Know-How: Doors/Windows

Mar 1, 2005 12:00 PM, by Mike Kennedy

Schools and universities strive to maximize energy efficiency in their facilities. But the potential gains may leak away through inefficient doors and windows.

Administrators should strive to select doors and windows that keep unwanted heat out of buildings during warm weather and keep cold air outside during the winter months.

The National Fenestration Rating Council has established an energy performance rating system to measure how well doors and windows help keep heat out in the summer and cold out in the winter. The categories:

• Visible transmittance measures how much light comes through a product. The U.S. Department of Energy's National Best Practices Manual for High-Performance Schools says that visible transmittance should be as high as possible, so long as it does not create glare or other visibility problems that can distract students or hinder learning.

• Solar heat gain coefficient measures solar heat gain through a window. In general, schools want products with a low heat-gain coefficient, but administrators must balance that desire with a school's need for high visible light transmittance.

• U-factor measures how well a product prevents heat from escaping. Low-e coatings, double or triple panes, and inert gas fills between panes can lower the U-rating and boost energy performance.

Two other rating categories are considered optional: air leakage, which measures how much air is able to pass through the window assembly, and condensation resistance, which measures how well a product resists the formation of condensation on its interior surface.

To ensure that energy-efficient doors and windows continue to perform, school maintenance staffs must check regularly for air leaks and make sure that doors and windows form a tight seal when closed. Workers also should routinely check caulking and weatherstripping, where leaks often occur.

NOTABLE

0.20 TO 1.20

The general range of the U-rating for window assemblies. The lower the number, the better the insulating value.

0 TO 1

The range of possible values for the solar heat gain coefficient. The lower the number, the less solar heat transmitted.

0 TO 1

The range of possible values for the visual transmittance measure. The higher the number, the more light that is transmitted.

0 TO 100

The range of possible values for condensation resistance. The higher the number, the better the product is at resisting condensation formation.

Source: National Fenestration Rating Council