A tight, well-insulated envelope. The Wisdom Way Solar village — a project of the Massachusetts nonprofit Rural Development Inc. — is a 20-unit development of two-, three-, and four-bedroom duplex units priced for low- to moderate-income buyers. The simple rectangular structures feature double 2x4 walls insulated with blown cellulose and conventional truss roofs insulated with R-50 cellulose at the attic floor.

Most of the units have no dedicated air barrier or vapor retarder. (A layer of polyethylene installed beneath the drywall of the first two units was omitted from those built later.) Thanks to careful air-sealing beneath the plates and at sheathing joints, however, they’re very tight: Blower-door testing put their air leakage at 280 to 300 cfm (...

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