Roger Lin’s Washington, D.C., house will have a roof of 12-inch-thick structural insulated panels (SIPs). By most standards, that’s a well-insulated roof. But Lin wants to add 2 inches of rigid foam on top of the panels to reduce thermal bridging.
He’s uncertain about the details. He has already installed roofing underlayment over the panels. Can he put expanded polystyrene foam on top of the underlayment and cap it with metal roofing? Or does he need a layer of plywood or furring strips over the foam before the metal roofing is installed?
His question, posted in GreenBuildingAdvisor’s Q&A forum, is the topic for this week’s Q&A Spotlight.
The discussion quickly turns to the risk of rot in the oriented strand board (OSB) skins of the SIPs, and two competing points of view emerge.
The foam will help protect the roof
GBA senior editor Martin Holladay likes the idea of using foam over the roof panels. “Installing a layer of rigid foam on top of your SIP roof to address thermal bridging is an excellent idea,” Holladay writes.
The foam layer should be airtight, he adds, so seams should be sealed with high-quality tape. Metal roofing should not be installed directly over the foam but on furring strips or a continuous layer of plywood. And if plywood is used, then Holladay recommends that Lin install 2x4s from soffit to ridge between the foam and the plywood to create ventilation channels.
“These ventilation channels will give you a cold roof — cheap insurance if you are going to the trouble of getting your details right,” he says.
The bottom line: the extra layer of exterior foam will keep the OSB warmer and dryer,…