U-Factor: is a calculation that describes the rate of heat loss. A window with a lower U-Factor has greater resistance to heat flow and a better insulating value. Note: United States uses inch-pound imperial values (IP) and other countries use metric values (SI). (Conversion is IP multiplied by 5.678, or SI divided by 5.678.).
Solar Heat Gain Coefficient (SHGC): is the total amount of direct solar heat entering the room and indirect heat gained from absorption in the glass. A value of 1 is 100% gain, 0 is total blockage.
Visible Light Transmittance: is a percentage of the visible light spectrum that passes through the window into the room's interior. A higher visible transmittance is desirable for maximum daylight.
Fade Transmission: is a weighted calculation using UV and visible light (also called Tdw-ISO). A lower number means better fading protection to interior objects like carpets and furniture.
The measure of an energy-efficient window is one that provides thermal comfort throughout the seasons of the year. Geographic location and window size are attributes that should be considered in order to select a glazing package that balances the need to keep rooms warm in the winter and cool in the summer.
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Window comfort analysis is derived using ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy protocols (www.ashrae.org) and NFRC research report "Window Performance for Human Thermal Comfort, February 2006" when applied to full year (8760 hour) weather data file for each location. A particular hour counts for discomfort if the window heat loss (winter night) or heat gain (summer day) increases thermal discomfort by more than 10 percentage points over a room without windows.
Occupant proximity and shade closure demonstrate intervention needed to get selected window package within the 10% thermal dissatisfaction limits.