Kevin Underwood, Technical Director of the British Woodworking Federation (BWF) explores the potential impact of The Future Homes and Buildings Standards 2023 Consultation on the calculation of U-values for windows and doors.
The Future Homes and Buildings Standards 2023 consultation outcomes will shape the future of the woodworking and joinery sector, as well as the broader construction industry, by establishing new energy efficiency requirements that will help the decarbonisation of newbuild homes.
In its October 2021 publication, Net Zero Strategy: Build Back Greener, the Government stated it: “aims to support action in the construction sector by improving reporting on embodied carbon in buildings and infrastructure with a view to exploring a maximum level for new builds in the future.”
Embodied carbon
When the ‘Future Homes and Buildings Standards: 2023’ consultation was published in December of that year the BWF was shocked to see that embodied carbon was to be out of scope. The consultation document stated: “Embodied carbon, the carbon emissions generated from the production and transportation of building materials, construction process and maintenance of a building – is beyond the scope of this consultation and the existing Building Regulations. We recognise, however, that embodied carbon is a significant contributor to the whole life carbon of a building and that it is therefore crucial that we take steps to address it. The government intends to consult on our approach to measuring and reducing embodied carbon in new buildings in due course.”
At the same time, the Government published a policy paper, the ‘Timber in Construction Roadmap’, where it said: “Timber in construction can reduce the whole life carbon, the full carbon impact, of our built environment. The carbon impact of the built environment is made up of both operational carbon from the type of energy or heating systems within a building and the embodied carbon resulting from the manufacture, maintenance and disposal of the construction products that comprise the building or structure. Use of timber can reduce the embodied emissions in a single building by 20% to 60%.”
As the operational carbon of a building reduces, the embodied carbon becomes more significant. By not considering embodied carbon the government has lost an opportunity to further reduce carbon in the construction industry.
In the Future Homes and Buildings consultation the Government considered that it could keep fabric standards largely the same as the levels set in the 2021 amendments to Approved Document L as:
- It considered that level set in 2021 would ensure that (with adequate ventilation) new homes would not generally experience damp and mould or excessive temperatures
- The 2021 fabric standards allow efficient low carbon heating, e.g. heat pumps, to function well
- As the electricity grid is decarbonising, and the efficiency of heat pumps significantly reduces energy demand, further reducing total energy use is relatively less important than switching to electric sources of heat in efforts to decarbonise.
The impact on U-values
The notional (target) and limiting (no worse than) U-values for windows and doors in new dwellings are set out in the table below. For the reasons already highlighted, these values are no different to what they are today. But it is not the values that pose a problem for manufacturers, it is the proposed change in the way that U-values are determined that presents a challenge to the sector.
| The Future Homes Standard 2025: dwelling notional buildings for consultation.
U-value (W/(m2.K)) |
Draft consultation-stage Approved Document L, Volume 1: Dwellings, Table 4.1 Limiting U-values for new fabric elements in new dwellings.
U-value (W/(m2.K)) |
|
| Windows and glazed doors | 1.2 | 1.6 |
| Other doors | 1.0 | 1.6 |
Currently, ‘Approved Document L, Volume 1: Dwellings’ allows the U-value of a window or door to be determined using standard sizes and configurations, but the Future Homes and Buildings consultation introduced a new national calculation methodology for dwellings called the Home Energy Model. This new methodology aims to improve the accuracy of energy modelling and consequently requires greater accuracy of the input data.
This is based on the government’s view that when determining U-values for windows and doors, the use of U-value calculations based on standard sizes and configurations, or default values was not sufficiently accurate. The proposals for new dwellings recommend that the U-value of windows and doors should be calculated using either the actual size and configuration of the window or door, or it should be measured using the appropriate hot box method. This proposal would require a huge increase in heat transfer modelling or testing to be undertaken by manufacturers. For new dwellings, the U-value of a door or window should be either calculated using the actual size and configuration of the door or window or measured using the hot-box method set out in BS EN ISO 12567-1.
Calculation challenges
The Government’s proposal for the determination of U-values, however, appear to have missed two essential points.
Firstly, that windows and external doors are regulated under the construction products regulation and must be conformity marked – either UKCA or CE. As such, the determination of U-values must follow the requirements of the designated product standard BS EN 14351-1.
Secondly, that there are no UK Approved Bodies that can undertake hot box testing to BS EN 12567-1 and the governments post-Brexit rules do not permit the use of test evidence from laboratories that are not UK Approved Bodies for the purposes of UKCA marking.
Article 4 of the assimilated EU Construction Product Regulation (305/2011) (the CPR) states that: “When a construction product is covered by a designated standard or conforms to a 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.”
This means that for windows and external doors their U-value, which is an essential characteristic, must be determined by a UK Approved Body and not determined by the manufacturer alone.
However, the Government is proposing that unique U-values are produced for each window or door installed into a new build home. This is therefore not only complicated by the number of calculations required but also by the requirements of the CPR.
Looking ahead, it is difficult for the sector to plan for what may not come to fruition. Until we see the final amendments to Approved Document L, we can’t be certain of what the guidance document will require. In the past, content put forward in a consultation, has not made its way into the final Approved Document so it is right for the sector to be cautious. The hope is that whatever changes are made, the sector is given sufficient time to put in place new systems to achieve the new demands.
Approved Document L could have a significant impact on our sector, and at the BWF, we are closely monitoring the proposed amendments. Once the document is finalised, the BWF will be ready to provide our members with the necessary guidance and support to effectively navigate the changes.
More at: www.bwf.org.uk
