By Guy Willis-Robb, Director, Harley Haddow
While the built environment races to meet net zero targets, the framework guiding how progress is measured remains worryingly static. The Energy Performance Certificate (EPC), long the compliance standard for UK buildings, is increasingly unfit for purpose, particularly for the country’s heritage stock.
At first glance, EPCs appear to offer a simple means of comparison. Yet beneath the headline rating lies an assumption-based model that often fails to capture how buildings actually perform. The problem is especially acute in older properties where materials, systems and patterns of use differ sharply from the modern templates on which EPC methodology depends.
With more than a third of UK housing built before 1944, heritage buildings make up a significant portion of the nation’s carbon challenge. Their path to decarbonisation cannot follow the same blueprint as new build, and the question must be asked whether reliance on EPCs is actively holding back progress.
Policy misalignment
EPCs estimate energy demand using standardised assumptions about occupancy, weather and systems. Buildings, however, are not static. They breathe, shift and perform differently depending on numerous variables. Dynamic simulation modelling (DSM) offers a more realistic alternative. By using real weather files, occupancy profiles and material properties, it creates a living digital twin of a building’s performance.
At Harley Haddow, these digital twins have been shown to sit within 5% of monitored meter data compared with the 30 to 35% variance typical of EPC estimates. The implications are significant. When engineers understand how a building actually operates, interventions can be targeted to achieve measurable carbon savings rather than compliance scores that may bear little relation to reality.
A balanced approach to intervention
Heritage projects must balance conservation with decarbonisation. Too often EPC-led approaches push for fabric interventions such as window replacement or added insulation that offer limited benefit and risk damaging historic value.
Dynamic modelling supports a more strategic hierarchy of intervention. Replacing an existing boiler with a heat pump, for instance, can cut emissions by around 30%, while fabric upgrades may deliver less than half that impact at greater cost and complexity. Understanding these trade-offs is only possible through data-driven simulation.
Projects such as The Royal Institution, the Horniman Museum, the Wallace Collection and The RIBA show this approach in practice. By testing options virtually, teams can identify measures that deliver the greatest impact while protecting architectural significance.
Beyond lifecycle assumptions
As the industry shifts towards whole life carbon assessment, heritage buildings require a different lens. Their embodied carbon has long been accounted for, meaning the priority must now be operational performance. Applying lifecycle methodologies designed for new builds risks misdirecting effort and resources.
If regulation is to enable meaningful retrofit of heritage assets, policy must recognise this distinction. Dynamic modelling should sit at the centre of how performance is measured, not on the margins as an optional add-on.
Towards performance-led regulation
Heritage buildings are often portrayed as barriers to decarbonisation, yet with the right analytical tools they can become exemplars. Precision modelling brings conservation and sustainability together, allowing design teams to quantify outcomes before works begin and giving clients confidence that every investment drives measurable improvement.
The sector must move beyond box-ticking and embrace evidence-based regulation that reflects how buildings truly perform. Dynamic modelling does not tell us how a building could behave, it shows how it actually does.