The U-value or U-factor explains how well a building element conducts heat. It is a measure of the rate of heat loss or gain through a material or assembly. It is also called the overall heat transfer coefficient. U-value measure conductance and is the inverse of R-value which measures resistance. Therefore, the lower the U-value, the greater a product’s resistance to heat flow and the better its insulating value.

U-values are often quoted for windows and doors as R-value does not translate well to windows and other fenestration products. In the case of a window, for example, the U-value may be expressed for the glass alone or the entire window assembly, which includes the effect of the frame and the spacer materials.

Units and Measures

U-value measures the rate of heat transfer through a building element over a given area, under standardized conditions. The usual standard is at a temperature gradient of 24°C, at 50% humidity with no wind.

SI units of W/(m²K) and US units of BTU/(h °F ft²)

Following are the units of U-value:

  • SI Units: watt per square-meter kelvin (W /m²•K)
  • Imperial Units: British thermal unit per hour Square feet degree Fahrenheit (Btu /ft²•°F•h)

U-value is the reciprocal of R-value.

Example Values

Following are some typical u-values for a range of window glazing types and frames:

Material U-Value
Single Clear Glass (Aluminium Frame) 7.9
Single Clear Glass (Timber Frame) 5.6
Single Tinted Glass (Aluminium Frame) 7.9
Single Tinted Glass (Timber Frame) 5.6
Double Glazing Clear Glass 3/6/3 (Aluminium Frame) 6.2
Double Glazing Clear Glass 3/6/3 (Timber Frame) 3.8
Single Low Emissivity Glass (Aluminium Frame) 4.4

Useful References


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