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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?

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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.

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If it were mine, I'd excavate access space around the partially covered walls, down to the footings, and hot-mop with roofing tar to a point slightly above finished grade... ESPECIALLY if you're going to apply sealer to the inside surface. You want to prevent hydrostatic pressure from pushing moisture into the wall from outside, and especially to prevent moisture from building up inside the basement wall (as it would if the inside surface is sealed but not the outside surface).

After hot-mopping and cooling, I'd apply 2" styrene (pink) foam board to the outside of the mopped wall, this time all the way up to the top of the wall, and have it covered with stucco or plaster so the sunlight doesn't eat it.

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  • I appreciate the thoughts, but you didn't actually answer the question. Even in your scenario, some moisture could one day come through the foundation wall. Which way does the interior insulation and barriers go?
    – David Pfeffer
    Commented Aug 17, 2014 at 13:16
  • In all scenarios, some moisture is guaranteed to find its way past any moisture barrier. In the case of hotmopping with tar, the tar is the front-line moisture barrier to SLOW DOWN the intrusion of moisture into the concrete. Vapor barrier should be on the HEATED side of the wall down to at least the frostline, but should not exist at all significantly below the frostline. This applies to foil-faced polyisocyanurate foam as well as to plastic sheeting over rock wool. Below the frostline, you want the wall to "leak" moisture (slowly) into the structure, where it can dissipate.
    – TDHofstetter
    Commented Aug 17, 2014 at 14:56
  • I'm realistically not going to be excavating the foundation wall for cost reasons. You seem to be knowledgeable about the topic and if you can, I'd appreciate a direct answer as well.
    – David Pfeffer
    Commented Aug 29, 2014 at 23:54
  • @DavidPfeffer... I've tried to be as direct as possible. I pointed out where the best place for the vapor barrier would be, and I also pointed out where the best place for the vapor barrier would be. If the moisture barrier cannot (within budget) be placed in the right place, then everything gets more complicated in compensation. In such a case, I'd recommend against any sort of sealant or moisture barrier because either will permit intruding moisture to build up in the concrete under hydrostatic pressure. You'll want that water to go someplace, not trap it anywhere.
    – TDHofstetter
    Commented Aug 30, 2014 at 1:27
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    @DavidPfeffer... the ground will never totally dry out, and the outside surface of the concrete will never have any less water content than the underground soil touching it. There will always be some hydrostatic pressure there. That holds true in any environment wetter than the Sahara. Sometime when you have half an hour, dig a hole two shovels deep in the soil near the basement wall. Feel the dirt from the bottom of the hole. That's how wet your basement wall is.
    – TDHofstetter
    Commented Aug 31, 2014 at 2:49

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