Out of all of the dual pane window choices on the market today it’s hard to tell which is the best. One of the biggest factors to find out about the dual pane window you select is it’s insulation level.
If your window needs to be replaced, now is the time to select a window that is Energy Star qualified, will lower your energy bill 7 to 15 percent, enhance the look of your home and reduce noise.
When shopping for your window replacement be sure to compare the energy performance rating. ENERGY STAR® has made energy performance ratings based on climate. Energy.gov lists out some methods for selecting the right ENERGY STAR® rating for your window;
The basic window guidelines are to select, orient, and size glass to maximize solar heat gain in winter and minimize it in summer.
In heating-dominated climates, major glazing areas should generally face south to collect solar heat during the winter when the sun is low in the sky. In the summer, when the sun is high overhead, overhangs or other shading devices prevent excessive heat gain.
To be effective, south-facing windows should have a solar heat gain coefficient (SHGC) of greater than 0.6 to maximize solar heat gain during the winter, a U-factor of 0.35 or less to reduce conductive heat transfer, and a high visible transmittance (VT) for good visible light transfer. See Energy Performance Ratings below to learn more about these ratings.
Windows on east, west, and north-facing walls should be minimized while still allowing for adequate daylight. It is difficult to control heat and light through east and west-facing windows when the sun is low in the sky, and these windows should have a low SHGC and/or be shaded. North-facing windows collect little solar heat, so they are used only for lighting. Low-emissivity (low-e) window glazing can help control solar heat gain and loss in heating climates.
In cooling climates, particularly effective strategies include preferential use of north-facing windows and generously shaded south-facing windows. Windows with low SHGCs are more effective at reducing cooling loads.
Some types of glazing help reduce solar heat gain, lowering a window’s SHGC. Low-e coatings—microscopically thin, virtually invisible metal or metallic oxide layers deposited directly on the surface of glass—control heat transfer through windows with insulated glazing. Tinted glass absorbs a large fraction of incoming solar radiation through a window, reflective coatings reduce the transmission of solar radiation, and spectrally selective coatings filter out 40% to 70% of the heat normally transmitted through insulated window glass or glazing, while allowing the full amount of light to be transmitted. Except for spectrally selective, these types of glazing also lower a window’s VT. See Window Types to learn more about glazing, coatings, tints, and other options when selecting efficient windows.
Some of the top energy efficient windows include Andersen, Pella, Jeld-Wen, ReliaBilt, Milgard, TAFCO and American Craftsman.
When shopping for a double pane window compare the R-Value and SHGC of the choices available.
Energy Performance 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
- Air leakage through and around them.
These properties can be measured and rated according to the following energy performance characteristics:
The R-value of a window measures its ability to resist heat flow. A window with a higher R-value is more effective at blocking heat transfer than one with a low R-value. To keep your home warm during the winter and keep expensive heat from escaping through the window panes, look for units with the highest R-value possible. Double-paned windows have an R-value ranging from 3 to 3.7, according to the Arizona Public Service Co., while triple-paned windows have an R-value of up to 5, making them more effective at blocking heat flow. This means that replacing double-paned units with triple-paned windows could reduce heating costs. It also means that homes with triple-paned windows will stay warmer during winter power outages because less heat will escape through the windows.
Compare Solar Heat Gain
In addition to R-value, it’s also important to understand how well a window blocks solar heat gain. This information is revealed by the solar heat gain coefficient (SHGC), which ranges from 0 to 1. A low SHGC means that the window does an excellent job blocking heat-producing solar rays, while a high SHGC means more solar heat gain into the home. In northern climates, a high SHGC may be desirable because it helps keep the home warmer. Homeowners in southern climates should look for windows with a low SHGC because it helps to keep cooling bills lower during the hot summer months. In general, triple-paned windows have a lower SHGC than double-paned units. Check the specs on each unit while shopping for windows to verify SHGC.
The 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 window, door, or skylight.
Solar heat gain coefficient (SHGC)
The SHGC is the fraction of solar radiation admitted through a window, door, or skylight — 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. For more information about SHGC and windows, see passive solar window design.
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 than one with a high air leakage rating.
The ability of glazing in a window, door, or skylight to transmit sunlight into a home can be measured and rated according to the following energy performance characteristics:
Visible transmittance (VT)
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. A product with a higher VT transmits more visible light. VT is expressed as a number between 0 and 1. The VT you need for a window, door, or skylight should be determined by your home’s daylighting requirements and/or whether you need to reduce interior glare in a space.
Light-to-solar gain (LSG)
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.