Kevin is working up the design for a new lakeside cabin in Tennessee, with plans currently calling for a scissor-truss roof and a vaulted ceiling over the kitchen and living area. He wants a trouble-free roof assembly, but local advice on insulation and air sealing has been hard to find.

The roof starts with scissor trusses set on 8-foot centers. Between them, running horizontally, Kevin would run 2x6s on 24-inch centers (the 2x6s set on edge), and insulate the bays with fiberglass batt insulation. The bottom of the 2x6s would be covered with tongue-and-groove pine boards. On top, he’d start with OSB sheathing, then add 2 inches of rigid foam insulation, a layer of 1/2-inch plywood, then foil bubble wrap and, finally, metal roofing.

He’s ruled out the use of structural insulated panels (SIPs).

“There doesn’t seem to be anyone around here, in southern Kentucky, that cares much about good insulation and sealed houses,” Kevin writes in a Q&A post. “I don’t have any education other than applying different parts of roofing projects I have seen online and what I have seen in person. Please point out problems with this idea or a better way.”

That’s the topic for this Q&A Spotlight.

This plan has some problems

GBA editor Martin Holladay responded to Kevin’s question before Kevin had fully explained his plan to install rigid foam above the roof sheathing. Based on Kevin’s description of fiberglass batts installed between the 2x6s, Holladay noted a glaring weakness in plans: the lack of an uninterrupted air barrier in the ceiling.

“You absolutely need an airtight ceiling to prevent interior moisture from entering your roof assembly and causing moisture problems,” Holladay writes. “That means (a) You can’t…