Annex 6.A Compensating U-values for windows, doors and rooflights
This annex gives guidance on how to calculate the average U-values 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 U-values 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 area-weighted average U-values identified in guidance provided that the average U-value calculated for all the windows, doors and rooflights is no greater than that relevant U-value.
The example which follows below illustrates how this trade-off can be calculated.
A semi-detached house has a total window area of 17.8m2 (including frames) and a total door area of 3.8m2. It is proposed to use 2 external quality timber finished fire doors with a U-value of 1.9W/m2K.
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 U-values in the windows and/or rooflights so that the average overall U-value of such elements does not exceed 1.6W/m2K (see table to clause 6.2.1).
Specifying windows and rooflights with a U-value of 1.5W/m2K can achieve this requirement, as shown by the following table and subsequent calculation:
Table 6.6. Average U-value calculation
| Element | Area (m2) | U-value (Wm2K) | 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 U-value, for the purpose of calculation the rooflight value is 0.3W/m2K poorer due to inclination from the vertical plane (see BR 443 - 'Conventions for U-value Calculations' http://www.bre.co.uk./filelibrary/pdf/rpts/br_443_(2006_edition).pdf).
This gives an average U-value of 34.19 รท 21.6, or 1.58W/m2K. The windows, doors and rooflights can therefore be considered to follow the objectives of the requirement for the insulation envelope.