Rigid Foam on the interior face?
Hi
I am working on plans for a new house and have been looking at a couple of options for the exterior wall assembly. Climate zone 5, coastal, very humid summers. Will probably be forced air heat and central AC for cooling.
Looking for approx. R-40 walls.
I am leaning towards a double 2×4 stud wall filled full with cellulose. I prefer the environmental benefits of cellulose over spray foam and rigid foam. It also seems like detailing/ flashing at windows/ doors etc are much easier with a double stud wall than rigid foam on the exterior.
However, the first GC I spoke to about the design noted that in our area labor costs are relatively high and that the thought of building double stud walls around the entire perimeter would be exceedingly high. He suggested rigid foam on the interior side (3/4″-1″) and then full cavity cellulose or spray foam. I haven’t read much about interior rigid. I would be a bit nervous about the drying potential to the interior particularly on a house with forced air cooling. Certainly, the construction details would be relatively straightforward.
I should add this house does not have an optimal footprint. It is one-story and “L” shaped in plan, 3500sf. I know this is not ideal, but site, program and other factors dictated the design. This certainly impacts using a double stud wall as there is a large percentage of exterior wall, which results in a lot of “lost” sf in the thickness of the wall.
Any thoughts on interior side rigid foam?
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Replies
Chris,
Interior foam is a bit of a pain for nail pops, hanging cabinets, window trim, etc. Look for a knowledgeable and progressive builder in your area that will build your house the way you want it.
Chris,
It's possible to build walls with interior rigid foam. Here are the disadvantages (in addition to those already mentioned by Doug):
1. With interior rigid foam, rim joists and partition intersections aren't as well insulated as with exterior rigid foam.
2. In a hot, humid climate, exterior rigid foam gives you added protection against inward solar vapor drive.
Martin,
You mention the advantages of exterior foam in a hot, humid climate, but any vapor concerns with interior foam in Zone 5?
The disadvantages don't seem as significant as I thought they might. Trim install with exterior rigid is not particularly easy and the rim joist issue seems to be the same as with a double stud wall.
It does seem like air sealing could be more difficult with interior foam, but maybe no different than a double wall.
I guess the biggest disadvantage of double wall and interior rigid is you are not keeping the sheathing and framing warm like you would with exterior foam.
Difficult decision because of the amount of exterior surface area.....
Thanks again for the info.
Chris,
No real vapor concerns.
I hear the argument about the prohibitive "extra cost" for double-stud walls, but it seems to me that the extra labor for walls is rather small in the big picture of building a whole house. For you professionals, how much time do you think double-studding adds, vs single-stud and exterior foam? I've only worked on a couple of houses, so my view is myopic on this.
Depends on foam thickness. Only 1"? no furring strips? 2" and furring strips? 2 layers of foam? I have experience putting 2 layers of foam on my house, but not a double stud wall. Foam took me quite a while, although doing at mostly after work, in the dark, didn't help. Nor did cutting almost every sheet. Generally speaking, the framing part is fast and easy. I would expect it to be slightly more expensive than a single layer of foam with furring strips and metal t-bracing. House shape/geometry would make a huge difference as well. But I'm thinking I would side with the OP on this, in new construction it would be easier to do the double stud wall, flashing and what not, with better insulation levels. A while back there was an article on a passive house built in Maine with I joists on the outside as insulation bays, built for $130/sf. Around here that is about the cost of a starter, builder level tract house. I'd guess the shell is typically 15-20% or so of the cost of the house, not including windows.
Aaron: Agreed. I just don't think the time to add another wall, and its nuances, is as extreme as some individuals would lead a person to believe. I suspect it is as much lack of familiarity on the builder's part as it is expense. However, I'd still like to hear what pros who have done both think. You spend so much time on windows, trim, stairs, etc, that I can't see the second wall being a huge deal. But, I'd like to know.
John,
Some of your questions might be answered by the NREL Net Zero Habitat For Humanity house.
http://www.nrel.gov/buildings/pdfs/43188.pdf
We've built several houses with double stud walls. In fact, I wrote a piece on one project for JLC a few years ago. It's easy, goes up fast, allows for flexibility, and avoids foam, which I think is always a goal worth pursuing in new construction.
I'd say that the cost of the 2nd wall is less than the materials and labor for the foam, its installation, the furring required, and the added difficulty for electrical, drywall, trim, etc.
Doug: I scanned that article, and did not quite see what I was looking for. It was an interesting read anyway; the walls are quite similar to my house. Dan: OK. Especially if one has a Larsen/Riversong-type 2nd wall, there really is not a lot to it. The numbers I ran for my area we very much in favor of the double stud set up, ignoring labor costs, which is the big unknown to me. I don't get paid either way, and hope I run into someone who builds/built both ways and has some hard numbers. Thanks both.
Repetition makes almost any build way faster and hence lowers the labor cost. I explain to local carpenters how in California, 500 home tracts, go up at the speed they do one home and they just can't picture it.
I have built both ways a number of times and definitely prefer double stud to exterior foam. While the foam may seem quicker, the detailing becomes the time lag. Also, I would read this before going the exterior route (3/4" may be to thin even with polyiso):
https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
Also, using rigid foam on the exterior and spraying foam on the interior creates the dreaded "foam sandwich". This would also be a much more expensive route than double wall construction.