Rigid Foam in between studs
I live and work in south-central Tennessee and Northern Alabama (Zone 3A). I am working on a wall insulation design for a group of houses that my company will build. We are going to manufacture the walls in our facility before they go to the site. We would like to build a standard stick frame (2x4 studs with sheathing). We would tape every seam on the outside and caulk seams on the inside. Between the studs, we are planning on installing rigid foam board instead of spray foam—we can make a very tight fit because of our jig. On the outside we will add 2” of rigid foam. That would bring the R value to around 35 at a much cheaper cost than spray foam, and I think our walls will be comparably tight and still be structurally strong.
In south-central Tennessee, the winters have a few nights in the teens, but the temperature never stays below freezing for very long. We have very hot and humid summers, and I want to make sure that moisture doesn’t collect in the walls. Having a vapor barrier on the outside of the sheathing, but inside of the exterior foam seems like it would keep moisture out in the summer when it is trying to come in. On the other hand, with 2” of foam on the outside, I can’t see the dewpoint causing moisture to develop on the inside of the walls during the winter. If we make the walls very tight (as we can with the jig) and seal them better than can be done on a jobsite, I am wondering if the moisture problem would still be an issue. I am also wondering if an additional vapor barrier is a good idea with foam at all. Maybe there is a better way on a similar budget.
Any input is appreciated. Thanks.
Posted Jul 12, 2012 8:41 AM ET
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