Windows, Doors & Beyond

U-factors and solar heat gain coefficients can be confusing when it comes to windows, especially depending on the climate you live in. Take into consideration one, two and three pane systems, three panes having the greatest resistance to heat transfer. Also you'll want to consider low-E (low emissivity windows) which reflect heat, not light, keeping spaces warmer in winter and cooler in summer and whether or not tinted windows would be of value. The following is a general guide by region through maze of U-factors and solar heat gain coefficients.

The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. The insulating value is indicated by the R-value which is the inverse of the U-value. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

Northern Climate: Select windows with a U-factor of 0.35 or less. If air conditioning loads are minimal, windows with U-factors as high as 0.40 are also energy-efficient if the Solar Heat Gain Coefficient is 0.50 or higher. Some double-glazed low-e products have U-factors below 0.30. Some three-layer products have U-factors as low as 0.15.

North/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

South/Central Climate: Select windows with a U-factor of 0.40 or less. The larger your heating bill, the more important a low U-factor becomes.

Southern Climate: A low U-factor is useful during cold days when heating is needed. A low U-factor is also helpful during hot days when it is important to keep the heat out, but it is less important than SHGC in warm climates. Select windows with a U-factor lower than 0.75 and preferably lower than 0.60.

Solar Heat Gain Coefficient (SHGC)
The SHGC is the fraction of incident solar radiation admitted through a window, both directly transmitted, and absorbed and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits.

Northern Climate: To reduce heating, select the highest SHGC you can find (usually 0.30-0.60 for the U-factor ranges required in colder climates) so that winter solar gains can offset a portion of the heating energy need. If cooling is a significant concern, select windows with a SHGC less than 0.55. Use a computer program such as RESFEN to understand heating and cooling trade-offs.

North/Central Climate: If you have significant air conditioning costs or summer overheating problems, look for SHGC values of 0.40 or less. If you have moderate air conditioning requirements, select windows with a SHGC of 0.55 or less. While windows with lower SHGC values reduce summer cooling and overheating, they also reduce free winter solar heat gain. Use a computer program such as RESFEN to understand heating and cooling trade-offs.

South/Central Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.

Southern Climate: A low SHGC is the most important window property in warm climates. Select windows with a SHGC less than 0.40.

To properly figure the size of HVAC system the installer should take into consideration the R-value of your walls, roof and foundation along with the SHGC and U-factor of your windows. Depending on the quality of your building envelope it will have a tremendous effect on the size of HVAC system you need to heat and cool your home, regardless of how large it is.