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As a builder in the Energy-Crafted Homes (ECH) program in Massachusetts, I’ve been putting together energy-efficient houses for years. Homes in the program have to meet stringent standards for total air leakage and total heating load. In typical houses, roofs are a big source of heat loss and air leakage. So we put a lot of effort into building airtight roofs with plenty of insulation. Cathedral ceilings are a particular challenge: When you have only the depth of the rafter to work with, achieving a high R-value, a good air and vapor barrier, and code-compliant venting takes a bit of thought.
If I’m building a big house but only a small section has a cathedral ceiling, I don’t go overboard. It’s the whole house, not each little part, that has to meet the standard, so putting a lot of work into improving the insulation of that little piece doesn’t pay off. I’ll usually settle for R-40 or so in a small section of ceiling.