Window systems and the new Part L – what the industry needs to know

John McComb, director of technical services for Reynaers

John McComb, director of technical services for the aluminium systems company, Reynaers, has urged specifiers, contractors and installers to prepare now, in readiness for the changes to Part L of the building regulations. It is anticipated that the changes announced on 15 December may come into legal force on 15 June 2022.

John said: The government has announced changes to Part L of the building regulations, which will come into force from June 2022. These are designed to reduce the carbon emissions of our buildings and will have significant implications for the design and construction of buildings – including window and façade systems.

What do the changes mean?
The Future Homes Standard will demand new buildings produce at least 75% less carbon emissions, when compared to current levels. To ease the way, the government has introduced new Part L regulations as an interim measure, in which all new buildings will need to generate 30% fewer CO2 emissions than the current regulations allow.

The new legislation means the thermal regulations for windows and openings are changing. The current U-values for the thermal transmittance of windows and openings into all settings (new and existing buildings) are to be upgraded. Take a look at the documents below to see the specific changes in U-value per application.

Approved Document L, volume 1: dwellings
Approved Document L, volume 2: buildings other than dwellings

What you need to do
We’d urge specifiers, contractors, fabricators, and installers to start to prepare now. We recommend you familiarise yourself with the Future Building Standards document, and ensure you understand implications for window systems, as well as other parts of the building envelope.

Also, consult with specialists, to understand how the change in U-value requirements will impact the aesthetics and cost of façade systems. This will enable you to begin planning future specifications and installations, so that you can select the most suitable products, that are fully compliant and meet the clients design requirements.

Rigorous evaluation and testing
Over the last year, we’ve rigorously benchmarked the new U-values against our current products and planned new products to ensure they comply. This involved a detailed finite elements calculation and analysis (FEA), using specific thermal modelling software. We also used a BISCO FEA analysis to calculate the thermal performance of all our products. As well as carrying out additional analysis associated with the British Fenestration Rating Council (BFRC), which is a labelling system for window products, to give our customers the utmost confidence in their purchasing decision.

Reynaers has a comprehensive range of products – windows, doors, sliding doors, folding doors, and curtain walls – that fully comply with the new regulations. Some of our products also exceed the new U-value requirement, providing additional thermal performance and making them futureproof, in lieu of additional regulatory changes in the future.

Be reassured with Reynaers
We’ve been preparing for these changes for well over a year, to ensure we can provide our customers with absolute reassurance that when they specify or install Reynaers products, they are fully compliant with the updated regulations. As a company, we’ve been closely involved in the development of the new regulations. I sit on various committees, and the company is also a member of the Curtain Walling and Cladding Technology (CTCT) group, which is actively involved, working in collaboration with trade bodies and the government.

To help support the industry, in the coming months we’ll be announcing a series of in-person and online events, as well as a range of guidance material, to make it as easy as possible for our customers to choose products that comply with the new Part L and meet the right performance and aesthetic standards. Visit to keep up to date.