by Dariush Arasteh and Peter Lyons
A prospective window buyer may be convinced to buy energy-efficient products on the basis of the energy saved, but the comfort provided by high-efficiency windows is an even more persuasive selling point. The increased thermal comfort offered is a benefit purchasers can enjoy right away. By understanding how windows influence thermal comfort, window manufacturers and distributors can meet clients needs for comfortable spaces.
Windows affect thermal comfort in several ways.
When its cold outside, a windows interior surface temperature drops. How far it drops depends on the insulating value of the window. The surface temperature of single-glazing, for example, will be very close to outdoor temperatures. The interior surface temperature of double-glazing will be much warmer but still significantly lower than room air temperature. A high-performance window with a low-emissivity (low-E) coating and argon gas filling will have an even warmer interior surface temperature. Superwindows with multiple low-E coatings and gas filling will have surface temperatures very close to the indoor air temperature. Frames, which can make up ten to 30 percent of the area of a typical window, also have noticeable effects; surface temperatures of insulating frames will be much warmer than those of highly conductive frames.
Warmer glass surface temperatures translate into more comfortable spaces for occupants during the winter because comfort is a function of radiant heat transfer among people and their surroundings. If people are exposed to the effects of a cold surface, they experience significant radiant heat loss to that cold surface, so they feel uncomfortable. People feel uncomfortable even when only partially exposed to a cold surface (radiant asymmetry). A familiar example of radiant asymmetry is what we experience when sitting around a campfire on a winter night. The side of our bodies facing the fire is hot while the side facing away is cold.
Cold surfaces also create convective currents or drafts in a room. (Many people mistake these drafts for infiltration through the window.) Both radiant heat loss and convective currents from cold window surfaces cause people to turn up thermostats.
During the summer, the interior surface temperatures of tinted glass, clear glass and clear glass with tinted film can get hot. Some tinted glass surfaces get as warm as 140 degrees F. These surfaces radiate heat to building occupants and also convect heat (create warm air that travels) to occupants.
Sunlight traveling through glass and striking occupants will also make them feel hot. Sunlight striking room surfaces heats up the space, further contributing to occupant discomfort. Just as people turn up the heat in response to cold window surfaces in winter, they may use air conditioning to counter the effects of warm window surfaces and sunlight in summer. But, if air conditioners are not sized or installed properly, some areas of a room may become comfortable while others will not.
Window manufacturers and distributors can respond to both winter and summer discomfort by offering high-efficiency windows. Windows with a spectrally selective low-E coating help prevent thermal discomfort in warm and cold temperatures (see figure two). These windows transmit only visible sunlight while reflecting all the invisible heat associated with it. Uncoated clear and many tinted glass products (bronze, gray, but not blue or green) transmit this heat. With spectrally-selective low-E products, occupants will feel less need to close blinds and drapes to maintain comfort. Because these products insulate effectively, they will also help prevent cold window surfaces in winter.
It is better to install efficient windows than to rely on heating, ventilation and air conditioning (HVAC) systems to solve thermal comfort problems. HVAC systems not only create non-uniform interior conditions, only partially relieving thermal discomfort, they also break down and may be unavailable on days of peak electricity demand.
Typically, thermal comfort experiments are conducted on a large number of human subjects who report their comfort levels under widely varying conditions in a lab set up to represent a room or office. Much of this research has been incorporated into the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) Standard 55. Research in progress at the Ernest Orlando Lawrence Berkeley National Laboratory in Berkeley, CA, in collaboration with the Center for Environmental Design Research at the University of California at Berkeley, aims to quantify window comfort information for use by the window industry.
However you look at it, efficient windows mean more comfortable spaces. With construction costs often well over $100 per square foot, its worth it to include efficient windows so that occupants can be comfortable in their living and work spaces.
Dariush Arasteh is a staff scientist for Lawrence Berkeley National Laboratory in Berkeley, CA. He has worked on issues relating to energy efficiency in windows for 15 years.
Peter Lyons, Ph.D., is a building physicist with Energy Partners in Canberra, Australia. He has worked on energy rating systems for windows for five years.
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