What Is The u-Value Of A Window?
The u-Value (expressed as Uw in windows) measures how readily a window system conducts heat. It is a measure of the rate of non-solar heat loss or gain through it. In simple terms, how much heat is gained from the outside in Summer, and how much heat is lost to the outside in Summer. The u-Value is sometimes called the u-Factor.
The rate of heat transfer is indicated in the terms of the U-value of a window assembly, which includes the effect of the frame, glass, seals and any spacers.
The lower the U-value, the greater a window’s resistance to heat flow and the better its thermal insulating value.
u-Value ratings are produced by tests under the Window Energy Rating Scheme (WERS). WERS provides a list of window manufacturers, and their WERS approved windows. The Ratings include the u-Value figures.
The u-Value is just one of the measures that tells us how well a double glazed window performs. You should also check up on Solar Heat Gain Coefficient, and Visible Light Transmittance.
u-Values for the technically- minded…
Video Tutorial on Calculating u-Values.
A simple formula can help quantify the impact of improved U-value:
- the amount of heat conducted through a glazed unit (in watts) equals the U-value (Uw)
- multiplied by the number of degrees difference in air temperature on each side (T)
- multiplied by the area of the glazing unit (A)
Uw x T x A = watts (W)
If your home has 70m2 of windows and glazed doors with aluminium frames and clear glass (i.e. U-value of 6.2), on a winter’s night when it’s 15°C colder outside, the heat loss would be about:
6.2 x 15 x 70 = 6,510W
That’s equivalent to the total heat output of a large gas heater or a 2hp air conditioner running at full capacity.
If you roughly halve the U-value of the window by selecting double glazing, you can halve the heat loss — in this example avoiding about 3,000W of heat loss, equivalent to the energy use of fifty 60W incandescent light bulbs.
So what is the difference between U-values and R-value?
R-value and U-value are essentially two sides of the same coin. R-value is usually quoted when discussing things such as wall and ceiling insulation values. The term does not translate well to windows and other fenestration products, so the industry prefers to use U-values. The two are actually inversely related. The higher the R-value, the better insulated are the walls and ceilings. The lower the U-value, the better job a window does in keeping out the heat and cold.