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Community and Q&A

Prepping Vent Penetrations in Hot Roof for Spray Foam

allstar784 | Posted in Green Building Techniques on

We modified our ceiling to be a cathedral ceiling and want to do a hot roof.  The existing roof (which is part of an addition), has 2 vents.  In order to do a complete seal from roof to top plate and down the walls, we need to seal off existing openings.

My question is, what material should I use to cover vent hole openings prior to spraying the foam?

I’ve seen what looks like kraft paper being used, rosin, and a whole heap of things however, I’d like to use what will best suit the longevity of the roof and ensure no future leaks.

I’m in Climate Zone 5A (Chicago).  The vent holes are ~5 x 5in.

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  1. Expert Member
    Akos | | #1

    You can pretty much cover it with anything that will stay in place while the spray foam is applied. All materials you listed above will work.

    If the vents are visible from the ground, I would cover them with something dark such as fiberboard sheathing.

  2. Expert Member
    NICK KEENAN | | #2

    Could you give more detail on what kind of vents you're talking about? Ridge vents, soffit vents? Flat roof or pitched? Do you want to seal the vents to be as weatherproof as the surrounding roof, or just keep the spray foam from leaking out?


    1. allstar784 | | #4

      Pitched Roof, standard vents, not soffit or ridge. Keep spray foam from leaking out and make sure the spray foam is continuous. I also want to ensure no pests become attracted to or attempt to access the foam as I've heard some horror stories about birds & instects boring holes into the foam, such as carpenter ants & bees.

  3. Expert Member
    BILL WICHERS | | #3

    It's common to use mineral wool or fiberglass batts stuffed into larger openenings. I've used housewrap myself multiple times. All you need is something to provide a backing for the foam to be sprayed against. Once the foam cures, the backer doesn't really do anything (assuming you don't have a really big gap you're trying to bridge).

    Since you say there are "hole openings", I'm assuming you have holes cut for vents, and not something like a ridge vent that uses a continous, open strip near the peak. If you have actual round or square holes cut out of the sheathing, I would patch those properly so that roofing crews don't put a foot through at some point. All you need to do is put some sheathing into the holes that is a rough fit (it doesn't have to be perfect), then either use a second, larger piece as a backing to tie things into the existing sheathing, or do a nice job and span the gap with some 2x4s between the rafters and use those to support the patch piece of sheathing. When the spray foam is applied, it will seal any gaps in the patch job -- the wood parts are only for structure here.


    1. allstar784 | | #5

      Thanks very much, Bill. I was thinking I could do something like that. I think I might cut out a piece of house wrap and then fill in the area with the backer as you suggest. I'm leaving the vents on, purely for aesthetics and because I don't want to climb out there to remove them.

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