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:
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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
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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:
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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.