The current EPC reform consultation has sparked debate over the level of survey required, and the best method for assessing a home's heat demand—specifically, the choice between the Fabric Energy Efficiency Standard (FEES) and the Heat Transfer Coefficient (HTC).
HTC measures a home's thermal losses—how much energy escapes per degree of temperature difference between inside and outside. It can be estimated via SAP calculations or directly measured through monitoring, its accuracy and practicality raise key concerns.
FEES provides a more holistic measure of a home’s heating and cooling demand per square meter. Crucially, it accounts for real-world factors such as solar gains, internal heat sources (occupants, appliances, hot water), and shading—elements that directly impact energy performance.
At first glance, a directly measured HTC figure from in-home monitoring might seem like the better choice. But both measurement and modelling have inaccuracies and impacts to consider before deployment. However, when considering accuracy, cost, and practicality for delivering Energy Performance Certificates (EPCs), the case for FEES is stronger.
Accuracy: The SMETER project found that monitoring was no more accurate than RdSAP in the monitoring of 40 social homes. And our experience of monitoring over the years has faced real-world challenges such as unreliable smart meter data, battery failure, resident movement of sensors, and heat transfer through party walls. All of these compromise the reliability of HTC measurements, and issues cannot necessarily be spotted from remote monitoring. Without a robust verification framework, the risks to accuracy are significant.
Cost & Practicality: HTC monitoring adds time, cost, and disruption for homeowners. It would require at least two home visits and an absolute minimum of 3 days monitoring (more for improved accuracy). It would also require significant scaling up of skills and number of Domestic Energy Assessors (DEAs). We think that monitoring adds more value later in the retrofit journey, when homeowners are engaged, and prepared for disruption; and it is in the residents and contractors’ interest to get accurate readings – particularly if post-works monitoring is also planned.
For EPCs to be both robust and practical, Cotality’s preferred approach is for survey data rather than monitoring. We support the MHCLG’s proposal that heat demand be assessed using FEES, thereby ensuring that real-world factors—occupancy, glazing, and shading—are fully integrated into the calculation.
But we also support the development of a regulated framework for monitoring that builds consumer, and other stakeholders', confidence in services that rely on the data. With that in place, we think performance monitoring has a great future to support homeowners, lenders and policy delivery.
#EnergyEfficiency #EPCReform #FEESvsHTC #NetZeroHomes #Retrofit
