The EPC wrapper is the policy configuration layer that sits on top of the Home Energy Model (HEM) core physics engine to produce Energy Performance Certificates. It applies standardised assumptions for occupancy, heating schedules, and fuel prices, then extracts the four headline metrics (Fabric Performance, Heating System, Smart Readiness, and Energy Cost) from HEM's half-hourly simulation results. The EPC wrapper is separate from the FHS wrapper used for building regulation compliance — both use the same physics engine but serve different purposes.
HEM's Modular Architecture
HEM is designed as a layered system. The core engine handles the building physics simulation — calculating heat loss, solar gains, heating system performance, and energy consumption at half-hourly intervals across an entire year (17,520 timesteps). Wrappers sit on top of this engine and define:
- Standardised assumptions — occupancy patterns, heating schedules, hot water demand profiles, and internal gains
- Input rules — what data is required, what defaults to apply when data is missing, and how to validate inputs
- Output metrics — which results to extract from the simulation and how to present them
- Compliance or rating rules — how to interpret the results (notional building comparison for FHS, A–G banding for EPCs)
This separation means wrappers can be updated independently. If the government decides to change EPC banding thresholds, only the EPC wrapper needs updating — the core engine and FHS wrapper remain untouched. Similarly, if the physics engine is improved, all wrappers benefit automatically.
FHS Wrapper vs EPC Wrapper
| Aspect | FHS Wrapper | EPC Wrapper |
|---|---|---|
| Purpose | Building regulation compliance for new homes | Energy Performance Certificates for all homes |
| Input data | Full design data from architect/assessor | Full data (new builds) or reduced data (existing homes) |
| Compliance method | Comparison against notional building | Four headline metrics with A–G banding |
| Key metrics | DPER, DFEE, carbon emissions | Fabric Performance, Heating System, Smart Readiness, Energy Cost |
| Occupancy assumptions | Standardised for compliance comparison | Standardised for fair comparison between properties |
| Applies to | New-build homes only | New-build and existing homes |
| Delivery | ECaaS API | ECaaS API |
Both wrappers run through the same ECaaS platform, ensuring every calculation uses the identical engine. Software providers build user interfaces on top of the API — the calculation itself is always centralised.
Standardised Assumptions
EPCs need to provide a fair comparison between different properties, so the EPC wrapper uses standardised assumptions that are the same for every assessment. These include:
- Occupancy: Standardised number of occupants based on floor area, following the same approach as current EPCs
- Heating schedule: Standardised heating periods and thermostat settings, applied consistently across all assessments
- Hot water demand: Standardised profiles for showers, baths, and tap use, modelled at individual outlet level in HEM
- Internal gains: Standardised assumptions for lighting, appliances, cooking, and metabolic heat from occupants
- Fuel prices: Standardised prices used for the Energy Cost metric, ensuring the rating reflects the property rather than current market conditions
- Carbon factors: Forward-looking factors based on the projected 2025–2029 grid mix
Reduced Data Methodology for Existing Homes
For existing dwellings, an assessor typically cannot determine every construction detail. The EPC wrapper addresses this through a modular input framework that is more flexible than RdSAP's rigid approach:
| Aspect | RdSAP (Current) | HEM EPC Wrapper (New) |
|---|---|---|
| Data model | Fixed questions in set order | Modular — record what you can observe |
| Unknown elements | Age-based defaults from lookup tables | Calibrated defaults from building characteristics |
| More detail available? | Limited ability to add detail | Additional data improves accuracy progressively |
| Calculation | Monthly steady-state | Half-hourly dynamic simulation |
| Software | Multiple third-party engines | Centralised ECaaS API |
| Consistency | Variation between software providers | Identical engine for every assessment |
The modular approach means that a basic EPC assessment, a more detailed building survey, and a full retrofit plan can all feed into the same HEM engine at different levels of detail. This creates a natural pathway from a basic EPC to a comprehensive energy retrofit strategy.
For more on the transition from RdSAP, see our RdSAP Transition page.
Delivery Through ECaaS
Both the FHS and EPC wrappers are delivered through the ECaaS (Energy Calculation as a Service) platform operated by MHCLG. This means:
- Every EPC assessment uses the identical calculation engine — no variation between software providers
- Software providers build user interfaces and data collection tools on top of the ECaaS API
- Updates to the engine or wrappers are deployed centrally and take effect immediately for all users
- The government retains control of the authoritative calculation, improving confidence in EPC accuracy and consistency
Frequently Asked Questions
What is a 'wrapper' in the context of HEM?
A wrapper is a policy-specific configuration layer on top of HEM's core physics engine. It defines standardised assumptions, metrics, and post-processing rules for a particular purpose. The FHS wrapper handles building regulation compliance; the EPC wrapper handles Energy Performance Certificates. Both use the same physics engine but with different assumptions and outputs.
Do the FHS and EPC wrappers use the same HEM engine?
Yes. Both wrappers use the identical HEM core engine, delivered through ECaaS. The core engine performs the half-hourly simulation. Each wrapper then applies its own standardised assumptions (occupancy, heating schedules, fuel prices, carbon factors) and extracts its specific metrics. The underlying physics is always consistent — only the policy overlay differs.
Can the FHS and EPC wrappers give different results for the same home?
Yes, because they use different standardised assumptions and output metrics. The FHS wrapper compares against a notional building and produces DPER and DFEE. The EPC wrapper uses standardised occupancy and produces the four headline metrics. A home that complies with the FHS would typically score well on all four EPC metrics, but exact ratings may differ.
How does the EPC wrapper handle missing data for existing homes?
The EPC wrapper includes a reduced data methodology for existing dwellings. When an assessor cannot determine a specific element, the wrapper applies calibrated defaults based on building age, type, and other observable characteristics. This is more flexible than RdSAP's rigid defaults — more detailed data can be entered where available for a progressively more accurate result.
Will there be other HEM wrappers in the future?
The modular architecture is designed for this. Potential future wrappers could include a retrofit assessment wrapper, a mortgage valuation wrapper, or local authority planning wrappers. The core engine and ECaaS platform stay the same — each wrapper simply adds different policy rules and output metrics on top of the same physics.
Related Pages
EPCs & HEM
Overview of how EPCs are changing under the Home Energy Model.
New EPC Metrics Explained
Full breakdown of the four new headline metrics.
RdSAP to HEM Transition
How RdSAP is being replaced for existing dwelling assessments.
What is the Home Energy Model?
Comprehensive introduction to HEM and its architecture.