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...
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