Life without windows?
Here’s the lowdown on a building envelope’s most complex component
Could you live in a home without windows? Probably not. But while highly desirable, windows are literally “thermal holes” in your building envelope. If you have an older home with single-pane windows, they could be responsible for as much as 50 percent of your heating and cooling loads! The R-value—a measure of a building insulation’s effectiveness—of a single-pane, wood-framed window is about an R-1 (the higher the number, the more effective the insulation). This means that it will transfer about 12 times more heat per unit area than a standard 2-by-4 wood-frame wall with insulation. Sure, many homeowners now have dual-pane, argon-filled, low-emissivity (low-e) windows, but these improvements only increase the R-value to about an R-4 at best.
Locked and loaded
Heat transfer through windows is a fairly complicated combination of all three modes (conduction, convection and radiation), making them the most complex component of the building envelope. Multiple panes (or glazings), air spaces between panes, special coatings and the frame all play important roles. During the summer, windows can significantly increase a home’s cooling load by direct solar radiation. They also have air infiltration (leakage) no matter how well-sealed and -caulked. Indeed, R-value alone does not tell the whole story and there are several other measures you should know about when selecting them …
Prescription diction Windows, glazed doors and skylights are collectively known as fenestration products. In California, these products must meet prescribed performance measures and be properly labeled according to the Title 24 Building Energy Efficiency Standards. The window-performance measures include U-factor, solar heat-gain coefficient (SHGC), visible transmittance and air leakage.
U-factor, or heat-transmission coefficient, is a measure of how much heat (excluding solar) passes through the glazing and frame of a window. (For the math-savvy, U-factor is the inverse of R-value: U=1/R.) U-factor is reduced by filling the gap between panes with a low-thermal-conductivity gas such as argon or krypton. Low-e coatings on glazing surfaces also lower the U-factor by reducing radiation heat transfer through the window.
SHGC is a number between zero and one that represents the percentage of solar heat that is transferred (by all modes) through the window. It is affected by low-e coatings and can be further reduced through tinting. Visible transmittance is also a number between zero and one that represents the percentage of visible light passing through the window. Since low-e coatings and tinting reduce visible transmittance, these factors must be balanced to get sufficient daylight. Finally, window air leakage is expressed in terms of cubic feet per minute (cfm) per square foot of window area.
Hit the zone
To meet the Title 24 “standard package” prescriptive requirements in our climate zone, new residential windows must have a U-factor of 0.40 or less, an SHGC of 0.40 or less, and air leakage of no more than 0.3 cfm/ft2. There are no requirements for visible transmittance. Head back to Sustainable Space in the comings weeks as I reveal ways you can significantly reduce window heat transfer beyond Title 24. I’ll also introduce some exciting new window technologies. In the meantime, happy holidays!