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Community and Q&A

How best to seal the gaps between chunky 4″ roofing foam slabs.

severaltypesofnerd | Posted in GBA Pro Help on

We’re replacing concrete tile with standing seam, and adding 4″ of solid foam above the deck. We’re well aware of

Installing Rigid Foam Above Roof Sheathing

PODCAST: How to Insulate an Unvented Roof


Climate is Zone 3 Maine, San Francisco Bay Area.

And ideally would have preferred to have two layers of 2″ foam, staggered.  But the contractor would only go with a single layer of 4″.  The question is how to handle the seams?

I’m thinking spray a bead of window & door flexible foam before sliding each 4×8 sheet of foam in place, then after the foam cures tape the top of each seam.  How does that technique sound for leaving some drying potential to the inside, and reducing thermal transfer through the seams?

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  1. Expert Member
    Michael Maines | | #1

    I think you'll find that it's easier to leave small gaps between sheets and fill them with foam after the sheets are fastened in place. Taping the joints is a good idea, except the roofing will probably get too hot for the adhesive. The sheets will shrink a bit over time; that's one reason it's best to use two layers as you wanted to.

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