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Closed-cell spray foam under fiberglass for high R-value roof assembly?

DakotaGreen | Posted in Green Building Techniques on

I was talking to a spray foam contractor and I was unfamiliar with his method. He applies 2″ of closed cell spray foam to the backside of the drywall on the ceiling and then covers the spray foam with blown-in fiberglass to reach a High R-value. The space is still vented.

Would this approach have concerns or issues? It is not one of the High-R roof assemblies I have heard of before and I can’t seem to find any articles discussing this particular approach.

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  1. Expert Member
    Akos | | #1

    This is a great approach, the spray foam does a great job of air sealing and the venting helps with drying from small leaks.

    The only thing I would want is a layer or house wrap under the spray foam. Getting plywood off spray foam is a pain but doable, drywall off would be a giant mess. Over the life of the house, drywall is something that does need to be occasionally taken down.

  2. GBA Editor
    Martin Holladay | | #2

    The approach you describe won't cause any moisture concerns. However, it isn't particularly environmentally friendly or inexpensive.

    The usual approach is to perform air sealing work by other means, and then to install a deep layer of blown-in cellulose or fiberglass. For more information on less expensive ways to air seal, see this article: "Air Sealing an Attic."

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