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Windows sometimes seem to be necessary evils. Although occupants demand daylight and views to the outside, windows may lose up to 20 times the energy the rest of the wall does. But significant improvements have been made in window technology and design that allow building owners to have all the advantages of windows, such as daylight and visibility, while reducing both heating and cooling loads as well as the electricity needed for lighting.
Windows face three energy challenges: the difference between external and internal temperatures, air infiltration, and the sun.
These energy challenges essentially come down to keeping heat inside or outside of the building. Understanding how windows meet those challenges means understanding some key window ratings.
A good place to start is the U-factor of a window. That’s a major consideration for any building where a heating system is required to make building occupants comfortable, says Ray McGowan, manager of technical services for the National Fenestration Rating Council (NFRC), the organization responsible for giving windows their energy and optical performance ratings.
The U-factor represents a measure of heat loss through the entire window system, including the frame. The lower the number, the higher the insulating value of glass.
U-factor is the inverse of another widely used measure, R-value. With R-values, a higher number is better.
Glass itself is a very poor insulator, and a single pane is given the base U-factor rating of 1.0. The addition of another pane and a space between the panes of glass improves the U-value of the windows to 0.5 at the center of the glass. Adding an inert gas, such as argon or krypton, either of which is a better insulator than air, doubles the insulating value of the window to 0.25.
Insulating ability is also affected by the frame material and whether it has thermal breaks built in. Aluminum frames without thermal breaks will decrease the insulating value of the window and carry a U-factor for a double-pane glass window system from 0.25 to 0.59, according to the Efficient Windows Collaborative. With the same window, thermal breaks increase the insulating value to a U-factor of 0.47. Using non-metal frames such as wood, fiberglass or vinyl and the same glazing technology further improves the U-factor.
Because the frame can have a big impact on the U-factor ratings of a whole dual-pane gas-infill insulated window system, the U-factor can range from 0.59 for the thermally unbroken aluminum frames to 0.26 for fiberglass window frames, according to the Efficient Windows Collaborative.
Of the three energy challenges, air infiltration is the easiest to contend with because nearly all new windows are built tight. “While infiltration is a problem with many older windows, most new windows, if installed correctly, shouldn’t leak,” says John Carmody, director for the Center for Sustainable Building Research at the University of Minnesota.
In large office buildings that have lots of glazing and primarily a cooling load, building owners aren’t concerned about the loss of heat through windows. The big issue is heat gain from the sun, even in the winter in northern parts of the country, says McGowan.
Gauging a window’s ability to control sunlight is possible by paying close attention to the solar heat gain coefficient (SHGC) and visible light transmission (VT) rating.
An SHGC rating on windows is represented by a number between 0 and 1. The lower a window’s solar heat-gain coefficient, the less solar heat it transmits. It is most effectively paired with the VT rating, which uses a fraction between 0 and 1 to indicate the percentage of the visible spectrum of light transmitted through the glass. The lower the number, the less light that is transmitted through the glass. A good window system will have a low SHGC and a high VT.
Any glass will have some insulating value and will restrict some visible light. A typical clear glass single pane window can have an SHGC of 0.82 and a VT of 0.88, according to the Efficient Windows Collaborative. Add a bronze or gray tint to the window system and it will reduce SHGC to 0.65 and VT to 0.56. A VT that low is cutting half of the light coming into a space and will appear quite dark, says Carmody. Ideally, VT numbers should be above 0.60 to 0.80.
Uses of new tints have dramatically improved SHGC and VT ratings of windows. For instance, windows with a light blue or light green tint can reduce solar heat gain, like the bronze tints, but raise the amount of visible light by absorbing less of the visible spectrum of light than bronze or gray tints do. Barbara Erwine, senior consultant with Paladino and Company, says green and blue tints are often so slight they have a negligible effect on the view from inside. If a clear pane of glass were put next to it, occupants would readily discern the difference. But, Carmody says, humans make adjustments so light coming in a slightly tinted window looks normal.
Despite the fact that low-e coatings have been around for more than a decade, they are still not well understood by most building owners or designers, Erwine says.
“The industry still seems to not understand clearly the range and uses of selective low-e coatings,” she says. “To make sure owners get the performance they expect from windows they should specify the exact performance required, not just a generic low-e coating.”
For instance, if it is important to control solar heat gain but also to maintain high visibility, those factors will have an impact on the type of low-e coating applied. All low-e coatings are not the same, Erwine ways.
Low-e coatings are often put down in layers. The number and type of layers determines the amount of infrared energy reflected, she says.
Also there are hard-coat and soft-coat low-e coatings. Hard coats are applied when the glass is made and are like glazing on ceramic pottery — very durable, says Stephen Selkowitz, director of the Lawrence Berkeley National Laboratory’s Building Technologies Department. Soft coats are applied after the glass is made and need to be protected within an insulated glass unit.
What’s often heard is that soft coat low-e provides a greater range of spectral selectivity and hard coat low-e generally offers higher U-factor protection. While that is true, McGowan says, people often make too big a deal of this.
“Hard coats are more useful for buildings in the north,” he says. “The soft coats generally offer better SHGC and are more appropriate for buildings in the south. But people like to wrestle with the physics of the window when it’s really just the numbers that are important and not the nature of the coating.”
Selkowitz agrees. “It’s better to stick to the properties of the windows and go by the number rather than worry about which coating is applied,” he says. “It’s possible, for instance, that a manufacturer of a frame can get the performance specifications wanted by the owner from either type of coating. Sometimes it’s better and cheaper for the manufacturer to make that decision so long as the performance the owner wants is there.”
The best thing is for the owner to stay involved in the specifications of window systems, and not just because they are large capital expenses, he says.
“Windows are critical to the energy, comfort and aesthetic performance of the building,” Selkowitz says. “Because the same numbers can be used to describe different windows and because there are different ways to achieve energy performance using tints or low-e coatings or a combination, it’s important to stay on top of the process. That way the owner won’t be surprised when the project is done.”
The Bottom Line
By careful specification of window systems, facility executives can resolve many of the conflicts that used to plague windows.
— David Kozlowski