If you're in the market for new home windows, you're probably hearing a lot about window performance. And if you're not well-versed in window technologies, you may be wondering just what that means. In a nutshell, performance ratings measure a window's energy efficiency. They tell you how well a certain window will regulate the flow of heat, light and air entering and escaping your home.
To understand window performance, you should be looking for two important labels: the ENERGY STAR label and the NFRC certification label. The ENERGY STAR label proves that a window has been independently certified and verified to perform at levels that meet or exceed energy efficiency guidelines set by the U.S. Environmental Protection Agency. The ENERGY ZONE climate zone map will help you choose windows specifically suited to perform in the type of climate in which you live.
The NFRC certification label verifies conformance with standards set by the National Fenestration Rating Council — a nonprofit organization that independently assesses and issues ratings for window products. NFRC ratings provide detailed information regarding a window's critical performance factors, expressed in four values:
U-factor: A U-factor, which ranges from 0.20 and 1.20, tells you how well a window will keep heat from escaping the inside of a room. The lower the number, the better the heat retention. A window with a U-factor of 0.15, for example, offers better insulating properties than a window with a U-factor of 0.30.
Solar Heat Gain Coefficient (SHGC): The Solar Heat Gain Coefficient, which ranges from 0 to 1, measures how well a window can resist unwanted solar heat gain (i.e., heat generated from sunlight coming into the window). The lower the number, the less you'll spend on cooling in warmer temperatures. A window with an SHGC of 0.35, for example, will allow more heat to pass through the window than a product with an SHGC of 0.27.
Visible transmittance (VT): Visible Transmittance, which ranges from 0 to 1, measures how well a window will light your home with daylight. The higher the number, the more light is transmitted. So, a window with VT rating of 0.50 will allow less light into your home than a window with a VT rating of 0.75.
Air Leakage: The Air Leakage rating, which ranges from 0.1 to 0.3, indicates how much air may be allowed through cracks in the window assembly. The lower the Air Leakage rating, the less air will pass through the cracks.
Condensation: You may also see a condensation rating on the NFRC label. This rating, which ranges from 0 to 100, is optional for manufacturers to include. The higher this number, the better the window resists condensation.
Alone or combined, certain window technologies can lower U-factor or SHGC values to increase energy efficiency. Low-emissivity (Low-E) glass coatings reduce both the amount of heat and the amount of UV radiation that allowed to pass through the window glass, for example. And double- or triple-pane configurations contain a layer (or layers) of air or inert gas that is sealed between the inner and outer panes to insulate the window and slow the passage of heat.
Ultimately, high-performing windows offer a number of benefits, including reduced energy costs and fabric fading, increased natural light, and improved insulation and comfort throughout your home.
Megan Linhoff is a reporter for Angie's List, a trusted provider of local consumer reviews and an online marketplace of services from top-rated providers. Visit AngiesList.com.