It’s fun to shop for new windows in Sacramento, imagining how the variety of frames will improve your house’s curb appeal. But your contractor or sales team will also want to talk energy-efficiency ratings—terms that can leave you feeling like an outsider looking in.
At Hall’s Window Center, we’re happy to walk you through the choices and terms, but here’s a cheat sheet from the Department of Energy in case you want to brush up ahead of time.
Energy Performance Testing, Certification, and Labeling
The National Fenestration Rating Council (NFRC) operates a voluntary program that tests, certifies and labels windows, doors and skylights based on their energy performance ratings. The NFRC label provides a reliable way to compare products.
The NFRC label can be found on all ENERGY STAR®-qualified windows, doors and skylights, but ENERGY STAR bases its qualification only on U-factor and solar heat gain coefficient ratings:
Heat Gain and Loss
Windows, doors and skylights can gain and lose heat through
• direct conduction through the glass or glazing, frame, and/or door,
• the radiation of heat into a house (typically from the sun) and out of a house from room-temperature objects, such as people, furniture, and interior walls, and
• air leakage through and around them.
We measure and rate these properties according to these energy performance characteristics:
• U-factor is the rate at which a window, door or skylight conducts non-solar heat flow. It’s usually expressed in units of Btu/hr-ft2-oF. For windows, skylights and glass doors, a U-factor may refer to just the glass or glazing alone. NFRC U-factor ratings, however, represent the entire window performance, including frame and spacer material. The lower the U-factor, the more energy-efficient the product.
• Solar heat gain coefficient (SHGC) is the fraction of solar radiation admitted through a window—either transmitted directly and/or absorbed, and subsequently released as heat inside a home. The lower the SHGC, the less solar heat it transmits and the greater its shading ability. A product with a high SHGC rating is more effective at collecting solar heat during the winter. A product with a low SHGC rating is more effective at reducing cooling loads during the summer by blocking heat gain from the sun. Your home’s climate, orientation and external shading will determine the optimal SHGC for a particular window, door, or skylight.
• Air leakage is the rate of air movement around a window, door or skylight in the presence of a specific pressure difference across it. It’s expressed in units of cubic feet per minute per square foot of frame area (cfm/ft2). A product with a low air leakage rating is tighter.
The ability of glazing to transmit sunlight into a home can be measured and rated according to these energy performance characteristics:
• Visible transmittance (VT) is a fraction of the visible spectrum of sunlight (380 to 720 nanometers), weighted by the sensitivity of the human eye, that is transmitted through the glazing of a window, door or skylight. VT is expressed as a number between 0 and 1; a product with a higher VT transmits more visible light. The VT you need for a window, door, or skylight should be determined by your home’s day lighting requirements and/or whether you need to reduce interior glare in a space.
• Light-to-solar gain (LSG) is the ratio between the SHGC and VT. It provides a gauge of the relative efficiency of different glass or glazing types in transmitting daylight while blocking heat gains. The higher the number, the more light transmitted without adding excessive amounts of heat. This energy performance rating isn’t always provided.
Drop in at Hall’s Window Center showroom at 11297 White Rock Road, Rancho Cordova, CA 95742 any time and we’ll be glad to translate what these terms mean specifically to your home.