What Is The U-Value Of A Window?

 Uvalue Window Heat Transfer

Image courtesy of www.thegreenage.co.uk


U-value is the measurement of a window’s conductivity to heat and indicates how much non-solar heat loss and gain the window allows. A window with a lower U-value has better insulation properties than one with a higher U-value as it allows less heat to flow through. The U-value takes into account the assembly of the window including the framing, sealing and spacing of the window as well as the glass that is used.

A formula can be used to understand the significance of a lower U-value:

* The heat flow that a window allows (in watts) is the U-value (Uw)

* Multiplied by the temperature difference (in degrees) on each side of the window (T)

* Multiplied by the area of the window (A)

Uw x T x A = heat loss (in watts)

For example, if a house has 75m2 of aluminium framed windows and doors with glazed clear glass (Uw of 6.2) and the temperature difference between inside and outside is 14°C the heat loss can be calculated by multiplying these numbers:

6.2 x 14 x 75 = 6,510 Watts

6,510W is equal to the heat output of a large sized gas heater or a two horsepower air conditioner on maximum.

However, if the windows were double-glazed the U-value of the window would be approximately halved which in turn means half the heat loss or 3000W in the above example. By saving 3000W of energy you are avoiding the equivalent energy usage of thirty 100W light bulbs.


Tutorial on Calculating uValues.

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