Future Homes Standard and Calculating U-values

In recent discussions with industry colleagues, the British Woodworking Federation (BWF) has explored potential challenges arising from the recent consultation on Approved Document L for England. This document proposes that each individual door or window in the external envelope of new-build dwellings should have an accurate U-value. The BWF aims to find a simplified, less daunting approach to providing these U-values.

In this blog BWF Technical Director, Kevin Underwood, discusses only one of many concerns there are relating to proposals for doors and windows made in the consultation draft of Approved Document L covering new-build dwellings in England.

U-values are an essential characteristic for doors and windows and as they are also regulated under the building regulations it is necessary to declare a U-value on the Declaration of Performance (DoP) that applies to a product family. For the purposes of conformity marking (CE and UKCA) the U-values can be calculated using the model door and window sizes given in the product standard for windows and external doorsets, BS EN 14351-1.

It is not a requirement of the building regulations that a door or window is conformity marked. However, the Approved Document to Regulation 7 (Materials and Workmanship) states, “The Construction Products Regulation requires that construction products that are covered by a harmonised European product standard or conform to a European Technical Assessment should normally have CE marking.” We assume this paragraph also applies to UKCA marking.

The Approved Document to Regulation 7 also states, in the context of conformity marking, “If the declared performance of a product is suitable for its intended use, the building control body should not prohibit or impede the use of the product.”

The recommendation that the U-values should be determined for each individual window or doorset installed into a new-build dwellings in England contradicts the recommendations for the provision of U-values for replacement doors and windows into existing buildings in England, and the recommendations for the U-values of doors and windows for both new-build and existing dwellings made by the devolved authorities in Wales, Scotland and Northern Ireland. In all these other cases the determination of the U-values should follow the principles of the Building Research Establishment’s BR 443 and allow the use of a single model doorset or window representing a range or product family.

BR443 provides information on the calculation of U-values for windows in section 11 and doors in section 14. The methods stated permit numerical calculations in accordance with BS EN ISO 10077-1 and -2 or simplified calculations in accordance with Annex F of BS EN ISO 10077-1. It also allows the use of the model door and window sizes given in Tables E1 and E2 of BS EN 14351-1.

It is interesting to note that both the proposed guidance in the draft Approved Document L for England and BR 443 also permit U-values to be determined through a hot-box test to BS EN ISO 12567-1. However, having have checked via the United Kingdom Accreditation Service’s (UKAS) website no accredited laboratory that offers this test in the UK could be found.

The great majority of windows and external doorsets will need to be conformity marked and the manufacturer will need to declare a U-value on a DoP and, unless the manufacturer is a micro enterprise, this U-value will need to have been determined by either a notified (EU) or approved (UK) body using a model door or window as permitted by BS EN 14351-1. When it comes to determining the U-values for each individual door or window BS EN 14351-1 states, “where detailed calculation of the heat loss from a specific building is required, the manufacturer shall provide accurate and relevant, calculated or tested thermal transmittance values (design values) for the size(s) in question.” It is interesting that this statement emphasises that the U-value for the specific size of window or door is provided by the manufacturer rather than a notified or approved body.

It is still to be confirmed whether, once a U-value for a model door or window has been declared for the purposes of compliance with conformity marking by a notified or approved body, the manufacturer can produce their own U-values for specific doors and windows. We need to consider Article 4 (2) of the Construction Products Regulation amended for use in the UK states, “When a construction product is covered by a harmonised designated standard or conforms to a UK Technical Assessment which has been issued for it, information in any form about its performance in relation to the essential characteristics, as defined in the applicable harmonised technical specification, may be provided only if included and specified in the declaration of performance except where, in accordance with Article 5, no declaration of performance has been drawn up.”

If, after reading this article, you wish to air your own concerns then please contact BWF Technical Director Kevin Underwood at kevin.underwood@bwf.org.uk.