protection for the walls, the only problem is that it is all but a foregone conclusion that the footing are in the wet ground. So they will always be wet, and that moisture will rise up into the vertical walls.

I hate to comment after work is done with anything that will concern you, but it sounds pretty good. Polyiso is not the best insulation for areas of high moisture, but it sounds like you're pretty well protected. I assume that black facing on the Polyiso is foil? That would provide you with waterproofing to prevent any mosture in the concrete from migrating into the wall assembly.

The biggest problem with basement foundations that cause mold is a misunderstanding of where the moisture is and where it wants to go. All too often (like almost always) wood studs are sealed in with the concrete by a poly vapor barrier, and that just doesn't work in basements. Vapor protection needs to go first against the concrete THEN the wood, so the wood can dry to the interior. The concrete is the wet part you need to worry about, not any humidity in the air, which is why vapor barriers are installed in above grade walls, and it works there because the walls can dry to the outside, which they cannot do below grade. 

mark mettrick
mark mettrick 1 year ago
Mike Rodriguez 4 months ago

I have a similar question,. Is having 2" xps foam with 2x4 wall filled with mineral wool r15 and no vapor barrier in zone 5 chicago land good? House was built in 1975 with concrete foundation

Mike Reynolds 4 months ago

Hi Mike, 2 inches of XPS is a perfect vapor barrier, so yes - I would use that against the concrete and then put a stud wall with batts against the foam with no other vapor barrier, just drywall.

Florian Sauter
Florian Sauter 2 months ago
Mike Reynolds 1 month ago

1 inch of EPS would significantly slow moisture movement, but it does not qualify as a vapour barrier. At 2 inches thick however, EPS is rated at 1 perm or 60 ng in Canadian Code, which is considered a type II vapour retarder.

I am slightly confused about the placement of insulation and what you mean by 'behind'. So to be clear - stick the EPS against the concrete, THEN the stud wall with insulation, and assuming you can get it passed by a building inspector (if you have inpsections), I would put batt insulation, then  drywall right on the studs. I would not add an additional poly vapour barrier as that will only serve to trap moisture in the wall. Let any moisture out into the basement since it can't dry to the exterior,  and let you your ventilation systems or dehumidifier do the job of handling moisture. Here is a page on how to insulate basements and avoid mold that could be helpful.