Exterior rigid foam vs. double stud wall
I’m considering 2 types of exterior wall assemblies on a new build in climate zone 6 and am wondering which you think is superior. R value is very important obviously, but breathability, durability, and cost efficiency is also important:
1. (from inside out) Drywall, 2×6 wall w/ 1.5″ open-cell spray foam against exterior filled with dense pack cellulose, sheathing, Tyvek Drain Wrap, 2.5″ XPS rigid foam, 3/4″ rain screen, and siding.
2. (from inside to out) Drywall, 2 2×4 walls w/ a 2″ gap filled w/ dense pack cellulose, sheathing, Tyvek House Wrap, 3/4″ rain screen, siding.
Also, would you recommend a vapor barrier on the inside wall of either of these systems?
Thanks for your help!
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Replies
David,
Option 2 wins for green builders, because Option 1 includes XPS, a type of rigid foam manufactured with a blowing agent that has a very high global warming potential.
If you decided to use EPS or polyiso instead of XPS in Option 1, then either Option 1 or Option 2 would be fine.
You don't want an interior vapor barrier with either type of wall. Here is a link to an article that explains why: Do I Need a Vapor Retarder?
Here is a link to an article that explains why walls with exterior rigid foam need to be able to dry to the interior: Calculating the Minimum Thickness of Rigid Foam Sheathing.
If you build Option 2, the wall would benefit from a smart vapor retarder like MemBrain -- but whatever you do, don't install an interior vapor barrier like polyethylene.
Who is doing the work? Exterior foam is a pain to install, detail and side over.
I used to be intimidated by the exterior foam, having done it in retrofits under vinyl. Because many contractors are unfamiliar and might screw it up. They key is to find a guy who knows how to do it, because it is not that difficult, just requires steadfast observance to detail. Like air sealing with caulk. I kind of think that you can skip the spray foam in the cavity as well, and just air seal with elastomeric caulk and tapes. Then do the dense pack.
DAVID - I agree with Joe's point about no need for the spray foam in Option #1. You may be considering it in order to form your air sealing layer, but taping plywood sheathing or using Zip system with tape would seem to cover the air sealing more efficiently and certainly at less cost. Some would tape the rigid exterior foam layers as the air sealing layer. Best of luck to you with the house.