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2 questions: Is 6 inches of closed-cell spray foam enough? And what to use on the deck before spray foam?

Jeff_Hudson_Valley | Posted in Green Products and Materials on

I’m taking this gut job of a house to the studs and rafters. We’re going to spray the rafters to 6″ closed cell spray foam. I’m planning on adding 2″ polyiso foam board in a few years when we redo the roof.
1) Any particular concerns for rookie mistakes? current roof deck looks to have some mold, but it’s probably because the house has been vacant for 4 years.  Some historic leaks, but not new (and we’re removing the chimney).
2) Is there a product (e.g. roofing felt) that people commonly put over the underside of the roof deck before spraying in case the sheathing needs to get lifted? Otherwise I imagine everything gets pretty much ruined. Someone recommended cheap 4 mil plastic or thin rigid board stuck against it to prevent a good portion of the sticking…
Thank you!

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  1. GBA Editor
    Martin Holladay | | #1

    For information on the pluses and minuses of sandwiching roof sheathing between two relatively impermeable layers of foam, see this article: "Sandwiching Roof Sheathing Between Two Impermeable Layers."

    Almost any available material will work to create a barrier between the underside of the sheathing boards and the spray foam: strips of cardboard, strips of asphalt felt, or 4-mil polyethylene will all work. In this case, cheap and easy is good enough.

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