The household energy costs in most U.S. homes could be reduced by up to 15 percent just by installing Energy Star windows. Since an average household spends more than 40 percent of its annual energy budget on heating and cooling costs, 15 percent energy savings equates to real cash. This is to say nothing of the environmental costs incurred when generating the energy required for heating and cooling America’s homes, which is largely provided by coal. Today, windows alone account for 3.5 quadrillion Btu in U.S. energy consumption, at a collective cost of $20 billion per year.
Top-performing Energy Star windows are increasingly common. In fact, they are really considered “the cost of admission” for quality construction of any kind. In fact, there is a push to accept only the upper reaches of Energy Star performance for benefits like tax credits. For instance, the 2010 federal tax rebate for windows didn’t even accept all Energy Star windows, but only an elite performance category within Energy Star.
To understand Energy Star windows, you must learn how to read the black-and-white National Fenestration Rating Council (NFRC) label that will be on every Energy Star window, declaring the window’s performance in a number of categories. Keep in mind that Energy Star rates windows on just two of these performance categories; the U-Factor and the Solar Heat Gain Coefficient. Following are descriptions of the important aspects of that NFRC label, as well as what to look for in target values.
U-Factor tells you how well the window insulates. The lower the U-factor, the better the unit insulates. U-factor values generally range from 0.25 to 1.25. In areas where you spend most of your energy dollars on heat, the U-factor is the number you should pay attention to.
Solar Heat Gain Coefficient tells how well the window blocks heat from sunlight. SHGC is measured on a scale of 0 to 1; values typically range from 0.25 to 0.80. The lower the SHGC, the less solar heat the window transmits. In areas where homeowners spend most of their energy dollars on cooling their homes, the SHGC number is the one that is most important.
Target values for U-Factor and Solar Heat Gain Coefficient? Look for a U-Factor of .30 or less and Solar Heat Gain Coefficient of .30 or less. This is the so-called “30/30” rule, and it was the minimum of what the 2010 Federal tax rebate required. Other important parts of the NFRC Label include:
Visible Transmittance measures the amount of light the window lets through. The higher the number, the more light gets in. VT is measured on a scale of 0 to 1.
Air Leakage measures the rate air leaks through the window joints. The lower the AL value, the less air leakage. Most industry standards and building codes require an AL of 0.3 cfm/sq. ft.
With hurricanes and tornadoes increasingly in the news, other notable values covered by the NFRC label are the DP or design pressure, which rates the window’s ability to resist wind. A DP9 (which is very low) is rated to be resistant to a 60-mph wind, whereas a DP15 is rated to resist an 80-mph wind. The NFRC label also will indicate whether or not the glass is impact-resistant, like the glass in your car windshield, where a plastic film is placed between the panes. In some windows, this same film can also block damaging UV light.
For more information about windows than your could read in a lifetime, visit NFRC.org or search “windows” at EPA.gov.