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What is a U-value?

The U-value of a window is a measurement of the rate of heat loss indicating how well your windows are keeping valuable heat in. It is expressed as Watts per square metre Kelvin W/m2 K. The lower the U-value the better the thermal performance of the glass. So called 'Low-emissivity' or 'Low E' glass offers improved thermal insulation denoted by very low U-values.