In Southwest Florida, 30 percent of the summer cooling costs are attributed to glass and windows. The use of glass as a home building material is becoming more and more popular. This rise in the use of glass causes special concerns for homeowners in a summer-dominant climate because glass and windows offer an avenue through which solar heat can enter the home.
All transfer of heat energy occurs as the result of convection, conduction or radiation. Convection occurs when heat energy, embodied in a substance, usually air, moves from place to place as the embodying substance moves. According to the Florida Solar Energy Center, convection caused by leaky window gaskets or door thresholds accounts for only three percent of the typical home’s air conditioning load.
Conduction occurs when heat energy moves from molecule to molecule through a substance. The greater the difference in temperature, the greater the flow of conducted heat.
Substances that resist heat flow are said to be good insulators, measured in R-values. Although glass has a low R-value, the typical summer temperature difference between outside and inside a home in Southwest Florida is very small (10 to 20 degrees Fahrenheit). The amount of heat that can be conducted into your home through a window is equally small. This is why multi-pane glass does not provide a reasonable payback in Southwest Florida, unless a “spectrally selective” window is chosen.
Radiation is the process by which most heat energy enters homes in Southwest Florida. Every object embodies or stores heat energy and some of this heat energy leaks away in the form of infrared radiation or radiant heat. This heat is very difficult to reflect because it travels in a straight line. Clear glass windows offer practically no resistance to radiant heat. Radiant and solar heat together account for 31 percent of the heat load on the typical air-conditioning system, with 26 percent of all heat loads coming right through the windows in the form of sunlight.
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