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Q.I'm looking at house plans that call for insulating a cathedral ceiling with closed-cell polyurethane foam sprayed against the roof sheathing. If we go this route, would it be possible to create a thermal break by fastening a layer of rigid foam to the underside of the joists before hanging the drywall?

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. But it does improve the thermal performance of the assembly, and provides a ventilation channel between the roof deck and foam within each rafter bay.

In a retrofit situation, of course, the spray foam will ordinarily have to be applied from below. In that case, convective heat loss can be minimized by wrapping the foam up onto the sides of the rafters. That profile prevents any direct contact between the air in the rafter bays and the cool surfaces of the rafters themselves. If the volume of foam used is equal to that of the flat application, the R-value at the center of the rafter bay will be slightly lower, but the reduction in heat loss at the foam-wrapped rafters themselves will give the assembly a higher R-value overall. (Some trimming of the overspray at the rafter edges will probably be needed before the rigid-foam thermal break can be applied and fastened).