Use of rigid polyisocynene on exterior wall system
I’m building a house in Wisconsin (zone 6) and plan to use an inner wall structure filled with dense-packed cellulose covered by 4 inches of rigid polyiso on the exterior, topped with battens for a rain screen. I believe polyiso on the exterior is endorsed by Joe Lstiburek, Martin Holliday here at GBA, and also the Journal of Light Construction. It would serve triple duty–thermal break, WRB, and drainage plane. But my architect does not agree, saying polyiso will prevent drying to the outside. True, but based on what I’ve read, I don’t think drying to the outside is necessary since the inner cellulose and stud wall will only have drywall and paint as a vapor barrier, allowing any moisture migration to the inside. Any feedback? Thanks
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Bill,
Here are two articles that describe the concepts behind rigid foam wall sheathing:
Calculating the Minimum Thickness of Rigid Foam Sheathing
How to Install Rigid Foam Sheathing
Bill,
You could also install up to six inches of rigid mineral wool, for similar thermal value and improved resistance to pests, fire and moisture accumulation. See this article: https://www.greenbuildingadvisor.com/blogs/dept/musings/installing-mineral-wool-insulation-over-exterior-wall-sheathing.
TJ,
I plan on using some Roxul on the interior, but mostly for sound proofing. Based on what I've read, I don't think mineral wool products are quite there for many exterior applications. To get the equivalent R value of foam would, as you suggest, require at least 6 inches of wool. That would be problematic for fastening an exterior batten system for my siding, which will be a combination of cement fiber board and stucco. Mineral wool is not recommended under stucco at all.