BWF Technical Director, Kevin Underwood clarifies the new requirements for U-values in new build and existing dwellings, and how U-values can be improved for windows and doors.
On the 15th of June new requirements for the U-values of timber windows and doors came into force in England for products supplied for use in existing dwellings. These requirements, which were published in Approved Document L volume 1 in December 2021, have come into effect for timber windows and doors a year after they were effective for other materials. This extension was agreed by the Government to give woodworking manufacturers more time to make the necessary changes to their products. This extension wasn’t applied to timber windows and doors supplied for use in new dwellings which had to comply with the new requirements from the 15th of June 2021, the same time as other materials.
Full guidance is given in Approved Document L volume 1:Dwellings, 2021 edition incorporating 2023 amendments. (AD L v1)
For new build dwellings, since 15th of June 2022
- The notional dwelling specification – (AD L v1 Table 1.1), i.e. target U-value.
- Windows and doors with a glazed area more than 60% – 1.2 W/(m2.K)
- Other doors – 1.0 W/(m2.K)
- The limiting U-values for new dwellings – (AD L v1 Table 4.1), i.e. U-values should not be worse than these.
- All windows and doors – 1.6 W/(m2.K)
For existing dwellings, since 15th of June 2023
The U-value of a replacement window or door in an existing dwelling should not be worse than the window or door being replaced and should meet the limiting values below.
The limiting U-values for existing dwellings – (AD L v.1 Table 4.2)
Windows – 1.4 W/(m2.K) or WER min. band B
Doors with a glazed area more than 60% – 1.4 W/(m2.K) or DSER min. band C
Other doors – 1.4 W/(m2.K) or DSER min. band B
There is an allowance for external fire doors which are permitted to have a U-value of 1.8 W/(m2.K)
WER stand for window energy rating and DSER stands for doorset energy rating. Energy ratings describe an energy balance between energy lost through the window or door, the energy needed to heat cold air that may have leaked passed the weather seals, and any solar heat gained from sunlight passing through the glazing. The methods for calculating Window Energy Ratings and Doorset Energy Ratings are set out in the Glass and Glazing Federation’s Glazing Manual Data Sheet 2.3, Guide to the Calculation of Energy Ratings for Windows, Roof Windows and Doors.
If a window, or fully glazed external pedestrian door, cannot meet the requirements of AD L v.1 Table 4.2 because of the need to maintain the character of the building, then either the glazing within the window or door should not exceed a centre pane U-value of 1.2W/(m2.K), or single glazing should be supplemented with low-emissivity secondary glazing.
I am often asked by members of the BWF how they can improve the U-value of their windows and doors. There are seven key methods available to them.
- Increase the thickness of any components. The thermal resistance of a component, i.e. how well it will insulate, is in proportion to its thickness.
- Reduce the thermal conductivity of any materials used. Materials with a lower thermal conductivity will provide a greater thermal resistance for a given thickness.
The thermal conductivities of some timbers are,
Norway Spruce – 0.11 W/(m.K)
Accoya® – 0.12 W/(m.K)
Generic softwood, Larch and Hemlock – 0.13 W/(m.K)
Dark Red Meranti, Sapele and Utile – 0.16 W/(m.K)
Generic hardwood, Eucalyptus Grandis and European Oak – 0.18 W/(m.K)
- Where timber doors have panels, or can be double boarded, there is an opportunity to form a sandwich with a core of insulating material – An insulating material with a low conductivity should be used. The thermal conductivity of thermal insulation can range from 0.5 W/(m.K) for cork to 0.022 W/(m.K) for a higher performing insulating foam.
- Incorporate air cavities or insulation into timber profiles – This can improve the overall thermal resistance of the section beyond that of a solid timber section.
- Change the low emissivity coating of a double-glazing unit which is usually located on the cavity side of the inside pane, to one that has a lower emissivity value – This can be as low as 0.1. The emissivity of a surface is rated from 1 for surfaces with the highest emissivity to 0 for surfaces with the least.
- Ensure that the double-glazing unit uses a spacer bar with a low linear thermal transmittance, or psi value (ψ) – This value can range from 0.08 for a steel or aluminium spacer bar to 0.03 W/(m.K) for a higher performing “warm-edge” spacer bar.
- Change the gas that is used to fill the cavity, or cavities, of a glazing unit.
Typical centre pane U-values for a double-glazing unit with a higher performing low-e coat with either a 10mm or 16mm cavity width, and using air, argon or krypton for the gas fill will be,
- Air 10mm – 1.6 W/(m2.K) – 16mm – 1.3 W/(m2.K)
- Argon (90%) 10mm – 1.3 W/(m2.K) – 16mm – 1.0 W/(m2.K)
- Krypton (90%) 10mm – 1.0 W/(m2.K) – 16mm – 1.0 W/(m2.K)
