Elmhurst Receives Home Energy Model Methodology Schema for Existing Homes
Elmhurst Energy has received the confirmed Home Energy Model (HEM) methodology for existing dwellings, marking a significant milestone in the transition towards the next generation of domestic Energy Performance Certificates (EPCs).
The methodology, currently known by the industry as HEMIE, sets out how the energy performance of existing homes will be calculated under the new EPC framework, which is being developed to replace the current Reduced Data Standard Assessment Procedure (RdSAP).
Our technical teams are now reviewing the methodology to understand the key changes and what they will mean for Domestic Energy Assessors (DEAs).
How Does the Home Energy Model Work and How Will It Apply to Existing Home EPCs?
The Home Energy Model (HEM) is the Government’s next-generation calculation methodology, designed to replace SAP and RdSAP. Initially developed to support the Future Homes Standard for new-build dwellings, HEM is also being adapted for existing homes as part of wider EPC reforms.
The recently released HEMIE schema sets out how HEM will be applied to existing dwellings, including the information used to calculate a property’s energy performance. It introduces a more detailed approach to energy modelling, while incorporating simplified inputs and default assumptions to support practical assessments. You can find out more about how the Home Energy Model works in our dedicated article.
What Are the Key Changes?
The Home Energy Model (HEM) introduces a more detailed and flexible approach to assessing a property’s energy performance than the current SAP and RdSAP methodologies. While the underlying calculations are significantly more sophisticated, HEMIE has been designed to simplify data collection by using defaults, automated geometry generation and inferred values wherever possible.
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HEMIE moves from monthly calculations to dynamic half-hourly modelling, allowing it to better reflect changes in energy demand, heating system performance, occupancy behaviour, renewable generation and electricity use throughout the day.
This provides a more realistic representation of how homes actually perform in use.
HEMIE uses significantly more detailed information about building geometry than current RdSAP methodologies.
Individual walls, roofs, floors, windows and doors can be represented with their own orientation, pitch and dimensions. The model also introduces enhanced treatment of complex features such as rooms-in-roof, dormers, bay windows and conservatories.
In addition, HEMIE will provide more sophisticated solar gain and shading calculations, accounting for nearby and distant obstructions such as neighbouring buildings, trees, balconies, roof overhangs and other shading features.
Rather than relying primarily on age-band assumptions, HEM can model building elements using documented U-values, simplified construction descriptions or detailed layer-by-layer build-ups. The model also introduces improved treatment of thermal mass, solar absorption and thermal bridging, enabling a more representative assessment of how building fabric affects energy demand and internal temperatures. Walls between a heated dwelling and an attached or integral garage will now be recorded as sheltered walls, with the adjacent space type set to “garage”.
Rather than relying primarily on age-band assumptions, HEMIE can model building elements using documented U-values, simplified construction descriptions or detailed layer-by-layer build-ups.
The model also introduces improved treatment of thermal mass, solar absorption and thermal bridging, enabling a more representative assessment of how building fabric affects energy demand and internal temperatures.
For example, walls between a heated dwelling and an attached or integral garage will now be recorded as sheltered walls, with the adjacent space type set to “garage”.
HEMIE includes much more detailed modelling of heating and hot water systems. Assessments can account for a wider range of technologies, including heat batteries, heat networks, smart hot water tanks and advanced controls. The model also considers heat distribution systems, emitter characteristics, buffer tanks, thermal stores and primary pipework.
Heating emitters such as radiators and underfloor heating are represented in greater detail, helping to improve the accuracy of system performance calculations. Emitter characteristics are determined through emitter size and type.
HEMIE improves the way low-carbon and smart technologies are assessed. It allows assessors to record more information about solar PV systems, including inverter details, and also supports features such as battery storage, PV diverters and smart hot water systems.
The methodology is designed to better reflect how modern homes use and manage energy, including any relevant constraints on electricity imported from or exported to the grid.
HEMIE provides a more comprehensive assessment of building performance by incorporating detailed modelling of cooling and ventilation systems, as well as the factors that influence overheating.
The methodology considers window opening behaviour, shading, thermal mass, infiltration, air permeability and trickle vents.
For example, air conditioning and cooling heat pump systems can be modelled using performance-based inputs and system sizing data.
Within HEMIE assessors will record the dwellings appliances such as, dishwasher, clothes drying appliance, separate fridge/freezer and oven/hob fuel type.
All other appliances are assumed using a standardised appliance profile, while lighting energy use and heat gains are calculated automatically with no assessor input required.
HEMIE also creates opportunities to incorporate measured building performance data where available. For example, measured Heat Transfer Coefficient (HTC) results from approved SMETER tests can be recorded alongside modelled values, providing additional information about a dwelling’s real-world thermal performance.
What Does This Mean for Energy Assessors?
For most existing homes, the information collected for a HEMIE assessment is expected to remain largely the same as that gathered by DEAs today, while benefiting from a more flexible and modular framework. Assessors will continue to record key details about a property’s construction, insulation, heating systems, glazing, ventilation and renewable technologies. Where precise information is not available, HEMIE may use suitable defaults, assumptions or estimated values.
As the methodology and supporting guidance continue to develop, Elmhurst’s technical teams are reviewing the changes to understand any potential impact on day-to-day assessment practices and to identify any additional information assessors may be required to collect.
Supporting Members Through the Transition
Receiving the confirmed methodology is an important step in understanding how the new EPC system will work and preparing for its introduction.
Elmhurst will continue to support members throughout the transition, providing guidance, training and resources to help them understand the changes and prepare for the new assessment requirements.
Members are encouraged to look out for further communications from Elmhurst, where we will share important updates, key information and any actions they may need to take.