What has changed?
Approved Document L sets out the energy performance requirements for buildings in England. The most recent updates have raised the bar considerably for both new and replacement window installations. Thermal performance, airtight sealing, and accurate energy calculations are no longer optional elements of a good installation — they are mandatory requirements that Building Control will expect to see evidenced.
The industry has moved beyond simple fabrication. Supplying and fitting a window with a good centre-pane U-value is no longer sufficient on its own. The whole installation — the junction detailing, the airtightness tapes, the thermal bridging at the frame — all of it now falls under scrutiny.
What does verifiable energy performance actually mean?
Building Control and clients increasingly expect documentation to back up performance claims. That means:
U-value calculations — the whole window U-value, not just the glazing unit, needs to be calculated and evidenced for the specific installation.
? (PSI) value calculations — the linear thermal transmittance at window junctions needs to be understood and accounted for. This is where thermal bridging at the frame-to-wall junction has a direct impact on the overall energy performance of the building.
Airtightness detailing — the specification and installation of the correct tapes and membranes at window reveals is now a compliance issue, not just a best-practice recommendation.
The challenge for installers and fabricators
Every building is different. In retrofit particularly, wall makeups vary, reveal depths differ, and existing construction is rarely consistent. Getting the calculations right and selecting the correct products for each specific situation requires technical knowledge that goes beyond traditional window installation expertise.
That’s a genuine challenge for businesses that have historically focused on fabrication and fitting rather than energy performance calculations and building physics.
How Lynvale can help
At Lynvale we work directly with window installers and fabricators to take the complexity out of Part L compliance. We supply the products — expanding foam tapes, airtightness membranes, and thermal break solutions — but we also provide the technical support to ensure they’re specified and installed correctly for each project.
Our team can assist with:
- U-value calculations for specific window installations
- ? (PSI) value calculations at window junctions
- Airtightness detailing advice and product specification
- Guidance on meeting Part L requirements for both new build and retrofit projects
If you’re an installer or fabricator feeling the pressure of Approved Document L and want straightforward technical support alongside quality products, get in touch with the Lynvale team.
