You may remember our post a couple of years ago promoting Fat Walls. In their 11/11 monthly newsletter, Energy Design Update recently reported on 15 different wall assemblies modeled through TRNSYS software. The walls were simulated in the climates typical to Atlanta, Pittsburgh, and Phoenix. Of the 15 walls, three tied in first place for an overall value of R-43. One of these walls fills a 2x6 cavity with closed cell polyurethane spray foam for a rather high price tag. The second wall involves 10" thick SIPS. The third wall is our option number four from the previously mentioned post with 2" more of foam. That is, a 2x6 wall with blow-in and 4" of outboard XPS foam. As we mentioned back when we wrote the initial post, this makes window detailing a bit of a bear. Attachment issues come into play as well. The advantage of this system is the standard wall framing and no loss of floor space inside the house.
A reasonable compromise might be 3" of foam. This allows the use of true 2x4 for bucking out windows while allowing 1/2" air space. Half inch furring strips can then be used over the foam for attachment as well as a rainscreen.
They also modeled a similar wall as our top choice, double 2x4, total 8" thick with 2" of outboard foam. Our results? R-40 with U-0.20 windows. Their results were R-38 with U-0.25 windows. As you should know, U-0.20 windows are slightly better than U-0.25 resulting in a slightly higher total wall R-value.
So apparently we know what we're doing!