Annex 6.A Compensating U-values for windows, doors and roof-lights
This annex gives guidance on how to calculate the average U-values for windows, doors, and roof-lights and supports the guidance to Standard 6.2. It may be used in the following cases:
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where it is not possible to input the individual U-values for all the windows, doors and roof-lights for the proposed new building into the calculation methodology
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for work to existing non-domestic buildings, namely replacements, alterations, extensions, and conversions (Standard 6.2), and
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for small stand-alone buildings such as one or two offices and a toilet located in an otherwise unheated warehouse.
Individual windows, doors or roof-lights may have U-values that exceed the relevant area-weighted U-values in the guidance to Standard 6.2 provided that the average U-value calculated for all the windows, doors and roof-lights is not greater than that relevant U-value.
The example which follows below illustrates how this trade off can be calculated.
An extension to a building has a total window area of 16.9m2 (including frames) and a total door area of 3.8m2. It is proposed to use two external quality timber doors with a U-value of 1.9W/m2K.
In order to meet Standard 6.2, the additional heat loss due to the use of the external doors should be compensated for by more demanding U-values in the windows and/or roof-lights so that the average overall U-value of such elements does not exceed 1.6W/m2K (see table to clause 6.2.11).
Specifying windows with a U-value of 1.5W/m2K can achieve this, as shown by the following calculation:
Table 6.8. U-value calculation
| Element | Area (m2) | U-value (Wm2K) | Rate of heat loss per degree (W/K) | ||
|---|---|---|---|---|---|
| Windows | 16.9 | x | 1.5 [1] | = | 25.35 |
| Doors | 3.8 | x | 1.9 | = | 7.22 |
| Roof-lights | 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, which is below 1.8Wm2K. The windows, doors and rooflights therefore meet the performance required for the insulation envelope under Standard 6.2.