What is a 'U' Value

A 'U' value is a measure of thermal transmittance, expressed as the rate of heat transfer through any element of a building, eg wall, roof, window, or indeed any structural component.

As the construction of these elements can vary so much through the design and choice of materials available, 'U' values can also vary and hence they need to be calculated specifically for each element.  In the case of a window and door, 'U' values can be calculated for the frame, glass, opaque panels and whole product.  It is the whole window or door that counts, not just the frame or glazing material.  Any variation in size, configuration, glazing specification, or frame materials, will result in a different 'U' value.

Source: Richard Bate, Technical Director of Build Check writing in the Glass & Glazing Products magazine August 2010