Convection occurs when heat energy, embodied in a substance, usually air, moves from place to place as the embodying substance moves. For example, hot air moving through a duct system carries heat energy from the furnace to the room being heated. However, according to the Florida Solar Energy Center, convection caused by leaky window gaskets or door thresholds account for only 3 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 conducted heat flow. 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 South Florida home is very small (10 to 20 degrees Fahrenheit). The amount of heat that can be conducted into your home through windows is equally small. This is why multi-pane glass and storm windows are poor investments for South Florida homeowners.

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.

Significant air conditioning savings can be attained by blocking solar heat before it reaches the windows.

• Awnings, storm shutters, shade trees and porch or lanai roofs are all very effective in blocking solar heat.

• To be 100 percent effective, the shading device must never allow direct sunlight to touch the window’s surface.

• Reflective glass or reflective glass coatings should be rated to reflect at least 65 percent of all solar heat to be considered efficient in Southwest Florida. Shades, drapes and blinds installed inside the home do

not interdict solar heat until after it has gained entrance to the home. Interior shades trap solar heat between the drape and the window glass until the heat energy warms the air in that space. This heat-laden air then convects, or flows up to the ceiling where it waits for the air conditioner to cycle on and draw it in though the filter. This process keeps intrusive solar heat from occupying the space in the middle of most rooms where humans usually reside. This enhances human comfort and creates an illusion of efficiency when, in fact, the load on the air conditioner has not been altered.