Thermal efficiency describes how much heat is gained during warm times of the year and how much heat is lost during cold times of the year. This is also known as thermal transmittance or U-value. The U-value does not consider the rate of air infiltration. An estimate of the thermal performance of differing window systems can be obtained from the American Society of Heating, Refrigeration and Air-Conditioning Engineers Handbook. Condensation can form on window glazing and frames when warm air meets a colder surface.
Condensation lowers a window's thermal capacity.
Condensation can also result in water damage to interior surfaces and materials. Sound deadening is another important issue in window selection. Windows are an important part of a building's sound barrier. In general, the more airtight the window, the less noise transmitted. The type of glass and air space between panes of glass in an insulated unit also affect sound transmission.