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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There is no building-science argument against your proposal, as long as you pay close attention to air sealing. (However, note that some builders worry about foam shrinkage -- a factor that may reduce the airtightness of a rigid-foam air barrier, even if the seams are well sealed at installation. To learn more about this issue, see the paragraphs below the "Do rigid foam panels shrink?" subhead on this page: Using Rigid Foam As a Water-Resistive Barrier.)
The major problem with your suggestion is that 4 inches of rigid foam doesn't provide a high enough R-value. The R-value of 4 inches of foam ranges from about R-15 (for EPS) to R-26 (for polyiso). That doesn't meet minimum code requirements anywhere in the U.S. -- even in Florida, where (according to the 2006 IRC) you need a minimum of R-30.