Posted By TexasICF on 22 Dec 2011 05:43 PM
Dana - whole wall value of r4 studs with r24 in the cavity with 20% framing factor (which is a bit low for better homes) is 12.
R = 1/((.2/4)+(.8/24))
Granted it only a diff of 1. But in the other example how does filling the cavity change the r value of wood from 4 to 5.5?
Only if you count the necessary structural sheathing and felt etc, as zero. R13 would be assuming a back-ventilated non-insulating siding such as vinyl. With rainscreen gap and wood siding (or even fiber-cement) with more insulation value (and additional air-films to stack up, if you're really going there) it would be somewhat more. You could a foil facer in a rainscreen gap and really knock yourself out (but I don't).
Granted you could probably add R1 for siding & ypsum to the ICF case too, but I've seen ICF analyses that give GENEROUS allowances for interior & exterior air films etc (which I count as zero, for comparative purposes, since all walls have 'em.) Was it you who posted a link to one of those earlier today, where
they stretched R21 to about R23 by adding in some air-film magic and wall board? ICFs don't have exterior sheathing that's required which is usually in excess of R0.5, so for estimation purposes I'll call it R1, making it more like apples-to-apples. Both have finish siding, both have interior finish gypsum, etc.
Regarding the R value of studs, the wood in the interior 1.5" of the 5.5" stud is at the interior temperature, surround on 3 sides by conditioned-space temperature air, which is 3x the exposure to interior it would have in a fully filled cavity. It's a heat sink from the more-conductive-than-air wood penetrating into the conditioned space. In a full-on 3-D modeling of the heat transfer, counting the four inches that pass through the ccSPF fully at R4 is slightly generous, but the extra heat transfer of the 5.5" stud with 1.5" in from the insulating layer compared to a 4" true-depth stud is "in the noise", R4 is close enough. With a full cavity only the stud edge is exposed to the conditioned space temp, and if it's a species that's truly R1.0 per inch at all temperatures, it'll be R5.5, since the sides are now covered with something more insulating with wood, and the temps along those interior 1.5" of side are now at a lower temp (in the heating season), not conditioned-space temp.
Taking it to the extreme, would you count a 50" deep stud with 4" of cc foam in the cavity at the sheathing as
R50 ? It would be even lower-R than the 5.5" deep stud case, due to the heat-sinking factor, but still about R4.