How Are 'U' Values Calculated

Approved document L1B (Conservation of fuel and power in existing dwellings) states: 'U' values shall be calculated using the methods and conventions set out in BR443 and should be based on the whole unit, ie in the case of a window, the combined performance of the glazing and frame.  The 'U' value for the window can be calculated for:

• the smaller of the two standard windows defined in BS EN 14351-1, or
• the standard window, configuration set out in BR443, or
• the specific size and configuration of the actual window

In practice thismeans that either a hot box test, or thermal calculation, must be carried out for each, and every window manufactured, or a standard size as laid down in BR443 which would cover various sizes as long as the compotent parts used in the window, are the same.  Door 'U' values can also be calculated using BR443 guidance.

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