Paul Scheckel, Author, The Homeowner’s Energy Handbook
A typical energy-efficiency project for many homeowners is to install new windows. It might seem like a no-brainer, but if you are among these homeowners, consider your options and motivations. Window replacement solely for the sake of energy cost savings is not often the best place to invest your efficiency budget. If the windows are damaged and in need of replacement anyway, it will often pay to spend extra and get the most efficient window you can. In most cases, this means a triple-glazed unit with three panes of glass, or two panes of glass with a coating on the glass, or a plastic membrane in the space between glass layers. If comfort is the issue, better windows offer an immediate payback. When I replaced a large, old, single-pane, double-hung window near my living room couch a few years ago, the increased comfort allowed the use of a space that was previously too cold and uncomfortable to use in the winter. This again was an immediate non-energy-related benefit but in terms of cost savings, it would never have made sense.
When it comes time to upgrade, put the right window in the right place for a good balance of efficiency, light, and solar heat gain. Before shopping, learn to read and use the performance label found on all windows. This label is provided by the National Fenestration Rating Council (NFRC), and is based on testing a sample from the window manufacturer. Performance varies with the number of glazing layers, glass coatings, and whether the space between layers is filled with a gas which reduces heat loss better than air does. One of the inert or noble gases is used, most often argon.
You may find that more than one type of window will offer the best performance. For example, you may want to capture the sun’s energy early in the morning while rejecting late summer afternoon sun. This approach would call for a window with a high solar heat gain on the east side and low solar heat gain on the west. Such a strategy is known as “orientation-tuned” glazing. Following is some guidance on reading the NFRC label. Ratings are based on the entire window unit, not just the glass area.
U-factor indicates the overall insulating value of the window. Lower numbers mean less heat transfer through the window. To find the R-value of a window, divide 1 by the U-factor. A single pane of glass has a U-factor of about .91, translating to an insulating value of about R-1.1.
Solar Heat Gain Coefficient (SHGC) is a measure of how much solar radiation (resulting in heat gain) is allowed to pass through the window. Higher values indicate greater solar heat gain. An SHGC of .45 means that 45 percent of the solar energy that is incident on the window passes through it.
Visible Transmittance (VT) measures how much visible light comes through a window. VT is expressed as a number between 0 and 1. A higher VT means more light is transmitted. A VT of .58 means that 58 percent of the sun’s visible light passes through the window.
Air Leakage (AL) is an air-leakage rating expressed in cubic feet of air passing through a square foot of window area. Much heat loss and gain occur by infiltration through small air leaks in the window unit.
Condensation resistance (CR) measures the ability of a window to resist the formation of condensation on its interior surface. CR is expressed as a number between 1 and 100, with higher numbers indicating better resistance to condensation.
Learn more about widow technology on the web at www.efficientwindows.org and obtain ratings of new windows at www.nfrc.org.
Paul Scheckel is an energy efficiency and renewable energy consultant and author of The Homeowner’s Energy Handbook. He lives in Calais, VT and is a partner at Shelter Analytics www.shelteranalytics.com.