Windows typically occupy about 15 to 20 percent 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 Lesson 5 that simple glass (1/8th 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
Instructions: Click the play button to see how heat loss occurs through a window.
Click here to open a text description of the Heat Loss Through Windows activity
Heat Loss through Windows
Warm inside air gives up energy through the window and becomes cold, sinking to the bottom of the room. Cold outside air hits the window pane, picks up energy, and then warm air rises. This is 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.
Here is a graph that compares heating costs for different types of windows. The figures are based on a typical home in Boston, MA, a relatively heating intensive place.
|Window Type||Heating Cost ($)|
|Single Pane, Clear Glass, Aluminum Frame||875|
|Single Pane, Tinted, Aluminum Frame||650|
|Double Pane, Clear Glass, Wood/Vinyl Frame||600|
|Double Pane, Clear Glass, Low SHGC, Low -e Coating, Wood/Vinyl Frame||575|
Similarly, poor windows allow the solar energy to penetrate through the windows and heat the space. The incoming solar radiation consists of infrared (IR), ultraviolet (UV), and visible waves.
The IR radiation, which is also called heat radiation, heats the space excessively and adds to the air conditioning in the summer time. Therefore, energy efficient windows are critical in summer time or even in places where the cooling requirement is high.
Instructions: Click the “play” button to observe the effects of solar energy on windows.
Click here to open a text description of the effects of solar energy on windows activity
Effects of Solar Energy on Windows
A cross-sectional view of a room has a window on the left wall and an air conditioner on the right wall. The sunlight coming through the window heats the room until the temperature around the air conditioner rises. When it does, the air conditioner turns on and cools the room by moving the heat back outside. The process repeats itself as long as the sun heats the room through the window.
Here is a graph that compares cooling costs for different types of windows. The figures are based on a typical home in Phoenix, AZ.
|Window Type||Cooling Cost ($)|
|Single Pane, Clear Glass, Aluminum Frame||800|
|Single Pane, Tinted, Aluminum Frame||750|
|Double Pane, Clear Glass, Wood/Vinyl Frame||675|
|Double Pane, Clear Glass, Low SHGC, Low -e Coating, Wood/Vinyl Frame||550|