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:
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.
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)|
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 ÷ 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.