Foam board insulation in basement

Ted Gorab Published: Oct. 3, 2019, 6:09 p.m.Last updated: Oct. 5, 2019, 10:25 a.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)

Mike Reynolds Oct. 4, 2019, 8:58 a.m. Reply

Hi Ted, can you please explain a bit better what the mold expert told you? by "on top" of the framing do you mean you were instructed to put wood framed walls directly against the concrete, then rigid boards afterwards? If so then I would say they are an expert in making mold grow perhaps. We must be misunderstanding something. 

The word 'Breathe' is also perhaps not the right word since it implies air movement, and that is anothter thing you don't want in an insulated wall assemly. But yes, we would put Rigid foam boards against the wall THEN the stud wall, you don't want to have organic material exposed to moisture, particularly having a source of moisture on one side and an impermeable or semi-impermeable layer to follow. By 'Breathe' I hope/think they meant 'allow it to dry'.

I'm not sure which page you saw but we have a few other pages that I will link here so you can read a bit more indepth, feel free to write back with more specific details as to what you were instructed to do and we will happily offer our two cents. 



Ted Gorab Oct. 7, 2019, 4:10 p.m. Reply

Hi Mike, thank you for the info. The mold guy just said to lay the Owen's corning board on top of the wooden frames that are nailed into the cinderblocks. To leave a little space for air. But you are saying to put the boards against the wall. My frames are already put up. And I either put the insulation  boards on top of frame or in between each frame against the cinderblocks.

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 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.