A.Contributing editor Paul Fisette responds: While there are many excellent sources that relate window area to square footage of living space, I'm not a fan of this approach. Conduction and air leakage are the main ways that heat escapes a building envelope, so any meaningful energy analysis should be directly related to these dynamics.
Conduction is a function of the surface area of the wall, the thermal properties of the wall assembly, and the difference between the inside and outside temperature. Surface area is critical; the footprint of the living area is not.
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