Water – Vapor and moisture barrier for “above ground” basement


I'm finishing the basement of my home in New Jersey, and I'm a bit confused about the general guidance for basement moisture and vapor barriers.

My "basement" is built from concrete block. However, two walls are completely above grade, one wall is 1/2 buried, and one wall is 1/4 buried. Most of the stuff I've found online talks about preventing ground moisture from diffusing into the wall. However, ground moisture won't be a problem at all on at least two walls.

I'm planning on insulating either with Roxul rockwool insulation or with polyiso foam. (I haven't decided yet.) The polyiso foam comes with a foil backer that behaves as a moisture and vapor barrier. If I use the foam, I would put a thin layer behind the studs and then thicker in between.

I also am planning on spraying on a concrete sealant such as RadonSeal prior to building the walls and floor. We've never had any water problems in basement (probably due to the limited below grade sections) but I figure that it can't hurt.

Where do I put a moisture barrier (if anywhere) and where do I put a vapor barrier? If I use foam, which side faces which direction?

Best Answer

You can put a vapor/moisture barrier up over rigid foam. A lot of new construction in the far north has this as a default. The idea is that you frame right outside of that. In that framing you are allotting a cavity to which moisture can move and evaporate.

Now if you are putting rockwool or other types of insulation in your framing then no you do not use a moisture barrier. This would in essence trap water which will condensate on the plastic and promote mold growth on drywall. I am not that smart this is just doing a lot of basements and reading the Building Science reports that are out there for the past 5-6 years.