Windows
Glass Terminology
Terminology to Help You Understand How a Window is Rated
R-Value: A window’s resistance to heat loss. This number is the inverse of the U-Value. The higher the R-Value and lower the U-Value, the more efficient the window is in reducing your winter heating and summer cooling costs.
U-Value: How well a product prevents heat from escaping a home or building, based on conditions of 0° outside and 70° inside. The lower the U-Value, the better the product is at reducing heat loss.
Solar Heat Gain Coefficient: How well a product blocks heat from the sun. The lower the number, the better the product is at blocking unwanted heat gain.
Visible Light Transmittance: How much light comes through a product. VT is expressed as a number between 0 and 1 - the higher the Visible Transmittance number, the more light shines through the product.
Ultraviolet Light Transmittance: The amount of Ultraviolet Light Transmittance in the solar spectrum that is transmitted through a window. This can cause fading of carpets, drapes, furniture, hardwood floors, etc. The lower the number, the better the window system is at reducing the fading potential of these materials.
Air Resistance (AAMA): Airtightness is measured by applying 75 pascals of pressure — the equivalent of a steady 25 mph wind to the outside of the window. Interior air infiltration is then measured in cubic feet per minute per foot of window joint.
*Results are from national tests under the following standards:
AAMANWDA1011.S2-‐97