Make the Most of Energy Efficient Windows

By Peter Fabris

As prominent architectural features, windows have a big impact on the character of home. Not only that, they greatly affect the energy efficiency of the house and the comfort of the residents. Find out how to make the most of energy efficient windows.

Indeed, choosing the best window for a particular use is an important, complex decision. Most double hung windows cost between $150 and $500 each and represent a considerable percentage of a materials budget in new construction or remodeling projects. Other types of windows—bay windows, for example—often cost much more.

Energy efficiency ratings

Many decisions should be considered when shopping for windows but identifying models with the most appropriate energy efficiency qualities for the application can be particularly complex. Manufacturers have different formulas for glass coatings and differing manufacturing techniques in a race to produce the most efficient windows at a competitive cost. Two labeling systems – Energy Star and National Fenestration Rating Council (NFRC) – are helpful markers to compare windows’ efficiency.

Both programs offer energy efficiency information based on laboratory testing. Energy Star is a kind of stamp of approval indicating that the product has met testing benchmarks measuring quality of performance. Windows that have the Energy Star label, a program backed by the U.S. Environmental Protection Agency, reduce energy bills by an average of 12 percent nationwide, compared to non-certified products, according to Energy Star’s website.

NFRC’s labels provide numerical values for heat loss and heat gain. Consumers should understand the NFRC’s ratings system in order to compare products. NFRC labels provide a U-value, which measures a window unit’s resistance to heat loss, and a solar heat gain coefficient (SHGC), which measures how much heat enters a home from sunlight. The range for U-value is 0.15 – 1.20. The range for SHGC is 0 – 1. In both cases, a lower number means better performance. NFRC U-factor ratings represent the performance of the entire window, including frame and spacer material.

NFRC labels are objective and credible, emphasizes says Tom Herron, senior director, communications and marketing, for NFRC. “We are an independent third-party rating organization that uses independent labs to perform testing,” he says. This means that the council can provide “unbiased ratings to help consumer make informed decisions.”

For clients who want the best performing products, look for Energy Star’s Most Efficient 2018 label. Windows with this label have U-values of .20 or lower.

Climate and energy efficient windows

Climate has a great impact on which windows are optimal for a project. In colder regions, a top performer in the U-value category would be most desirable. In a hot, sunny region, a top SHGC window with significant sun exposure would be a higher priority. The Energy Star website, which offers lists of Energy Star-rated windows for four regions of the country, is a good reference source for climate performance.

“Some windows are designed to perform well across many climates,” Herron says. They are suitable for many areas of the United States. In an extreme climate, though, such as the subarctic region of Alaska where heating is paramount and cooling is unnecessary, you might want to look for windows with the lowest U-factor and a high SHGC to retain as much heat as possible while allowing maximum solar heat gain.

Energy savings from high-performing windows differs by climate, Herron points out. In temperate regions with lower heating and cooling costs, the savings is typically lower than in hot or cold climates. So, spending more for a high-performing window in a region with more extreme temperature variation would create a larger payback over time.

The decision about how much to spend on energy efficient windows based on potential energy savings will vary from owner to owner according to their budget and priorities. There is one upgrade, though, that is a no-brainer from an energy savings standpoint. “If you were to replace single pane with double pane, that’s where you get the most savings,” Herron notes. “Anything you do from there is more incremental.”

Window orientation

To make the best choice for each application, consider which direction the window is facing. South-facing windows should have a SHGC of greater than 0.6 to maximize solar heat gain during the winter, and a U-factor of 0.35 or less to reduce conductive heat transfer, according to Energy Star.

East- and west-facing windows face the sun when it is low in the sky, so they tend to let in more direct sunlight. These windows should have a low SHGC and/or be shaded in most areas of the U.S. North-facing windows collect little solar heat, so the SHGC rating is less important than the U-factor.

The future

The next big advance in the marketplace is likely to be smart window technology—glass that changes to regulate the amount of sunlight passing through. For example, scientists at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) recently developed solar windows that use thermochromism—the property of substances to change color due to a change in temperature. The prototype technology allows window glass to transform from transparent to tinted, and convert sunlight into electricity in the tinted state.

The innovation, called SwitchGlaze, will be cost efficient, according to the Energy Department. The cost of adding SwitchGlaze solar cell layers to traditional windows could be cancelled out by solar energy payback, the department says. It will likely be at least a few years before this development makes its way to the windows market, but it’s promising enough to keep an eye out for it.

Tax credits expired

A couple of years ago, the federal government offered tax credits for people who upgraded their homes to make them more energy efficient, and replacement windows were eligible for these credits. Most federal tax credits for consumers, with the exception of solar panels, expired at the end of 2016, though. “At this time, there are no known plans to reinstate  federal tax credits for energy efficient windows,” according to the Energy Star website.

There are rebates on windows offered by offered by Energy Star partners in some areas. The Energy Star web site provides a feature, searchable by zip code, to find out if a manufacturer’s rebate is available in specific locations.

“Consumers owe it to themselves to know what they are getting,” Herron says. And contractors owe it to their clients to help them understand the various options, costs and payback in energy savings when installing energy efficient windows.