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Should I vent my polyiso insulation?

jberks | Posted in General Questions on

I have put down my roof, except for the top roofing membrane.

Here is my roof assembly:

9.5″ Joists with R31 Glasswool Batting
3/4″ T&G plywood decking
Roofing underlay (IKO Armourguard Ice and water sheild)
3″ + 3″ Polyiso Insulation, R 34, staggered Joints, joints filled with closed cell foam.
1/8″ asphalt protection top board, seams caulked.
(Not done yet) Polyurethane liquid applied roof membrane.
(TBD) 2.5″ Live vegetative roof assembly.

I was showing a friend who is a commercial roofer, and he mentioned that if my Polyiso gets wet, later one when I put down the membrane, its going to create a moisture sandwich and the Polyiso will rot and bubble. It has already been exposed to a bit of dew and ice in the mornings as its getting cold here in Toronto.

So that begs the question, how dry is dry enough for polyiso, and how do I prevent bubbling in the future?

He had mentioned installed a roof vent, essentially venting just the polyiso. I had put down the underlay material as per Big Joe’s article on Green roofs, thus I made small scuppers/notches at the deck level of the parapet for any condensation or potential future roof leaks to escape. Should I make them bigger to attempt to allow some vapour venting?

As always, Any advice is appreciated.



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  1. GBA Editor
    Martin Holladay | | #1

    The issue you raise is a tricky one for commercial roofers. You are definitely sandwiching the polyiso between two impermeable layers, and you definitely want to keep the polyiso as dry as possible during the roofing process.

    When I attended a presentation by Mike Steffen about the roof assembly at the Orchards at Orenco project (you can read about the roof assembly here: Roofing and Cladding for the Orenco Passivhaus), Steffen admitted that he was nervous about the issue you raise.
    The best he could come up with was something like "Only work in sunny, dry weather, and work fast."

    I don't think there is any way to vent this type of roof assembly -- at least no easy, effective way -- and it's worrisome that your polyiso may already be damp.

  2. jberks | | #2

    Hi Martin,

    That's a good article, thank you. I am doing many similar things on my project.

    Its funny, had I known polyiso was so finnicky with moisture, I might have opted for XPS instead. My mistake.

    I suppose at this point, I could drill some holes along the parapet. Basically allowing some venting between the Deck flashing and roof membrane. The parapet will be flashed with aluminium, so any holes I drill will be covered, but perhaps the vapour would still be able to escape around the flashing as its not really airtight. I prefer this method as the polyiso is fibergalss faced, and having horizontal venting 'should' allow for better vapour transfer through the polyiso as the edges aren't faced.

    Another possibility is to drill holes at the underside of the decking at the roof overhangs, and have some venting in the soffits. Since the overhang is not part of the building envelope, I don't mind compromising the deck flashing there.

    It's sort of a gamble in that I don't know if these will work. But potentially worth the effort as a "just in case" precaution.

    Do you see any issue with these?
    Or any issue with venting polyiso in general?



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