We're building a new house in New Hampshire. We insulated the 1st floor joists with batting last winter so the minimal amount of heat we had in the basement to prevent the foundation from cracking wouldn't create condensation on the exposed (cold) 1st floor above. We had terrible condensation anyway and had to remove all batting because of beginning signs of mildew and mold.

We're faced with the same situation this winter. Will foam board insulation be a better solution?

(We know the best solution is to sell the old house and finish this new house and just move in, but THAT is out of our hands!)

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    So your build doesn't yet have a roof and exterior walls? – The Evil Greebo Oct 18 '11 at 12:32
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    The first and last sentences have me confused, is the problem in the house that you're building? – BMitch Oct 18 '11 at 12:33

It sounds like you're saying your basement is heated, but nothing else is (possibly the floor above is exposed to the outside).

What you've done is put in insulation below the subfloor to try and keep the heat in the basement. This is a good idea, however, any time you have warm meeting cold, you're going to get condensation, and so you need a vapour barrier on the warm side only. This prevents the moisture in the warm air from getting into the cold zone.

If you have the vapour barrier on the cold side, you'll get condensation on the vapour barrier itself, between the insulation and vapour barrier, and this will lead to mold (and it's possibly a worse thing to do than have no vapour barrier).

If you have vapour barrier on both sides, you'll also run into trouble as if any moisture DOES make its way in between, there is no drying and no way for it to get out.

If you're using batt insulation, the best thing to do is put a 6mil vapour barrier on the bottom side of the joists, and seal it as best you can around the edges. Ideally you want a continuous barrier with no gaps. The vapour barrier on the ceiling should be connected to the vapour barriers on the walls, and then is typically sealed to the floor using acoustic sealant.

Foam insulation works as a vapour barrier as well, but you need to ensure all gaps between each piece and around the edges are sealed using air-tight tape (eg, Tuck tape).

Realize though that having this vapour barrier in place once you're finished construction will actually be detremental. You don't want to vapour seal between heated rooms, as it inhibits natural drying and air/moisture movement. If you end up having the basement unheated (or minimally heated), then it becomes the cold zone compared to your first floor and so your vapour barrier will be installed on the wrong side.

  • See I was thinking that the 1st floor is exposed to the elements, which means that in humid mornings the cold, exposed stone is collecting moisture from the warmer, moister air around it, causing condensation. IE -she's getting dew on her exposed floor... – The Evil Greebo Oct 18 '11 at 16:20
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    I've got the same impression as Greebo, and that they are trying to build a home over the period of multiple years, which isn't how you build a home. As Holmes would say, "take it all down," and "do it right the first time." – BMitch Oct 18 '11 at 16:40
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    If that's the case, that the first floor subfloor is actually directly exposed, then I agree - that's the wrong approach altogether. For a few days, it will probably be fine, but more than that and there will be more problems than it's worth. House wrap or tarp will protect it somewhat from rain/dew, but then you'll have to deal with water pooling, which will cause rot and basically require replacing at least all the plywood and possibly the entire structure. I agree with BMitch (and Holmes).. once you start building structure, you have to build all the structure, and make it water-tight. – gregmac Oct 18 '11 at 17:41

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