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Is a “hot roof” ok?

Dougherty17 | Posted in Green Building Techniques on

We have 2″ x 6″ rafters in our existing farmhouse. We have been planning to have them completely filled with close cell spray foam to meet building code, and then use 3/4″ thermax between them and the sheet rock to prevent thermal bridging. 

We were planning to have the spray foam applied right to the the plywood underside of the roof, making it a ventless,”hot roof”. Or had thought of putting in some kind of cardboard or material layer between plywood and spray foam so it wouldn’t stick sirectly to the roof in case of need for repairs in the future.

I am wondering if anyone has experience with a hot roof, or is it more important to loose some r value for venting purposes? The house is a cape with no attic space. 

We plan to reshingle with fiberglass reflective shingles that can withstand the higher roof temps in the future. 

Thanks, Dave

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Dave.

    Though your plan will likely work, without knowing where you are, it is not possible to know if you are installing an adequate level of insulation, whether or not it meets the building code. Also, and this may just be an issue of how you wrote the sentence, but the rigid insulation needs to be installed as continuous sheets below the rafter to prevent thermal bridging, not "between them."

    Reroofing offers an opportunity to take a different, and perhaps more effective approach: rigid foam above the roof sheathing and fibrous insulation in the rafter cavities. In this way you could minimize the use of closed cell spray foam and get a better, thermally-broken assembly.

    This article will give you a lot of information to consider: How to Build an Insulated Cathedral Ceiling

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