Industry urged to confront persistent myths around u-values and Part L compliance

According to Paul Booth, R&D Director at AluK, the final months of 2024 saw a welcome shift in industry sentiment around U-Value compliance, as more voices began echoing what AluK has long maintained: that the current approach to U-Value calculations across parts of the supply chain is not only inconsistent, but in many cases, fundamentally flawed.

At the heart of the issue lies a worrying trend. Some systems companies are passing down data that simply does not add up. Whether this is due to oversight or a deeper misunderstanding of the regulations remains unclear. What is certain, however, is that relying on incorrect information leaves customers exposed to the very real risk of non-compliance with Building Regulations – a risk no responsible supplier should permit.

While errors in calculation are a pressing concern, they are far from the only issue. A number of persistent myths continue to circulate in the market, contributing to confusion about what Part L compliance actually requires.

Misconception one is that energy ratings such as DSERs and WERs can be used to demonstrate compliance on new-build developments. This is incorrect. Energy ratings apply only to replacement installations and extensions – not new homes. Using them as evidence of compliance in new builds is a misstep with potential regulatory consequences.

The second myth relates to the use of the CEN standard window shown in BS EN 14351 – the 1230 x 1480 single opener – as a proxy for U-Value compliance. This too is false. The current version of Part L offers no scope to use this configuration. Instead, calculations must follow guidance set out in the Building Research Establishment’s BR443, using actual window sizes and configurations or specified standard sizes aligned with recognised formats for casement, sash, and roof windows.

Thirdly, there remains a mistaken belief that bifold doors can be assessed using a French door sample configuration to demonstrate U-Value compliance. Again, Part L is explicit: when calculating U-Values for a product range, the configuration used must be the worst-performing within that range. Moreover, the calculation must use defined standard sizes based on the overall area of the door – a far cry from the simplified French door approach still being employed by some.

Finally, some continue to claim that CE/UKCA marking is unnecessary for windows and doors as long as the product complies with Building Regulations. This is simply incorrect. All windows and doors placed on the UK market are legally required to carry CE or UKCA markings, and thermal performance is among the essential characteristics that must be declared.

At AluK, we remain committed to raising industry standards, with regulatory compliance central to that mission. Admittedly, Part L presents challenges. Sliding doors, for example, can be particularly difficult to improve due to the offset nature of thermal breaks in their sashes and frames. Yet innovation is not standing still. Our own S140 Lift and Slide system incorporates enhanced thermal breaks to meet – and in many cases exceed – current regulatory requirements.

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However, not all systems on the market have kept pace. Older, unenhanced designs continue to be sold, despite the growing difficulty of aligning them with today’s standards.

As the industry looks ahead, the path to compliance lies not in shortcuts or misinterpretations, but in a collective effort to understand and apply regulations accurately. Dispelling these myths is a necessary first step.