Shandy Heil, Reichel, Claudette Hanks, Attaway, Denise | 1/10/2007 7:43:01 AM
Houses can be designed so that occupants can get the most or least light and/or heat generated by the sun. (Figure 1)
West- and east-facing glass can have nearly five times the solar heat gain of north-facing glass and more than triple that of south-facing glass. Although the amount of radiant heat at west and east exposures is the same, west is most important to protect because it occurs during the hottest time of the day. Design to minimize west and east glass and wall surface, and shade it. Try to place most of the home’s glass area within 20 degrees of due south or north. (Figure 2)
Skylights are not recommended because they receive too much sun and are difficult to shade. Light tubes (domed glass roof fixtures coupled with an insulated reflective tube) are the exception. They can provide a lot of diffuse, reflected light without the heat gain.
Choosing roofing and siding colors with good solar reflectance (25% or more) can result in tangible cooling energy savings. This is particularly true for roofing materials such as light-colored tile and metal, now available with solar reflectances up to 75%. Dark colors generally absorb heat from sunlight, whereas light surfaces reflect heat and reduce cooling costs. A new option is a “cool color” metal roof coating or shingle granule technology that gives darker colors the heat reflectance of light colors.
ENERGY STAR-labeled roofings have a solar reflectance of at least 25% (for sloped roofs). For optimal performance in a hot climate, choose a roofing with a high solar reflectance (> 50%) and a high emissivity (> 80%). Materials, such as painted metal, with high emissivity release heat more rapidly to the atmosphere (cool off at night) than low-e materials like unpainted metal. (Figure 3)