A. Bruce Harley, technical director of Conservation Services Group in Westboro, Mass., and author of Insulate and Weatherize, responds: In this notoriously leaky and difficult-to-insulate area, both the insulation and the air barrier must be continuous to prevent outside air from moving freely through the knee wall and into the rest of the house. While fiberglass batt insulation is effective when installed properly, it won't stop air infiltration, regardless of how much is added to the knee-wall cavity. Without a continuous air barrier, outside air can enter through soffit vents, wash through insulation, and flow through joist bays.
While it's possible to insulate the knee wall and floor, I prefer to align the thermal boundary with the weather shell of the house by insulating the rafters, particularly if there's mechanical equipment, ductwork, or storage in the knee-wall space. Sprayed urethane foam can be used to provide both insulation and air barrier; note that vent...
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