There are a number of interactive factors which all work together to determine the energy efficiency of a window or door.
1) Measurement Values:
R-value and U-factor are the two common measures used to rate how well a specific material or system resists or conducts heat flow. R-value measures the resistance to heat flow. In windows and doors, however, R-value can be misleading, since it applies only to specific parts, such as the frame or center of glass.
U-factor measures the ability of a material or system to conduct the flow of heat. The lower the number, the better its insulating ability. U-factors for windows and doors most often refer to the insulating quality of the entire system (the glass, frame and spacer combined). Therefore, it is a more useful measure of energy efficiency. U-factor is based on a scale of zero to 1. Lower U-values are more cost effective on climates where the temperature difference between the outside and the comfort of your home is large, such as in colder climates where this difference can be in the 50o – 70o F.
Sometimes, values are quoted for the glass alone or for the framing material. These numbers can be very misleading since they look at individual components rather than windows as a system. For glass, many times people throw out center-of-glass measurements, which overstate the performance of glass as installed in a specific window unit. They are not indicative of glass or the window performance as a whole. This center-of-glass value drops off as you move towards the edge of the glass. The amount of drop off is dependent upon a number of variables. It is not a good idea to compare the performance of one product to another based upon glass or frame values alone. Most manufacturers have run NFRC (National Fenestration Rating Council) thermal performance tests on an assembled unit which is a much better indicator of the unit’s overall performance compared to glass or window frame values alone.
2) What Is Solar Heat Gain Coefficient (SHGC)
The second critical measurement is the Solar Heat Gain Coefficient (SHGC), which measures a window’s ability to reduce heat gain by blocking the sun’s heat-producing rays. It is also based on a scale of zero to 1. A high coefficient will allow more sunlight, and heat, into the house. A low coefficient will block more sunlight to reduce summer heat gain. Windows and doors with lower SHGC are important in climates where air-conditioning is used offset the heat from the sun coming into your home.
3) Air Leakage
The third way a window can lose energy is through Air leakage. The air infiltration rate for a window is measured in cubic-feet-per-minute (CFM) and refers to air leakage that occurs through cracks in the window assembly. The lower the number, the less air will pass through. Air tightness in windows and doors used to be more of a consideration than it is today. The small amount of air leakage through windows and doors provides an acceptable (and sometimes necessary) amount of ventilation for your home.
The final influencing factor is installation. Poorly or improperly installed windows can be a major energy consumer. Improper installation can cause air to leak around the window frame. Poorly caulked windows can also be an energy waster. If the installer does not understand the design characteristics of an individual window, he can influence the contact and compression of the weather stripping, resulting in poor performance.