Is there anything I can do about a vapor barrier on the wrong side of a basement wall?

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Iain B
Updated: Feb. 8, 2021

I have a problem with a basement that was finished about 1999. The house itself is from 1938 with a field stone foundation. When the finished the basement they put pt wood directly on the concrete, put R19 batts between the studs, and covered the whole thing with a poly vapor barrier and then drywall. So the vapor barrier seems to be on the wrong side to allow drying.

I had one corner where baseboard was showing signs of deterioration so I pulled that off and found that the baseplate (? whatever the studs are resting on) is rotted out down there. The studs seem fine apart from the lower inch or so. About six feet is completely gone in one direction from the corner. The other direction is deteriorated but not bad for about 3 feet. So far my plan is to replace the baseplate, repair the studs, put in rockwool for the last foot or so of insulation, put drywall back up and call it a day.  Save money to redo the whole assembly.

My question is: Is there anything else I can do about that vapor barrier without tearing out the whole wall? I've read enough of the basement articles to know that on a new wall I should put a vapor barrier right against the foundation, then the wall assembly. Ideally with a perimeter drain as well. But this isn't new and that's not gonna happen for a while. Thanks!

Responses (8)

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Iain B 3 years ago
Mike Reynolds 3 years ago

The insulation woudl be preventing some heat loss, but it doesn't work as well when its wet so it's probably not performing perfectly. So if anything letting it dry out would probably improve the performance a bit.

The only concern I would point out is mold and air quality, but if it isn't too severe and no one is experiencing any adverse effects then you probably can push it back a few years, and for sure keep the dehumidifier running. 

Your foundation wouldn't really be drying to the outside though if its below grade. That's the problem with vapor barriers in basements, the industry seems to have forgotten that foundations are backfilled with dirt, and dirt is wet, so they simply can't dry to the exterior below grade.

 I would make a few vent holes if you can, and keep running the dehumidifier and don't sweat it too much. As you said, you only just discovered it by chance, and millions of people have the exact same thing going on and don't even know it. So keep an eye on it, best of luck Iain! 

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Iain B 3 years ago
Robbie Anderson
Robbie Anderson 3 years ago
Mike Reynolds 3 years ago

Hi Robbie, 

1 - no you don't need those metal pins in the foundation so cut them off.

2 - 2 inches of EPS insulation is an effective vapor barrier, so no you don't need any additional protection, if you're going with only 1 inch then you might want to first install a 6-mil poly barrier. 

3 - If I'm getting what you mean and the poly at the top of your wall is keeping moisture in the concrete, that's not a bad thing. Concrete is very porous so it will forever absorb moisture from the ground and deposit it into walls if allowed, or dry to the interior. Without being separated from the ground with a membrane your concrete will always be wet and that's no problem as long as you take steps (like the rigid foam or poly) to prevent your walls from being saturated with moisture. 

Robbie Anderson
Robbie Anderson 3 years ago

Thank you thank you thank you!!  I broke the pins off, doing 2" and no other barrier, and cutting the extra poly off and letting those top edges dry out a little more.  I now suddenly found a leaky furnace vent pipe and have a rotted top sill plate.  I'm carefully cutting and replacing a 3' chunk of it and will then finish the project.  Thank you!

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Nina C 1 year ago