Building Standards technical handbook 2017: non-domestic buildings

The Building Standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004 and are available in two volumes, Domestic buildings and Non-domestic buildings. This publication is available in html and also in PDF format (in 'supporting documents' ).

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Annex 6.A Compensating U-values for windows, doors and roof-lights

6.A.0 Introduction

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:

  1. 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

  2. for work to existing non-domestic buildings, namely replacements, alterations, extensions, and conversions (Standard 6.2), and

  3. 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.

6.A.1 Example of trade-off between windows, doors and roof-lights

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


  1. 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'

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.

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