Foil-faced or Fiber-Faced Polyiso Exterior Insulation (Climate Zone 6)
I’ve been perusing the forum and can’t seem to find a straight answer. Located in Maine (climate zone 6). I’m looking into ordering some reclaimed polyiso foam board for exterior insulation but it is fiber faced and I am not sure if it will work well in my wall assembly. I was planning to do 3 inches using two 1.5in layers.
1. Interior wall will be pine tongue and groove (not planning to do drywall)
2. 2×6 construction
3. R-23 rockwool batts
4. ZIP sheathing (7/16in)
5. 3in polyiso (two layers 1.5in, seams staggered)
6. 3/4in rain screen
7. Wood siding (likely pine)
I’m concerned about permeability with the fiber faced polyiso insulation. With the ZIP sheathing, I already have a WRB. Based on what I’ve read it sounds like I would need the WRB on the outside of the polyiso if it was fiber faced. Is this the case? Would foil faced insulation be a better option here? Also, is there any advantage to doing two layers of foam or should I just do one 3in layer?
Thanks in advance.
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Replies
I've used fiber face for walls without issues. I like the WRB outside the foam as it provides an extra layer of protection plus with outie windows it makes all the flashing details simple as everything is a single plane. You don't have to have the WRB there, I just find it easier.
The fact that fiber faced is permeable is a bonus, it helps with drying into the rain screen cavity.
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I don't see a problem here. I think having a WRB over the polyiso, and a foil facer can act as one, is a plus, and foil faced polyiso is easier to tape. Aside from those two things, I don't see a problem using the fiber faced stuff here. As Akos mentioned, the little bit of drying ability provided by the slightly permeable fiber faced polyiso is a benefit here.
I think you'll be fine either way. I would pay a little more attention to details to make sure no bulk water sneaks behind your rainscreen's outer siding layer, but that's about it.
Bill