2×8 Wall True R Value
In Zone 5.
So we know that building science states we should have 27% of our insulation on the exterior of a wall if we are using rigid foam + insulated stud cavities. Earlier I posted a question considering if 2″ of Rigid is enough in zone 5 if the 2×8 stud wall is filled with R30 mineral wool. More or less the answer is not quite, but 2.5″ would work.
However, I forgot to mentioned the studs are 16″ o/c and thermal bridging would reduce the true r value of this wall from r-30 to maybe r-22 range? Would this mean, technically speaking, that 2″ of rigid foam *is* sufficient for the wall integrity, or does the 27% calculation only relate to the r value of the insulation material already installed and not the wall’s performing R Value? I feel like there was an article on this elsewhere, but I can’t find it. I am not wanting to under-build by any means.
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In short- it's the R value of the cavity insulation not the "whole-wall R" that matters from a dew point control perspective.
There isn't (much of) a moisture accumulation problem where the much lower R studs meet the sheathing, since the sheathing stays warmer at that location due to the thermal bridging of the wood. The R-value that counts is at the center of the cavity between the studs, where the R-value of the fluff renders the sheathing much colder.
The whole-wall R includes the thermal bridging, the R value of sheathing, the siding, the wallboard, and even the interior & exterior air films, not all of which is inside the susceptible sheathing.
With just the 2x8 16" o.c. framing (presumptive 25% framing fraction ,typ) a half inch of CDX or OSB, cedar shingles and half inch gypsum with R4.15/inch rock wool (R30) in the 7.25" cavity comes in around R20.5 whole-wall, not R22. If you add another layer of OSB or plywood it's R21-ish, if you add 2" of R5.5/inch foam it's R32-ish, etc. But the insulation parts of the wall are still R30 and R11, for a center cavity insulation value of R41. R11/R41=26.8%, which barely makes it (kinda). If you (rightly for zone 5, in this application) derate the 2" foam to R10, it doesn't, at least using the IRC as a guideline.
At the warm edge of zone 5 it probably makes it, but you'd have to run a more rigorous dew point analysis, or model it with WUFI, etc.to really have confidence.
Nicholas,
The short answer is that the minimum R-value rules for exterior rigid foam are based on the nominal R-value (the R-value label of the insulation) of the fibrous insulation installed in the stud wall, not the actual whole-wall R-value of the stud wall.
Here is a link to the article you are looking for: Combining Exterior Rigid Foam With Fluffy Insulation.