Shingle roof over closed-cell foamed rafters
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| Posted in Green Building Techniques on
Martin – I’m the architect for a new house. The roof will be:
Wood shingles over
Horizontal strapping over
Vertical strapping over
Grace Ice and watershield over
Plywood sheathing over
Closed cell foam
Do you recommend this? Will there be trouble within the sheathing ’cause it’s trapped between two non-breathing membranes: Grace Ice and Watershield (top) and closed cell foam (below)
Many thanks,
Louis Mackall
(you may remember me from Trident launchings in Groton!!)
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Replies
Louis,
Yes, I remember working on your house in Guilford. We had to rent a gas-powered concrete saw to cut through your concrete walls and concrete siding panels. That was a lot of fun!
Concerning your current question: I think that ordinary asphalt felt would make a better roofing underlayment for your roof than Ice & Water Shield. Since you are planning good ventilation channels between the roof sheathing and the roofing, and since you are choosing a vapor-permeable roofing, your roof assembly can dry to the exterior -- which is a valuable asset. Don't shut down this drying path with Ice & Water Shield.
It sounds like your roof assembly doesn't address thermal bridging through the rafters. Sometimes I sound like a broken record -- but it doesn't make much sense to install expensive closed-cell spray foam between rafters when the rafters are thermal bridges. A couple of layers of rigid foam above the roof sheathing are always a good idea. If go that route, switch to a vapor-permeable insulation like open-cell spray foam or cellulose between the rafters.
For more information on designing a roof with rigid foam above the roof sheathing, see How to Install Rigid Foam On Top of Roof Sheathing.