Window frames, although they are relatively small in comparison to the overall window area, do have an impact on the thermal efficiency of the window. Wood and vinyl frames have the best insulating properties. To improve the thermal efficiency of metal frames, manufacturers build-in thermal breaks. Because most window frames today are fairly energy efficient, selection is typically made more on a basis of durability and appearance than it is on just energy efficiency. The third way in which windows have an impact on the energy use in educational facilities is through infiltration.
Infiltration is the leakage of air between and around window frame and glazing components.
Infiltration rates are directly related to the quality of the window construction and are lower for fixed windows than they are for operable ones. Most high quality windows have infiltration rates of 0.5 cubic feet per minute, or less, per square foot of window opening. Another factor that must be considered when evaluating window options is the window's maintenance requirements. Maintenance budgets have always been tight in educational facilities. Even during times when funding for education was expanding, maintenance budgets failed to expand as rapidly.