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Updates to Welsh Approved Document L and F were published on 7^th April. These follow the shift announced in the publication of the Future Homes Standard for England (24/3/26) to the new Home Energy Model.

This effectively introduces a move away from the Standard Model and SAP calculation, introducing a requirement to evidence actual rather than ‘theoretical’ performance for each individual window.

“The Home Energy Model introduces the requirement to assess each window to its actual size and configuration to arrive at an area weighted average”, explained Kevin Jones, Head of technical, Glass and Glazing Federation.

“That includes all the items that sit within the opening, for example transoms, mullions, dummy sashes, astragal and Georgian bars, add-on cills, head vents and other components.

“These weren’t visible under the Standard Model but are visible under the Home Energy Model.

“While the Welsh Home Energy Model will be based on the same calculation as that across the border in England, there is a significant difference in that the limiting u-value – in shorthand, the lowest acceptable level of thermal performance – is going to be 1.4 w/m²K, compared to 1.6w/m²K in England.

“Depending on the design of the window and the building fabric as a whole, that could make it more likely that developers will have to move to triple-glazed options to meet regulations.”

The changes come as governments in Cardiff and Westminster attempt to close a gap in performance between theoretical building product performance – and what they deliver in real-life applications. 

The changes will come into force on all new schemes started after 4^th March 2027 unless the work has been registered for an initial notice or relevant notification provision before 3^rd March 2027 and work starts before 3rd March 2028 in which case the old regulations will apply.

For any works where initial notice or relevant notification provision is not registered before 4^th March 2027 and work does not start prior to 4^th March 2028 the new regulations will need to be followed.

“While there are transitional arrangements in place, the changes to the Welsh Approved Document L and in particular, that limiting u-value of 1.4 w/m²K are going to drive housebuilders to focus very closely on the weighted average as a whole.

“It would be our expectation that that will push a lot of developers to specify triple-glazed windows, which clearly has big ramifications for cost and future window design.”

GGF members will be able to take a look in more detail and have the opportunity to ask questions at the summer launch of the new GGF Wales regional event, details of which will be released soon.

For support, contact the GGF Technical team at technical@ggf.org.uk  or visit