The Welsh Government has introduced a major update to building regulations that could significantly reshape window and door specifications in new homes. With the publication of revised Approved Documents L and F on 7 April, Wales formally adopts the new Home Energy Model (HEM) for calculating u-values, marking a clear departure from the long-standing Standard Assessment Procedure (SAP).

The change aligns with broader UK ambitions outlined in the Future Homes Standard, but Wales is taking a notably stricter approach.

At the heart of the update is a shift from theoretical to real-world performance measurement. Under the Home Energy Model, each window must now be assessed based on its actual size and full configuration. This includes all components within the opening, such as mullions, transoms, Georgian bars, and ventilation elements, which were previously not fully accounted for.

According to Kevin Jones, Head of Technical at the Glass and Glazing Federation, this represents a fundamental change in methodology. By introducing area-weighted average calculations, the model ensures that every design detail contributes to the final thermal performance rating.

Crucially, Wales has set a more demanding limiting u-value of 1.4 W/m²K, compared to 1.6 W/m²K in England. This tighter threshold raises the bar for compliance and is expected to accelerate the adoption of higher-performance glazing solutions.

For many developers, this will likely mean a move toward triple-glazed units. While double glazing may still meet requirements in certain scenarios, the inclusion of additional frame elements and real-world variables makes compliance more challenging. Triple glazing, with its superior insulation properties, is increasingly seen as the most reliable route to meeting the new standards.

The regulatory update also aims to close the long-standing gap between designed and actual building performance. By focusing on in-situ effectiveness rather than lab-based assumptions, the Welsh Government is pushing the industry toward greater accuracy and accountability.

Transitional arrangements have been put in place. The new regulations will apply to all projects commencing after 4 March 2027, unless they are registered before 3 March 2027 and begin construction before 3 March 2028. Projects that do not meet these conditions will be required to comply fully with the new framework.