A. Henri Fennell, of H C Fennell Consulting in North Thetford, Vt., responds: It's true that the wood rafters are the weak link in the insulation system you describe. In the absence of a thermal break, heat from the living space can flow through the drywall into the rafters - which are much more conductive than the surrounding foam - and pass through the rafters to the outdoors. Adding a layer of rigid foam under the drywall would significantly reduce the conductive heat loss. However, this is not an ideal solution, because convective movement of the air between the spray foam and the rigid foam within the rafter bays will speed the transfer of heat from the warm ceiling to the cooler rafters.
Thermal-break insulation performs most efficiently when it's contiguous with the primary insulation. Therefore, a better solution would be to apply the spray foam from above, directly against the surface of the rigid foam. That approach can complicate job scheduling, since the roof has to be left open until the foam contractor has left the site....
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