While the energy efficiency of windows is highly technical, shopping for high-performance windows is made much easier by following standardized window performance measurements determined by the National Fenestration Rating Council and shown on a label affixed to most new window units.
U-value (also known as U-factor) is the rate of heat transfer expressed as a decimal number from 0 to 1. Lower numbers mean less heat transfer. A single pane of glass is about U-1.0. A typical double-glazed window would be around U-0.40, and triple-glazed windows would be around U-0.20. The reported U-value takes into account both the glazing and the frame. Solar heat gain coefficient (SHGC) tells how much of the sun’s heat is allowed to penetrate the glass. SHGC is also expressed as a decimal from 0 to 1. Lower numbers mean less solar heat gain. Visible transmittance (VT) indicates how well you can see through the glass. These numbers also range from 0 to 1, with higher numbers meaning better visibility. Air leakage (AL) shows how well the window seals against drafts around the operable sash. These numbers are expressed in cubic feet per minute (CFM). Again, lower numbers mean less leakage and better performance.
These four metrics should guide your window selection. Insist that your window supplier indicate these values on the estimate.
Window technology is improving rapidly. Select the most cost-effective windows with a heat loss rate of U-0.25 or lower. In some situations and climates, this may be advanced double-glazed windows, while other situations may call for triple-glazed windows. Affordable triple-pane windows are beginning to appear on the market with heat loss rates as low as U-0.10. Let window specifications and your energy model be your guide.
Low U-values are achieved using three important elements:
Consider triple pane, inert gas-filled windows with approximately 0.20 U-value, such as Harvey Tribute, Thermotech, Atrium Northwest, Cascade, Solar View, Intus, Milgard or Alpen. Some double-pane windows achieve U-values close to 0.22 and offer a good balance of high performance and lower cost.
More heat flows through the frame than the insulated glass unit. This has two implications. First, look for windows that have the smallest frame profile, because the frame has a higher thermal transmission than the glazing. Second, it is more energy efficient to use fewer, larger windows with the same glazing area than more, smaller windows, because larger windows have a higher glass to frame ratio.
Operable windows can be strategically placed to allow for optimal summer cross ventilation, which will reduce the warm-weather cooling load. Make use of the “stack effect” to ventilate and cool the building, by placing operable windows near high points to allow warmer air to escape and near low points to draw cooler air in.
All operable windows should be casements or awnings where possible because they seal better than sliders or single-hung windows. Select fixed windows for locations not specifically designed for natural ventilation. Fixed windows, followed by casement and awning windows, are the most airtight, and have better U-value compared to similar sliding or single hung windows. Fixed windows also cost less than casements.
Window area is a key factor in overall building heat loss. Even U-0.20 (R-5) windows will lose heat up to seven times faster than the walls. One way to express window area relative to the size of the house is by calculating the ratio of the window-to-floor area (WFA). The average WFA is about 18% to 22% in production homes and 30% to 40% in custom homes. Utilizing energy modeling, you can reduce the WFA to the 16% to 18% range, which is about optimum for minimizing heat loss. High performance walls are more airtight, and better insulated than any triple-glazed window, and because walls are far less expensive than windows, reducing window area also saves money.
With fewer windows, take care to place them for optimal light, summer ventilation, southern exposure for solar heat gain, and for views. With good design zero energy homes can be just as livable with lower WFAs as standard homes with higher WFAs..
Where possible, place 50% to 60% of the window area facing south. Most of the common living area should be on the south side of the floor plan where it receives light and heat from the sun. Unless shaded in some way, windows facing east and west tend to gain too much heat when it isn’t wanted, even in northern climates.
Ideally, windows facing south should have a high solar heat gain coefficient to gather valuable heat in the winter. Windows facing east and west would have low solar heat gain to reject heat during summer. Experts have suggested that designers specify the SHGC accordingly, and this makes sense. However, be sure to conduct an energy model to make sure the additional window value is worth the potentially higher costs and hassle.
Design a solar shading strategy that allows the sun to heat the building when needed and reject heat to avoid overheating. Fixed overhangs must be a compromise between similar sun angles in spring and fall when the heating or cooling requirements are much different. Consider using a shorter fixed overhang of 12 to 18 inches along with moveable shading, such as awnings, sun screens, or vegetation. This will allow greater heat gain during spring and less heat gain during fall. Calculate the roof overhang using the handy Susdesign website.
It is difficult to determine whether or not insulated shades are cost-effective, as manufacturers’ data on their functional U-value has been scarce to date. Hunter Douglas has claimed that their Duette shades have an R-value of about 4.0, which could significantly increase the R-value of the windows, especially at critical times of heat and cold. Look for the new labeling system from the window covering industry that clarifies the insulating value of specific products. In order to function properly, insulating window shades must fit tightly to the window opening to minimize air mixing with the room, although making them too tight-fitting may also make them difficult to operate. While insulated shades definitely resist heat loss, they require daily operation, making it difficult to quantify potential benefits. Visit Energy Savers for more information on energy efficient window treatments.
Insulated fiberglass doors are currently the best option for the price and are the most cost effective. Be sure that they have tight gaskets and air seals. The glazing on the door can be a source of air leakage and should be checked during the blower door test. If possible minimize glazing on doors, specify that they have high R-value glass, and verify that there are no air leaks around the glass. Consider a multi-point latching mechanism that will hold the door tight against the weatherstripping. This is especially valuable for doors that are exposed to sun and weather.
Where possible, place 50% to 60% of the window area facing south. Most of the common living area should be on the south side where it receives light and heat from the sun. Unless shaded in some way, windows facing east and west tend to gain too much heat when it isn’t wanted, even in northern climates.
Ideally, windows facing south would have high solar heat gain to gather valuable heat in the winter. Windows facing east and west would have low solar heat gain to reject heat during summer. Experts have suggested that designers specify the SHGC accordingly, and this makes sense. However, be sure to conduct an energy model to make sure the additional window value is worth the potentially higher costs and hassle.
Design a solar shading strategy that allows sun to heat the building when needed and reject heat to avoid overheating. Fixed overhangs must be a compromise between similar sun angles in spring and fall when the heating or cooling requirements are much different. Consider using a shorter fixed overhang of 12 to 18 inches along with moveable shading, such as awnings, sun screens, or vegetation. This will allow greater heat gain during spring and less heat gain during fall. Calculate the roof overhang using the handy Susdesign website.
It is difficult to determine whether or not insulated shades are cost-effective, as manufacturers’ data on their functional U-value is scarce to date. Hunter Douglas has claimed that their Archetella shades have an R-value of about 4.0, which could significantly increase the R-value of the windows, especially at critical times of heat and cold. In order to function properly, insulating window shades must fit tightly to the window opening to minimize air mixing with the room, although making them too tight-fitting may also make them difficult to operate. While insulated shades definitely resist heat loss, they require daily operation, making it difficult to quantify potential benefits. Visit Energy Savers for more information on energy efficient window treatments.
Insulated fiberglass doors are currently the best option for the price and are the most cost effective. Be sure that they have tight gaskets and air seals. The glazing on the door can be a source of air leakage and should be checked during the blower door test. If possible minimize glazing on doors, specify that they have high R-value glass, and verify that there are no air leaks around the glass. Consider a multi-point latching mechanism that will hold the door tight against the weatherstripping. This is especially valuable for doors that are exposed to sun and weather.
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