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. On big expanses of cathedral, however, I want close to R-50, and a near-perfect air and vapor barrier. I try to be well within the ECH standards, not barely inside the line. I’ve tried a lot of different techniques over the years, and I’m still experimenting. In this article, I’ll discuss several ways to get a high-performance cathedral roof and give you a close look at our latest method.

This method seemed like a simple idea when I tried it. On a cathedral roof that needed only a 10-inch-deep I-joist for structural reasons, we went with a 16-inch I-joist, installed vent channel under the sheathing, stapled a reinforced plastic vapor barrier to the rafter faces, strapped over the plastic with 3/4-inch strapping, and blew the...

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