What Does the Fraction Mean in Window Shades?

Definition of Fraction

• Energy from the sun passes through a window. A shade or other window covering reflects some of that energy back through the window again to the outside. The total amount of the sun's energy that enters equals 1. The amount that remains to heat the room is called the fraction. It is a number larger than zero but less than one. This fraction of retained heat is usually expressed in the form of a decimal, called the solar heat gain coefficient or SHGC. A lower decimal means less heat remains in the room.

The Fraction for Various Shades

• Shades come in a variety of types, from lighter and more translucent to darker and more opaque. The American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc., tested blinds of three types. The actual measured solar heat gain coefficient for light opaque shades came to 0.34, while that of light translucent shades came to 0.38. The dark opaque shades had a measured coefficient of 0.70. Choose darker shades, therefore, to keep more heat in the room. Choose lighter shades to reflect more heat outdoors.

Considerations for Use

• The U.S. Department of Energy gives advice for placement and use of interior window shades for the best energy savings. Avoid air leakage by mounting the shades as close as possible to the window and with a minimal gap at the sides. In winter, keep shades up in the daytime on windows with southern exposure. Keep those shades down at night. In summer, keep sunlight out by covering sunny windows with lowered shades. To profit from the rated fraction or solar heat gain energy coefficient, the shades must be fully down.

Special Shades

• Choosing special shades also increases potential energy savings. For maximum flexibility, choose reversible shades with one light and one dark side. Keep the light side facing outside in summer and inside in winter. When the light side faces the window, more heat will go back outdoors. When the dark side faces the window, more heat will stay in the room. Pleated and cellular shades also provide extra insulation by trapping air within their cells.

Advantages Over Other Methods

• Shades, venetian blinds and similar window coverings have several advantages over other methods of controlling energy loss. Because shades are adjustable, the amount of heat they allow in can be changed at will. Window glazing is effective at blocking heat, but you can't change it with the weather. Although scientists have developed window tinting that adjusts with the sun, it is not yet completely satisfactory or affordable. Outside shutters and shades successfully control heat and are adjustable. However, they often cost more than indoor shades and have a shorter useful life because of weather exposure.