Windows particularly can have an impact on energy consumption (heating and cooling costs) because they can pose a triple threat -- heat moves through a window assembly by conduction through the glass, convection/leakage through gaps and cracks in the framing and assembly and joint with the wall, and by radiation by pulling heat from warm room-temperature objects, including people, to the cold space – that's why when you stand next to a window you feel cold in the winter and warmer in the summer (in winter heat move from warmth, that would be you, toward the cold, and vice versa in the summer).  Understanding how windows are made and the technologies available can help you make the best choices for your window.


U.S. Department of Energy

The National Fenestration Rating Council is a voluntary organization that rates windows, skylights and doors for the following characteristics related to energy efficiency and other functions:

U-Factor or U-Value, and the use of  Low-E Coatings and Gas Fills
Solar Heat Gain Coefficient
Visible Transmittance
Air Leakage
Condensation Resistance

Energy Star labeled windows rely on these ratings.

This Article also covers:
Skylights
Daylighting
Putting it all together - Guidelines for your Windows and Skylight Project
Improving the Energy Efficiency of Existing Windows
Doors
New York City Landmarks Preservation Commission