PERFORMANCE: Human Factors
Energy & Cost
Codes & Standards
Thermal comfort is determined by air temperature, relative humidity, air movement, mean radiant temperature, the presence of direct solar radiation, 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. 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. Though drafts can be a result of leaky windows, often drafts 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. 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.
Direct sun has fairly obvious impacts on thermal comfort. During warm weather, solar radiation can cause discomfort. 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 cold weather, they may use air-conditioning to counter the effects of warm window surfaces and sunlight in summer.
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.
Summer Glass Temperatures (°F)
There are hundreds of glazing systems available in the market today, with varying combinations of glass panes, special coatings, and tints. The Facade Design Tool models the performance of 10 glazing systems and two retrofit films, representative of the breadth of options available. The attributes of these glazing systems is described in the table and chart below.
For ease of comparing the performance of glass features, all high-performance glazing systems in the Facade Design Tool are modeled with an argon fill. In general, energy performance from similar windows with an air fill will be about 2-5% poorer.
*Whole window properties are based on an NFRC standard test size and simulated in WINDOW6 with frame specified per each glazing system.