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Can I add rigid foam during shingle replacement if roof deck is closed-cell foamed?

musician235 | Posted in Green Building Techniques on


After reading Joe L’s article on “over-roofing” I’m considering applying closed cell foam to the roof deck with the intention to add rigid foam board above the roof deck down the road when it’s time to replace the shingles. I understand that there’s a chance of water being trapped in between the two layers since each is water impermeable, but I want the high r value and don’t have room to add the foam underneath the ceiling. Will it work? Thanks for your input.

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  1. Expert Member
    BILL WICHERS | | #1

    Yes, it will work, but as Joe says it’s “risky”. Basically you’re ok as long as nothing leaks. Many roofs with ccSPF applies to the underside of the sheathing have vapor impermeable membranes on top which is a similar situation. I believe Joe has also written something along the lines of “its risk, but we also do it all the time” regarding roofs sealed on both sides.

    If you do put rigid foam on top, I’d be very careful to detail and flash everything very well to be extra safe.


  2. Expert Member
    Peter Yost | | #2

    Hi Matthew -

    Bill is right on. Have I designed/built roof assemblies that have little to no drying potential in a low- to no-vapor permeability sandwich of materials? Yes, BUT I only do it as a last resort AND only do it on very simple roofs for which I also am convinced that it will get actually constructed the way is is designed and specced. And by simple I mean: geometrically AND with few to no roof penetrations.


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