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.
Air exchange is calculated by measuring the rate at which a volume of air moves through the skin of the house, the heat capacity of the air, and the difference between the indoor and outdoor temperature. What is important here is the volume of air contained within the indoor conditioned space and how often it is replaced with unconditioned outdoor air. This rate of exchange can be expressed as air changes per hour (ACH) or as a rate of exchange in cubic feet per minute (cfm), but in any case, you're looking at volume and rate, not square footage of living space.
While a rule of thumb that relates window area to footprint of living area may provide homeowners and builders with a quick ballpark estimate, it dumbs down the analysis. This approach might make sense for lighting design, but not for energy analysis.
Consider a simple example: A builder constructs two homes with identical square footage of living space. However, the first home has 8-foot walls and casement windows, while the second has 10-foot walls and double-hung windows. Not only does the second home have more surface area and thus higher conductive heat flow through the walls, but it will also be leakier. The impact of these conditions is best expressed in terms of surface area and leakage rates, not window area per square foot of living space.