John McComb at Reynaers Aluminium UK discusses how the new, more rigorous method of calculating window performance is reshaping the fenestration industry.
Since the latest Future Homes Standard was published outlining the changes to Part L and Part F of the Building Regulations, much of the early industry commentary has understandably focused on what those changes will bring. However, now that the standard has finally arrived, perhaps the more important discussion should be about how energy performance must now be calculated.
Following a lengthy delay, the Future Homes Standard was finally published in March 2026, with much of the industry response to date focused on the detail of the extensive consultation. However, for new-build homes under Approved Document L1 (ADL1), the impact will prove to be far more significant than a series of regulatory revisions, representing instead a fundamental shift in how compliance is demonstrated.
In commercial fenestration projects this kind of detail oriented, project specific calculation has been considered best practice and commonplace for many years. The Future Homes Standard now requires the residential sector – particularly new-build housing – to adopt the same approach. Where compliance had previously relied on standard sizes and blanket energy ratings, every window, door and patio door must now be calculated on its specific dimensions and composition. In addition, hardware such as couplers must now be accounted for and distinguished between load-bearing and non-load bearing.
Why do these design implications matter? As each element must now be calculated individually, any change to a single U-value, even a small one, can significantly impact the entire build. For instance, if a door is widened by even a few millimetres to achieve compliance under the new standard this could reduce the square footage of an adjacent room. This is the sort of design decision that Tier 1 developers should be considering now.
The Government has confirmed that compliance will initially be assessed using SAP 10.3, while the longer-term intention is a transition toward the Home Energy Model (HEM). Once implemented, HEM will introduce a fundamentally different approach to energy assessment.
This transition represents far more than a software update. It marks a significant change in both the process and methodology used to demonstrate compliance in the new-build residential setting, requiring considerably greater levels of design accuracy, coordination and performance forecasting from the earliest stages of a project. Given the long lead times associated with specification, procurement and construction within the new-build sector, the implications of this new calculation methodology must be considered well in advance of formal implementation.
With the HEM model only being as good as the data provided, every window, door and facade element needs a verifiable U-value, G-value and frame factor, including couplers, add-ons, sills and thresholds – if suppliers don’t have the data ready, the building design will stall.
Design decisions being made today will directly influence HEM outcomes in the years ahead. Projects specified for completion in 2027 are already in the design stages, with key specifications, such as fenestration systems, being fixed now. These early choices set parameters that will ultimately determine the HEM results in 2027. With Approved Document L’s dates fixed in March and September 2027 the capability for Tier 1 developers to respond is deceptively short when you factor in the need to address verified data, in-house simulators, training of staff, and build supplier relationships. The methodological shift is significant: HEM pushes critical decision-making further upstream in the design process. Once key parameters are set, changes become increasingly difficult and costly, making early-stage decisions effectively locked in and difficult to reverse without substantial financial and programme implications.
The shift in methodology moves the burden of compliance and detail down the supply chain. Fabricators will be relied on to provide installers with the right data sets, and installers are then liable for ensuring the correct information is delivered to clients and assessors. Providing complete support for a project will need the right software, comprehensive data sets, and access to in-house simulation capability. Once a number is produced, interpreting that number and reconciling it with other impactful building regulations requires a high level of experience and competency across the board. This is where the calculation challenge becomes a competency one.
The combination of the calculation changes and competency requirement has exposed a market under pressure and decarbonisation cannot be achieved by improvements in fabric values alone; it requires a holistic approach that also recognises the ongoing decarbonisation of the electricity grid, and the Future Homes Standard is a necessary step towards achieving the UK’s net zero commitment by 2050. The new methodology that the Home Energy Model has introduced will play a significant part in how we as an industry deliver on our part of this commitment. The Future Homes Standard is not only a new target to meet, but also a shift in the way we work, and the market needs to be ready to implement that change now.
John McComb is technical director at Reynaers Aluminium UK