Wales has followed England in shifting from SAP-based U-value calculations to the new Home Energy Model—but with a stricter 1.4 W/m²K limiting U-value that will likely push housebuilders toward triple-glazed windows. The change forces per-window performance modeling rather than theoretical product data.
A Regulatory Shift That Reshapes Window Specification
The Welsh Government has introduced the new Home Energy Model for U-value calculations on new build windows and doors, and the practical consequence is significant: the shift to triple-glazed units becomes substantially more likely on Welsh housing schemes. Updates to Welsh Approved Document L and F were published on 7th April, following the publication of the Future Homes Standard for England on 24 March 2026, and effectively introduce a move away from the Standard Model and SAP calculation, introducing a requirement to evidence actual rather than 'theoretical' performance for each individual window.
For architects, fabricators, and developers working in either jurisdiction, this is not a minor administrative tweak. It changes how a window has to be proven to perform—and at what threshold.
What the Home Energy Model Actually Changes
The most important departure from the previous regime is the move from generic product ratings to size- and configuration-specific calculations. As Kevin Jones, Head of Technical at the Glass and Glazing Federation, explained: the Home Energy Model introduces the requirement to assess each window to its actual size and configuration to arrive at an area weighted average, including all the items that sit within the opening—transoms, mullions, dummy sashes, astragal and Georgian bars, add-on cills, head vents and other components.
In other words, the brochure U-value of a standard reference window is no longer the basis for compliance. Every opening on a drawing is now its own thermal calculation, with frame ratios, divider geometry, and accessory components all factored into the area-weighted average.
The Welsh-English Divergence
Here is where Wales has set itself apart from England, and where specifiers need to pay close attention. While the Welsh Home Energy Model will be based on the same calculation as that across the border in England, the limiting U-value is going to be 1.4 W/m²K, compared to 1.6 W/m²K in England—and 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.
That 0.2 W/m²K gap may sound small in isolation, but when combined with the new requirement to account for every transom, mullion, and decorative bar in the area-weighted average, double-glazed units with high-spec warm-edge spacers and argon fills will struggle to clear the bar consistently—particularly on smaller windows or designs with heavy frame ratios.
Why the Regulators Made the Change
The underlying motivation is the long-running performance gap problem. 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.
For years, SAP-based modeling has been criticized for crediting products at idealized lab values that as-installed windows rarely achieve. By forcing compliance to be demonstrated on the actual specified window, regulators are pushing the industry to design assemblies that perform in situ, not just on a datasheet.
Practical Implications for Industry Professionals
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Housebuilders and developers: The changes to the Welsh Approved Document L, and in particular the limiting U-value of 1.4 W/m²K, will drive housebuilders to focus very closely on the weighted average as a whole, and the expectation is that this will push a lot of developers to specify triple-glazed windows, which has big ramifications for cost and future window design. Triple-glazed IGUs add weight, depth, and material cost—affecting frame engineering, hardware sizing, lifting logistics, and pricing.
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Window fabricators: Product portfolios optimized around double-glazing may need rapid reconfiguration. Frame depths, sash sections, hinge load ratings, and gasket profiles built around 24–28mm IGUs will face pressure to accommodate 36–44mm triples.
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Architects and specifiers: Schedule-level U-value compliance is no longer a paste-from-datasheet exercise. Each opening's geometry—including decorative grilles or Georgian bars—now affects compliance, which has real implications for design language on traditional and heritage-inspired residential schemes.
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Glass suppliers: Demand for thin-triple IGUs, vacuum insulating glass, and high-performance warm-edge spacers will accelerate. Suppliers who can deliver triple-pane performance without the conventional weight and depth penalties stand to gain.
Timeline
Projects need to plan around the transition window now. The changes will come into force on all new schemes started after 4th March 2027 unless the work has been registered for an initial notice or relevant notification provision before 3rd March 2027 and work starts before 3rd March 2028, in which case the old regulations will apply.
That gives developers roughly nine months to lock in projects under the existing rules—or to begin re-specifying glazing packages for the new regime. For North American manufacturers watching UK code evolution as a leading indicator of where IECC and provincial codes may head next, the Welsh move to per-window, area-weighted compliance is worth tracking closely.