Annex 6.E Compensating U-values for openings
6.E.0 Introduction
6.E.1 Example of trade-off for windows, doors and rooflights in the Elemental Method

annex 6.E

Compensating U-values for openings
[Appendix E]

6.E.0 Introduction

Within the Elemental Method it is possible to have windows, doors or rooflights with U-values that exceed the Elemental U-values in Table 1 to 6.2.1. The average U-value however of all of the windows, doors and rooflights taken together should not exceed 2.0 or 2.2 depending on the type of frame (also subject to the carbon intensity of the heating system). The example that follows illustrates how this can be done.

6.E.1 Example of trade-off between windows, doors and

rooflights in the Elemental Method

A building is to have a total window area of 16.9 m² (including frames) and a total door area of 3.8 m². It is proposed to use solid wooden doors with a U-value of 3.0 W/m²K. In order to use the Elemental Method, the additional heat loss due to the use of solid timber 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 2.0 W/m²K.

Windows with a U-value of 1.7 W/m²K can achieve this requirement, as shown in the following table and subsequent calculation:

Average U-value calculation

Note:

These U-values correspond to double glazed windows or rooflights with a wood or PVC-U frame, with a 16 mm argon-filled space between the panes and a soft low-emissivity coating on the glass. Note that although the windows and rooflights have the same design the rooflight U-value is 0.2 W/m²K higher than the window U-value.

This gives an average U-value of 41.84 ÷ 21.6, or 1.94 W/m²K, which is below 2.0 W/m²K. The windows, doors and rooflights can therefore be considered to follow the objectives of the Elemental Method.