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Window Coverings

Window coverings include shades, blinds and curtains. Shades can be internal (probably the most common) or external. External shades are more effective if the shades are being used in a cooling-dominated climate to keep solar heat gain out the house. Internal shades will help, but external shades keep any radiation from entering the house. Window coverings can be very effective at cutting the cooling loads on a house during the summer, which generally will outweigh the additional lighting load that will likely be needed to compensate for the lack of natural light.

Some new types of windows are able to achieve the same shading effects as window coverings without the coverings. Electrochromic windows contain material that darkens when an electric potential is applied. A control system could send the signal to tint the windows based on a schedule or based on logic tied to lighting sensors. While this is still a new and expensive technology, the emergence of home control systems makes this a promising future technology.

A picture of electrochromic windows is shown. In this photo, the tinting is turned off and the windows look like ordinary windows. A second picture of electrochromic windows is shown. The electrochromic tinting is fully turned on and only a small amount of light can make it through.

Electrochromic windows without any tint
Credit: NREL PIX 18736

Electrochromic windows with full tint
Credit: NREL PIX 18753

Other types of advanced windows include photochromic windows and thermochromic windows. Photochromic windows dim in response to light. This technology is found in glasses that tint in bright sunlight and become clear indoors. However, if used in a home, photochromic windows would tint on a bright, but cold winter day and would eliminate the natural heating that helps to offset heating costs in the winter time. Thermochromic windows tint as a response to heat, which is a better metric for when tinting would be appropriate. In previous example, a sunny and cold winter day would not trigger tinting, but the hot afternoon sun in the summer would cause the windows to tint. The heat required to cause tinting is high enough that only direct sunlight will cause tinting, which means that east-facing windows would tint in the morning, south-facing in the middle of the day and west-facing in the afternoon. This lack of control over the tinting means that thermochromic windows may be more accepted in a commercial building setting, rather than a residential setting.