Rigid foam board on underside of rafters?
What is the building science opinion on fastening rigid foam insulation board to the bottom edges of rafters in an attic insulation retrofit?
I’ll provide more details on this proposed system. The foam board would be up to 4 in. thick and include a foil facing (radiant barrier) on both sides. To keep the insulation continuous, additional foam insulation would have to be installed over exterior wall plates and against gable ends. But essentially this technique aligns the home’s thermal and air barriers just beneath the rafters. Unlike the unvented cathedralized attic that’s created by spraying foam between rafters and directly against the roof deck, this rigid foam retrofit maintains attic ventilation.
Has such a system been tried before? What are the pros and cons?
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