10.1: Windows and Heat Loss

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Windows typically occupy about 15 to 20% of the surface area of the walls. Windows not only add aesthetic looks and often a very important aspect of a home, but also a very significant component of home heating and cooling costs. Windows lose more heat per square foot of area in winter and gain more heat in summer than any other surface in the home.

We already discussed in Chapter 5 that simple glass (1/8 inch) has a very low R-value (0.03). So even if the walls are well insulated to an R-value of about 13 to 19 and the windows have poor R-value, most of the heat escapes through the windows and the purpose of having a well insulated wall is lost.

It is estimated that in 1990 alone, the energy used to offset unwanted heat losses and gains through windows in residential and commercial buildings cost the United States $20 billion (one-fourth of all the energy used for space heating and cooling). However, when properly selected and installed, windows can help minimize a home's heating, cooling, and lighting costs.

Heat Loss through Windows

The following video shows how heat loss occurs through a window.

Although energy is spent heating the air in the room, windows can make the temperatures uncomfortable. However, by making the windows efficient, a significant amount of the energy and money can be saved.

Figure 10.1.1 compares heating costs for different types of windows. The numbers are based on a typical home in Boston, MA, a relatively heating intensive place.

Figure 10.1.1. Heating costs for different window types

Similarly, poor windows allow the solar energy to penetrate through the windows and heat the space. The incoming solar radiation consists of infrared (IR), ultraviolet (UV), and visible waves.

The IR radiation, which is also called heat radiation, heats the space excessively and adds to the air conditioning in the summer time. Therefore, energy efficient windows are critical in summer time or even in places where the cooling requirement is high.

The following video explains the effects of solar energy on windows.

Figure 10.1.2 compares cooling costs for different types of windows. The numbers are based on a typical home in Phoenix, AZ.

Figure 10.1.2. Cooling costs for different window types