ENERGY EFFICIENCY
The Learning Center
There are a number of interactive factors which all work together to determine the energy efficiency of a window or door.
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. For glass, companies often 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 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. A high
coefficient will allow more sunlight, and heat, into the house. A low coefficient will block more sunlight to reduce summer heat gain.
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.
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.
2. How do you measure the energy efficiency of a window or a door?
< back to list of questions about energy efficiency
3245 Miracle Drive
Murrysville, PA 15668
724-387-2991
PA Contractor ID
PA001856
The Learning Center
There are a number of interactive factors which all work together to determine the energy efficiency of a window or door.
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. For glass, companies often 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 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. A high
coefficient will allow more sunlight, and heat, into the house. A low coefficient will block more sunlight to reduce summer heat gain.
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.
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.
2. How do you measure the energy efficiency of a window or a door?
< back to list of questions about energy efficiency
3245 Miracle Drive
Murrysville, PA 15668
724-387-2991
PA Contractor ID
PA001856
