Fiber-faced polyiso for continuous insulation
Been reading GBA for a while, although this is my first post:
When using fiber-faced roofing polyiso for continuous wall insulation, can you install the polyiso outboard of the WRB? A large recycler in my area (usually) has reclaimed roofing polyiso at a reasonable price (4×8 sheets at 2 inch thickness, $0.52/sq. ft.), and reclaimed material seems sort of ‘green’.
Please consider the following assembly, inside to outside:
– Drywall with latex paint
– 2×6 framed wall, dense-pack cellulose cavity fill
– 1/2 inch plywood sheathing, taped for air-tightness
– Water resistive barrier (vapor open membrane – housewrap, drainable?)
– Windows installed at sheathing plane
– Fiber-faced polyiso, 2 to 4 inches
– 1×4 furring strips, screwed to framing
– Cladding (cedar, engineered wood, maybe metal)
I’m looking for a reliable, proven assembly using locally available materials wherever possible. I see advantages in locating the main air barrier, WRB, structural sheathing and window flanges all at the sheathing plane, advantages like simplified flashing details and a somewhat protected air barrier.
1) Will fiber-faced material withstand the occasional incidental wetting it will likely see, or does it need to be protected by the WRB? Moving the WRB to the outside may mean moving the windows out too.
2) Assuming outside is OK, do you need drainable housewrap, or something to provide a small gap, between the sheathing and polyiso?
3) Is fiber-faced more vapor-permeable that foil-faced, enough so to permit some outward drying?
4) I live in a somewhat termite-prone area (just east of Denver, CZ 5B, 6100 HDD, 15 in. annual rainfall). Does that make exterior foam insulation of any kind a risky idea? Note – There is also a local distributor for Rockwool Comfortboard, but that goes for $2.30/sq. ft. at 2-inch thickness, 4 times as much as the reclaimed roofing polyiso. I’m also interested in fiberboard (Gutex etc.), but I’d rather get that from a source closer than Europe. I hear GO-Lab in Maine is expecting to go into production next year.
FWIW, an acquaintance gave me a few leftover sheets of 2-inch roofing foam that look an awful lot like R-Max Multi-Max FA-3 or equivalent (see attachment). I left a broken piece outside for 2 years (still out there). It’s been rained, snowed, hailed, and UV’d on, and it still looks pretty much intact – no cracking, swelling, warping, or delamination. The fiber facing is starting to look a little ‘furry’, but otherwise it still looks usable. Attached Multi-Max spec sheet lists Water Vapor Transmission as < 1.5 Perm, although it doesn’t say at what thickness.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part