Annex 6.A Compensating Uvalues for windows, doors and rooflights
This annex gives guidance on how to calculate the average Uvalues for windows, doors, and rooflights and supports the guidance to Standards 6.1 and 6.2. It may be used with the elemental methods provided and, in particular:

the simplified approach in the guidance to Standard 6.1, where it is not possible to input the individual Uvalues for all the windows, doors and rooflights for the proposed new dwelling into the methodology (usually SAP 2012), and

for work on existing domestic buildings, namely, conversions, extensions, replacements, alterations, and conservatories (clauses 6.2.6 to 6.2.12).
Individual windows, doors or rooflights may exceed the relevant areaweighted average Uvalues identified in guidance provided that the average Uvalue calculated for all the windows, doors and rooflights is no greater than that relevant Uvalue.
The example which follows below illustrates how this tradeoff can be calculated.
A semidetached house has a total window area of 17.8m^{2} (including frames) and a total door area of 3.8m^{2}. It is proposed to use 2 external quality timber finished fire doors with a Uvalue of 1.9W/m^{2}K.
In order to meet Standards 6.1 and 6.2, the additional heat loss due to the use of the poorer external doors should be compensated for by more demanding Uvalues in the windows and/or rooflights so that the average overall Uvalue of such elements does not exceed 1.6W/m^{2}K (see table to clause 6.2.1).
Specifying windows and rooflights with a Uvalue of 1.5W/m^{2}K can achieve this requirement, as shown by the following table and subsequent calculation:
Table 6.6. Average Uvalue calculation
Element  Area (m^{2})  Uvalue (Wm^{2}K)  Rate of heat loss (W/K)  

Windows  16.9  x  1.5 [1]  =  25.35 
Doors  3.8  x  1.9  =  7.22 
Rooflights  0.9  x  1.8 [1]  =  1.62 
Total  21.6  34.19 
Notes:

Note that although the windows and rooflights have the same Uvalue, for the purpose of calculation the rooflight value is 0.3W/m^{2}K poorer due to inclination from the vertical plane (see BR 443  'Conventions for Uvalue Calculations' http://www.bre.co.uk./filelibrary/pdf/rpts/br_443_(2006_edition).pdf).
This gives an average Uvalue of 34.19 รท 21.6, or 1.58W/m^{2}K. The windows, doors and rooflights can therefore be considered to follow the objectives of the requirement for the insulation envelope.