Two new certifications have been launched for window and doorset manufacturers, fabricators and service providers to meet the requirements of the Future Homes Standard, which will change the way U-Values for windows and doors must be calculated and declared in new dwellings from next year.
The new certifications have been created by BSI, the UK’s national standards body, in response to the Future Homes Standard, published in March as part of the UK Government's wider commitment to reaching net zero by 2050 by setting new energy efficiency requirements for new-build homes.
It introduces a significant change to how the thermal performance of windows and doors is assessed. Under the current rules, U-Values are calculated using a standard benchmark size and specification, a simplified approach that is not reflective of the product actually installed in most cases, leading to a performance gap between design and reality.
From 24 March 2027 in England and 4 March 2027 in Wales, Approved Document L of the Building Regulations will require a declared U-Value for the actual size and style of every product installed in a new home, a far more rigorous and technically demanding approach. A one-year transition period may apply on some projects.
To support the industry in navigating this change, BSI has developed two new Certificates of Conformity. BSI U-Value Certificate of Conformity for Window and Doorset Calculations) - is aimed at window and doorset fabricators and systems designers who carry out finite element analysis and thermal calculations. It provides independent third-party assurance that the underlying data inputted into U-Value calculation software is accurate and compliant with Approved Document L, BR 443, and international standards EN ISO 10077-1 and EN ISO 10077-2.
BSI Certificate of Conformity for U-Value Calculation Programmes of Windows and Doorsets - is aimed at software and technology providers whose calculation programmes are used to generate U-Values. It certifies that the programme computes results correctly in accordance with the formulas in EN ISO 10077-1, Cl.6.4.
This certification has been developed following nearly two years of close collaboration with stakeholders, including pilot clients involved throughout the development process.
Used together, the two create an end-to-end chain of assurance, from the accuracy of the underlying thermal data through to the software used to generate product-specific U-Values. Where a calculation programme uses input data certified under the U-Value Certificate of Conformity for Window and Doorset Calculations, BSI can additionally issue a UK Approved Body test report, enabling U-Value outputs to be used on a Declaration of Performance for UKCA marking.
Mark Barsby, Certification Technical Expert – Windows and Doors, BSI said: “Ensuring homes are energy efficient is critical to the UK’s transition to a lower carbon economy. The Future Homes Standard represents one of the most significant changes to how the thermal performance of windows and doors is assessed and declared in a generation. The use of robust, independently evaluated data and calculation tools can assist manufacturers in demonstrating to stakeholders that their calculation methods are transparent and in accordance with the new Approved Document L.
“These certifications are designed to provide the market with a clear, credible and standards-based level playing field for specifiers seeking independently verified U-Value calculations. We have incorporated input from stakeholders over the past two years to develop a solution that meets the needs of fabricators, systems designers and software providers alike.”
The regulatory change forms part of the UK Government’s wider drive to improve the energy efficiency of new homes, running the updated Standard Assessment Procedure (SAP 10.3) in parallel with the new Home Energy Model (HEM), which will operate alongside SAP 10.3 for at least two years pending its formal launch. Under both SAP 10.3 and HEM, the thermal performance of each window and doorset must reflect its specific building opening size, configuration and materials, making product-by-product U-Value calculations essential for accurate modelling.
BSI will host a dedicated webinar on 1 July 2026 to introduce both Certificates of Conformity to the industry, covering the new Approved Document L, what is changing and how it will work in practice. For more information on the event, please visit https://page.bsigroup.com/new-u-value-webinar
For more information on the BSI U-Value Certificate of Conformity for Window and Doorset Calculations and the BSI Certificate of Conformity for U-Value Calculation Programmes of Windows and Doorsets please visit the website.
