What Is The uValue 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. The rate of heat 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 insulating value.
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.
Tutorial on Calculating uValues.