From a green-built standpoint, windows rank second only to the design of an HVAC system in overall technological complexity. Heating, cooling and lighting consume 67% of all the electricity that’s generated. Windows and doors comprise a large portion of heat loss in a wall and lower the overall R-Value of the wall. Installing windows and doors correctly will help eliminate air infiltration and water leaks.
Window Energy Performance
There are three primary factors in a window’s energy performance:
– frame construction
– spacer material that separates individual panes of glass
– through the glass (by radiation)
– across the spacer material that separates the two glass layers at their edges and
-through the frame of the window (by conduction)
-through the movement of air in the space between the glass (by convection)
-between the moveable or operable frame components (by air leakage)
Rating Windows for Performance
The National Fenestration Rating Council (NFRC) was formed to standardize the claim of energy efficiency by window manufacturers.
U-factor – for the entire window, distinct from a center-of-glass rating.
Solar heat gain coefficient – represents the amount of heat that is transmitted through the glass. The lower the number, the less heat transmittance.
Visible light transmittance – is the “sunglasses” effect. The lower the number, the darker things will appear through the glass.
Air leakage – is rated in cubic feet of air passing through a square foot of window area. The lower the number, the less air will pass through cracks in the assembly. This may be left blank by manufacturers of lower-quality windows.