The U-Value

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:

{| border=“1” |- | 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 |- |}

== References ==

http://www.wers.net/faqs#uvalue http://en.wikipedia.org/wiki/R-value_%28insulation%29#U-value http://www.daviddarling.info/encyclopedia/U/AE_U-value.html http://en.wikipedia.org/wiki/Thermal_conductivity