Condensing surface in a double-stud wall
We are in the current design phase and I am curious as to the exterior sheeting condensing surface in double stud walls. Our design is a basic staggered 2×4 wall (2×8 top plate)w/ dense pack cellulose. Our main heating system will be in floor hydronic heating. In regards to studies done; is there a significant difference in hydronic heating aiding the drying effect more than other types of heating. Just curious if we should still consider exterior poly isofoam/rainscreen. We will be building in Marine 4 just outside Seattle. Thank you
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First of all, can you tell us your name? (I'm Martin.)
Q. "Is there a significant difference in hydronic heating aiding the drying effect more than other types of heating?"
You may want to read this article: "Exterior Rigid Foam on Double-Stud Walls Is a No-No."
Thank you Martin for the quick response and informative article. I'm Nick, sorry for not updating the profile yet.
>Just curious if we should still consider exterior poly isofoam/rainscreen. We will be building in Marine 4 just outside Seattle.
I'd say "yes". Without the foam a rainscreen would be even more important in your neighborhood.
Then there is thermal performance thermal bridging of the top & bottom plates & window framing is still pretty significant, and the enter-cavity R of about R27 (before discounting for thermal bridging) isn't very spectactular. A 2x4 16" o.c. wall is well understood, cheap to build. Adding 3" of exterior polyiso would have the same wall thickness as a 2x8 staggered stud wall, and would outperform it, at a "whole-wall-R" of about R30 (AFTER thermal bridging), which is more than the nominal R value of 7.25" of dense-packed cellulose even without discounting for thermal bridging. A 3" layer of foam isn't too hard to build, but experience helps. Using reclaimed roofing polyiso (there are vendors in your area) the material cost of the foam would be about the same as 1" of virgin stock foil faced goods.
Without running the math I suspect an easy-to-build 2x6/R20 24" o.c. + 1" of exterior polyiso is comparable to or even higher performance than the staggered stud 2x8, and more resilient due to the warmer average heating season temperatures at the sheathing.