We use cookies to collect anonymous data to help us improve your site browsing experience.

Click 'Accept all cookies' to agree to all cookies that collect anonymous data. To only allow the cookies that make the site work, click 'Use essential cookies only.' Visit 'Set cookie preferences' to control specific cookies.

Set cookie preferences

Your cookie preferences have been saved. You can change your cookie settings at any time.

Publication - Advice and guidance

Building standards technical handbook 2020: non-domestic

Published
2 December 2020
Directorate
Local Government and Housing Directorate
Topic
Building, planning and design
ISBN
978-1-78544-329-9

The building standards technical handbooks provide guidance on achieving the standards set in the Building (Scotland) Regulations 2004. This handbook applies to a building warrant submitted on or after 1 March 2021 and to building work which does not require a warrant commenced from that date.

View supporting documents Supporting documents

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

Notes:

  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' 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.

Back to top