Basement Wall Insulation


Old 02-15-12, 03:53 PM
Thread Starter
Join Date: Jul 2010
Location: US
Posts: 7
Basement Wall Insulation

I am looking to insulate 8" poured concrete walls in my basement. They're about 1/4 above grade and 3/4 below grade. The walls are out of plumb and I will have to construct a 2x4 wall and will fill it with bat insulation for an achieved R value of hopefully 13.

How can you guarantee that the fiberglass stuff won't soak and have problems with moisture/mold? We don't live in a very wet climate here in Montana and yet I have heard several times that one needs to first create a reliable moisture barrier, such as 1.5" XPS sheeting.

Since the XPS sheeting will cost additional space and a considerable amount of money, I hope that I can get around having to install it. But obviously, if there's a risk of a water problem, it'll have to get done

Thanks all!
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Old 02-28-12, 08:37 AM
Join Date: Aug 2009
Location: NY
Posts: 326
Most importantly is what is the state of the wall now? If it's really wet that can be a problem, particularly if you have no interior drain along its edge to take excess water and drain it into gravel below the slab.

It's true that batt is not optimal and if you have a lot of wetness in walls it will soak and it will not dry out. Many, many homes do have bats and do ok, though.

You could do foam insulation alone and forget batts. I wouldn't worry about the 1.5" lost, or if you go with 2" thick the 2 inches, you'll just never notice it once the walls are up.

If you do foam, whether batts or not, you'll need some way for moisture from the wall to get away, should it come in. If you do foam and batts, don't do a vapor barrier at all: foam on the wall, then the batts. This way any vapor that gradually comes through the foam can dry into the batts and through them ultimately into the living space as desired. If you have a lot of water, though, it will get in behind the foam and have to go somewhere, which is why I wondered if you had a drain against the wall. The XPS foam looks water proof but it does allow small amounts of vapor through, so if you've only got some wall sweating here and there it should be able to pass through.

There are a lot of great articles at this site--here is one of them:

You should find out for your state the minimum R-value allowed in a basement. I will note for you that with your poured foundation the benefit of it over, say, hollow cinder block (like I have) means no airflow within the wall itself. You'll lose the majority of heat higher up on the wall and 2" of XPS foam is R-10 value and would do a decent job. The dirt for the bottom 3/4 of your wall insulates a decent bit itself.

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