Q. What’s the best way to insulate over an exposed 2x6 tongue-and-groove cathedral ceiling on a log house in Colorado? Some have suggested using SIPs, but others have advised that, because this particular roof is cut up with valleys and dormers, I would lose the labor savings associated with SIPs and end up with a very expensive roof. Another suggestion was to box out the roof with 2-bys, then spray foam insulation into the grid and cover it with plywood. A final suggestion was to put down foam panels right over the 2x6 plank ceiling, seal the joints, and lay plywood over that. Unfortunately, no one has given me details for these applications. Can you advise?

A.Contributing editor Henri de Marne responds: Two of the above suggestions sound okay. You can build a grid over the 2x6 deck and spray foam (urethane or Icynene, for example). I would opt for using 2x6 sleepers and 4 inches of foam, and providing an air space of 1 1/2 inches between the insulation and the plywood. Although it may be difficult to provide venting from soffits to ridge, considering the valleys and dormers you mention, it can often be done by cutting slots in the sleepers at strategic points to allow some airflow.

Some technicians I have worked with say that no air space is needed. They suggest filling the entire space and nailing the plywood over the filled space. We just did such a job on a large house here in northern Vermont where there was no way to provide ventilation on some of the roof’s sections. The problem I see with filling the entire sleeper space with insulation is that it causes thermal stress to the shingles and may, in some cases, void the warranty on the shingles. But, if you choose to use this system, you can use a 2x4 grid instead of a 2x6 grid or fill the 2x6 grid. The other drawback is that there will be a heat loss through the sleepers.

Another system I have used with success, also in northern Vermont, is to first lay 6-mil plastic over the deck as a vapor retarder and air barrier (very important with board decking). Because it is slippery and dangerous to work on, the poly can be unfurled as the rigid insulation is laid and fastened, starting at the top if that’s easier. Fasten at least two layers of 2-inch-thick rigid extruded polystyrene foam panels directly over the deck, staggering the joints. Make the installation of the panels tight, because they will shrink as they age. Use canned foam to seal joints at valleys, hips, and anywhere else it is needed.

Next, screw 2x2s through the insulation and into the decking and roof framing where possible. Be sure to tie the tops of the 2x2s from opposite sides of the roof as they meet at the ridge with metal straps (Simpson LST or MST Strap Ties) to prevent their creeping down with the weight of the roof assembly. Then nail the plywood sheathing, felt, and shingles. This will give you complete insulation of the roof deck and provide an air space, giving relief to the shingles as required by most manufacturers. If at all possible, also provide ventilation from soffits to ridge.