Issues in Window Selection: Human Factors
Daylight | Glare | View | Thermal Comfort
Thermal Comfort
Thermal comfort is determined by air temperature, relative humidity, air movement, mean radiant temperature, the presence of direct solar radiation (insolation), and occupants' clothing and activity levels. Windows affect human comfort in several ways. During cold periods, exterior temperatures drive interior glass surface temperatures down below the room air temperature; how low the glass temperature drops depends on the window's insulating quality. If people are exposed to the effects of a cold surface, they can experience significant radiant heat loss to that cold surface and they feel uncomfortable, even if the room air temperature is comfortable (Figure 2-43). The closer they are to a window, the more they will feel its influence. The fact that this heat loss occurs on one side of the body more than the other is called radiant asymmetry, and this leads to further discomfort. A familiar example of radiant asymmetry is the experience of sitting around a campfire on a winter night. The side of the body facing the fire is hot, while the side facing away is cold. In the case of a cold window, a person may be cold in warm clothes in a 70 degrees Fahrenheit room air temperature if part of the body is losing heat to a cold window.
Drafts near windows are the second major source of winter discomfort. Many people mistakenly attribute drafts to leaky windows when in fact they are the result of cold air patterns initiated by cold window surfaces. Air next to the window is cooled and drops to the floor. It is then replaced by warmer air from the ceiling, which in turn is cooled. This sets up an air movement pattern that feels drafty and accelerates heat loss (Figure 2-45). Cold-temperature-induced drafts occur at the same time as radiant discomfort. This emphasizes the need for insulating windows that maximize interior glass surface temperatures under cold environmental conditions.
Drafts can also be caused by windows with significant air leakage. These leaks can be a result of poor installation and/or ineffective weatherstripping. Such drafts correlate directly to air infiltration levels (see energy-related issues in this chapter). Radiant heat loss, convective currents from cold window surfaces, and drafts from air infiltration leaks all cause people to turn up thermostats during cold periods. Because this action may have little effect on increasing comfort levels, it can be wasteful and costly.
Direct sun has fairly obvious impacts on thermal comfort as well. During cold periods, solar radiation (within limits) can be a pleasant sensation. During warm weather, however, it invariably causes discomfort (Figure 2-46). People often close shades or blinds to block sunlight even though this means they can no longer enjoy the view. Just as people turn up the heat to compensate for cold windows in winter, they may use air-conditioning to counter the effects of warm window surfaces and sunlight in summer. If air conditioners are not sized or installed properly, some areas of a room may become comfortable while others are not, causing significant waste of energy.
The glazing surface temperature increase due to solar radiation depends on the absorptance of the glass and environmental conditions. Typical clear glass windows do not absorb enough solar radiation to cause a significant difference in surface temperature. With tinted glass, surface temperature increases can be significant. While poorly insulated tinted glass may actually feel quite comfortable on a cold sunny day, this practice is not recommended-the comfort consequences on hot summer days can be disastrous. During warm periods, the interior surface temperatures of poorly insulated tinted glass and clear glass with tinted film can get hot, as high as 140 degrees Fahrenheit. These surfaces radiate heat to building occupants and can also create uncomfortable convection currents of warm air.
In this analysis, it is assumed that the two major effects of windows on local thermal discomfort are asymmetric radiation from hot and cold window surfaces and from direct sunlight. Hourly values of predicted percentage dissatisfied (PPD) were computed based on the mean radiant temperature given these two effects, room air temperature, humidity, air speed, and ASHRAE Standard 55-92 values for clothing and activity levels. This definition of PPD is more stringent than that defined for ASHRAE 55-92 because of the inclusion of direct sun effects on local discomfort. Nevertheless, it is assumed in this book that the PPD must be less than 20 percent to comply with the Standard.
Figure 2-47 illustrates the impact of window type on thermal comfort in Chicago. Only Windows A and B exceed the 20 PPD level. Since there can be thermal discomfort during both heating and cooling periods, the PPD is influenced by climate and window properties such as U-factor and SHGC, as well as window orientation, size, and shading conditions.
Operable windows can contribute to improved thermal comfort. People have the opportunity to regulate their own thermal environment and use air movement from natural ventilation to feel cooler at higher temperatures. Even though operable windows may not save significant amounts of energy, the fresh air and sense of control they provide contributes to occupant satisfaction.