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Community and Q&A

Another cathedral cieling quesiton: polyiso board vent channels and CC polyurethane foam adhesion?

user-7202783 | Posted in General Questions on

I’m converting a vented attic (presently insulated on the floor with R-19 pink fiberglass batts) to a cathedral ceiling (keeping it vented to the ridge).  My existing rafters are very shallow (2×6) and I have limited headroom to work with so every inch counts.   At the same time, my roof pitch is pretty low (~12 ft of run for ~3 feet of rise, or about 12 degrees?) so it’s definitely important for me to retain a good-sized ventilation channel.  As such, I’m considering either using a pre-fabricated channel (i.e., smartvent, 2″ channel) that wastes as little space as possible, OR fabricating my own vent channels out of recycled polyiso board, so that the channel material itself is serving as an insulator.   

My plan is to then apply about 2.5-3″ of CC spray foam over the venting channels, to achieve ~ R 17-20?  Then, based on suggestions I’ve seen elsewhere in the forum,  I was considering running another set of 2x4s (or ideally 2×6’s if space permits) perpendicular to the existing rafters (creating a thermal break) and stuffing these with rockwool (of the appropriate depth) – giving me another R-15 (or, in the case that I can fit 2x6s, maybe recycling my R-19 fiberglass).   Then enclosing everything with painted 1/2″ drywall.    This assembly should give me between 1/3 and 1/2 my total R value outside of the “fluffy” insulation, which I think leaves me safe from condensation issues.

I know this is imperfect for zone 4, but it’s still a considerable improvement over the existing insulation scheme, and I am limited by practical considerations.  (When I replace the roofing shingles in the future, I’ll put another 2″ of foam board/nailbase over the exterior roof sheathing, and that should get me close to code).  Though I’m still open to other suggestions…

Finally to my question – if I build the vent channels out of polyiso board, I want to be sure the sprayed-in CC foam adheres to it well.  With that said, I don’t imagine it would bond well to foil facing?  Should I look for different facing materials, or are there versions of polyiso that are only foil faced on a single side?  I’d think that the CC foam would bond well to the “raw” exposed polyiso?

Thanks in advance for sharing your input!

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  1. Expert Member
    BILL WICHERS | | #1

    Closed cell spray foam bonds well to pretty much everything. It will stick like crazy to the foil facing of a piece of polyiso.

    Several years ago I had spray foam applied to the underside of the roof sheathing in an area of my house that has a cathedral ceiling. Everything that was still in the room when they applied the spray foam to this day has bits of foam stuck to it. Those things include an extension cord, a drill bit case, several polyethylene buckets, and some scrap wood. Probably some other things I’ve forgotten about. The spray foam continues to this day to stick like crazy to all of those things.


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