For Designers

Comfort

Although energy-efficient windows can make up for their cost premium by savings on energy bills, the increased thermal comfort they offer in winter and summer is a benefit that begins immediately. Thermal comfort of a space directly affects the quality of the space and its usefulness. Monitoring studies on houses with efficient windows invariably elicit comments such as, "I could sit next to the windows on a cold night without having to wear a heavy sweater!"

Thermal comfort is determined by air temperature, humidity, air movement, mean radiant temperature, and the presence of direct solar radiation. Surface temperatures in a room, which determine the mean radiant temperature, can have a big effect on thermal comfort. Even when room air is maintained at a comfortable temperature, occupants may experience significant discomfort as a result of radiant heat exchange with window surfaces. Window surface temperatures fluctuate much more significantly than those from other surfaces in a room. As a result, windows have the potential to "make or break" the thermal comfort of a space. Generally in winter, the more efficient a window is, the less the window surface temperature will deviate from the conditioned room air temperature and the less discomfort an occupant will feel.

Comparing Windows Based on Thermal Comfort
The importance of any measure of thermal comfort must be put into perspective. The climate will determine to what extent either winter or summer comfort will be a priority. In addition, thermal comfort will matter more in situations where window areas are larger and when people will be seated close to the windows.

To enable comparisons between windows, the Center for the Built Environment at University of California Berkeley has proposed a method for determining a Winter and Summer Thermal Comfort Index shown in the table below (Huizenga et al. 2006). The Winter Comfort Index represents the minimum exterior temperature that will provide indoor comfort for a given window. The index is nearly 60°F for single glazing (U=1.02). This means that the window has the potential to be uncomfortable at outdoor temperatures below this level. The index for double glazing (U=0.48) is reduced to 44.2°F and clear triple glazing (U=0.30) is reduced to 28.2°F. Double glazing with either high- or low-solar-gain low-E coatings further reduce the Winter Comfort Index to 20.8°F and 16.7°F. Triple glazed low-E options perform the best with Winter Comfort Indices of -18.4 to -21.5°F meaning that they remain comfortable as long as it is above these subzero temperatures.

The Summer Comfort Index developed by the Center for the Built Environment can be determined two ways, The first approach only includes diffuse solar radiation, assuming a person in direct sunlight would either move or adjust the shades in the room. The second approach includes direct as well as diffuse solar radiation. The diffuse rating is shown and discussed here. As shown in Figure 2-30, the Summer Comfort Index is around 1.00 for clear glazings whether they are single-, double- or triple-glazed units (SHGC=0.60-0.80). Bronze-tinted single-glazing (SHGC=0.62) actually has a worse Summer Comfort Index (1.06) than the clear glazings because of its increased heat absorption and surface temperature. Different types of low-E coatings perform very differently in terms of summer comfort. Double-glazing with a high solar-gain low-E coating (SHGC=0.59) has a Summer Comfort Index of 1.00 while a double-glazed unit with a low-solar-gain low-E coating (SHGC=0.36) has a much lower Summer Comfort Index of 0.53. In triple-glazed units, the high-solar-gain low-E unit (SHGC=0.45) improves to a Summer Comfort Index of 0.82 but is still well above the 0.51 index for low-solar-gain low-E (SHGC=0.31).

 

 

 

 

It is better to install efficient windows than to rely on heating, ventilation, and air-conditioning (HVAC) systems to solve thermal comfort problems. Not only do HVAC systems often create nonuniform interior conditions, only partially relieving thermal discomfort, but power outages on peak days can lead to extremely uncomfortable conditions.

A window with poor energy-performance characteristics will also be an uncomfortable window. A window with good energy-performance characteristics will generally provide greater thermal comfort than a poorer energy performer. Paying more for better-performing windows is often viewed only in relationship to fuel cost savings; however, thermal comfort is a clear benefit and should be considered an additional value worth paying for when selecting windows. There is little point in spending money to improve or add a room that is unusable at times because of thermal comfort problems.