Double Walls

This video details a variation of double wall construction which has evolved in Fairbanks, Alaska to increase home energy efficiency.
In all three of Alaska’s climate zones, residential building codes mandate the use of a 6 mil polyethylene vapor retarder. Since this vapor retarder also serves as a home’s primary air barrier/pressure boundary, it should be installed as airtight as possible to prevent excessive air leakage and moisture movement into exterior wall and roof assemblies during Alaska’s extended heating seasons. In a generic double wall system, however, wiring and electrical outlets still penetrate the vapor retarder, and thus require additional air sealing measures to reduce air leakage through and around these penetrations. The double wall system described here utilizes an outer 2x6 wall and an inner 2x4 wall, with a continuous layer of 2” foam board separating the two walls. The vapor retarder is located behind the inner (2x4) wall and is located directly over the layer of 2” foam board, where it remains intact and protected.
The layer of foam board serves as backing for the vapor retarder and also provides a robust thermal break for the 2x6 exterior wall, elevating the r-value of the outer (cold-in-winter) side of the vapor retarder to a minimum of R-30. This allows the inner 2x4 wall on the inner (warm-in-winter) side to be fully insulated with fibrous insulations and remain safely above the dew point. Electrical outlets, ducting, plumbing, etc. can all be installed conventionally inside the 2x4 wall and remain protected from freezing. This insulation technique substantially increases thermal efficiency, bringing the total wall R-value over 40.
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