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Can you install foam boards directly against basement walls?

Ted Gorab Published: Oct. 3, 2019, 6:09 p.m.Last updated: Dec. 1, 2019, 5:50 p.m.

I read in one of your articles when putting up rigid fome 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 (4)

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Ted Gorab Oct. 7, 2019, 4:11 p.m. Reply

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
Mike Reynolds Oct. 8, 2019, 3:38 p.m. Reply

Wow, that is a terrible idea. Your 'mold guy' is for sure  in the business of growing it,  not preventing it.  

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.