Tried foam board behind a panel in a damp basement and got a different problem

So I had this job in a 1920s house near Seattle where the basement wall was always a little musty. I put some 1 inch foam board behind the alarm panel to try and keep moisture off the back of it. Worked okay for about 6 weeks, then I came back and the foam had this weird black mold growing right where it touched the concrete. Learned that foam traps moisture against the wall instead of letting it air out. Any of you guys dealt with mold behind a panel in a damp spot?

2 comments

2 Comments

cole_walker264d ago

I remember reading a study from the Building Science Corporation a few years back that said closed cell foam directly against concrete in a damp basement is basically asking for trouble. They found that the vapor drive from the concrete gets trapped between the foam and the wall, which creates the perfect environment for mold. The numbers showed it can happen in as little as 30 days if the moisture levels are high enough. The only real fix I've seen work is putting a vapor barrier like 6 mil poly between the foam and the concrete, then taping all the seams. Even then, you gotta make sure the wall is as dry as possible before you button it up.

grace_perry444d ago

Oh man, I've ripped out more of that closed cell foam from basements than I care to count. The worst one I saw was a finished basement where they sprayed it right on the block, and within six months the drywall was covered in black spots from mold. You could smell it the second you walked in the door, musty and damp. The vapor barrier is non-negotiable if you're going that route, but honestly, I've had way better luck just using rigid foam board with taped seams and leaving a small air gap. It's a pain to cut and fit, but at least you can see what's happening behind it, you know? The 30 day timeline sounds about right too, I've pulled back panels where the concrete was still sweating even after it was supposed to be "dry.