High R-Value Windows Are Top Pick To Improve Home Energy Performance

Authored by: Ann V. Edminster, M.Arch., LEED AP+ Homes

Written by green-home building expert, Ann Edminster, this resource examines two scenarios to help architects, building owners, builders, and residential energy modelers understand cost-effective options and energy variables that can have the greatest impact on home energy performance. As part of research for her book, Energy Free: Homes for a Small Planet, Ann looks at two different types of residential properties in three different climate zones to answer two important questions:

Question #1: If I have a budget of $15,000 to spend on retrofitting a home to improve energy performance, what gives the biggest bang for the buck?

The energy performance gains of three different options were studied to find out which options gives the biggest bang for the buck:

		OPTION 1 Increase efficiency: improve attic and under-floor insulation, thoroughly air-seal the enclosure, and upgrade lighting and appliances to ENERGY STAR-compliant models.
		OPTION 2 Replace windows: replace all existing windows with R-5 windows. Two base case scenarios were examined, one with R-1 (single-pane) windows and the second with R-2 (double-pane) windows in the existing home.
		OPTION 3 Install solar system: install a 2-kW photovoltaic array on the roof.

Result: R-5 windows are the most cost-effective energy-saving solution for residential home retrofit projects when replacing single-pane windows and are cost-competitive with other options even when replacing dual-pane windows.

According to Ann’s study, R-5-and-above windows represent a game-changing entry into the residential replacement window market and into the broader realm of energy efficiency retrofit options. While in the past, window replacement was not typically viewed as offering a good return on investment from an energy perspective (though often attractive for reasons of improved comfort), it should now be considered routinely for home energy retrofit projects.

Question #2: If I’m building a new home, which energy variables have the greatest influence on the overall energy performance of a new home?

Energy variables studied included building orientation, wall insulation, roof insulation, window area, window R-values, window solar heat gain coefficient (SHGC), and HVAC (heating, ventilation, and air conditioning) equipment efficiency.

Result: Window R-value often has the greatest influence on energy performance compared to other common energy variables. High R-value windows (R-5 or greater) provide excellent energy savings.

In all but one scenario modeled, window R-value (the inverse of U-factor) had the greatest influence range of the variables studied. High R-value windows are likely to provide excellent performance benefits in many projects, particularly those with a high window-to-floor area ratio.

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