HEM Standards Reference: International Standards Underpinning the Home Energy Model

The Home Energy Model (HEM) is built on a suite of international standards from the ISO 52000 family and the European EN series. The core standard is BS EN ISO 52016-1:2017, which provides the hourly dynamic thermal simulation method that HEM extends to half-hourly resolution. Additional standards govern solar irradiance calculations, fabric thermal performance, ventilation modelling, and heat pump performance testing. Together, these standards give HEM a rigorous, peer-reviewed scientific foundation, a significant departure from SAP's simplified monthly methodology, which derives from the BRE Domestic Energy Model.

Standards Mapping to HEM Modules

The table below maps the principal standards to the HEM module each underpins and the technical paper that documents its implementation. Others appear in individual modules: BS EN 15316-4-3:2017 for PV output, BS EN 15316-5:2017 for hot water storage tanks, ISO 15927-4 for the FHS weather file, BS EN ISO 12241:2022 for pipe and duct insulation, and BS EN 13203-2 for combi boiler losses.

BS EN ISO 52016-1:2017: The Core Standard

BS EN ISO 52016-1:2017 is the foundation of HEM's calculation engine. It defines the hourly method for calculating the energy needs for heating and cooling, internal temperatures, and sensible and latent heat loads in residential and non-residential buildings. This standard replaced the older BS EN ISO 13790:2008, which provided only monthly and simplified hourly methods and which SAP still cites among its references.

The standard specifies a node-based thermal network model where each building element (wall, floor, roof, window) is represented by a series of thermal resistance and capacitance nodes. At each timestep, the model solves the heat balance equation across all nodes in each zone, accounting for:

• Fabric heat loss through the thermal envelope: conduction through walls, floors, roofs, windows, and doors
• Ventilation heat loss: energy carried away by air exchange between inside and outside
• Solar gains: direct and diffuse radiation entering through glazing and absorbed by opaque fabric
• Internal gains: heat from occupants, lighting, appliances, and cooking
• Thermal mass effects: heat absorbed and released by the building fabric over time

HEM extends the standard's hourly method to half-hourly resolution (17,520 timesteps per year), providing finer granularity for modelling fast-responding technologies such as heat pumps, battery storage, and smart controls. The dynamic thermal modelling at this resolution means HEM captures effects that SAP's monthly averaging cannot represent, for example how a heavyweight concrete floor absorbs solar gains during the day and slowly releases stored heat overnight. For the full calculation methodology, see How HEM Calculates.

BS EN ISO 52010-1:2017: Solar Irradiance

BS EN ISO 52010-1:2017 provides the method for converting measured or modelled climate data into the solar irradiance values needed by the thermal calculation. It specifies procedures for determining the direct and diffuse components of solar radiation on arbitrarily oriented and tilted surfaces, accounting for the sun's position throughout the year.

Within HEM, this standard underpins the solar gains module documented in HEM-TP-08. At each half-hourly timestep, HEM calculates the solar irradiance on every building surface based on the weather data, surface orientation, tilt angle, and shading from surrounding obstructions. This feeds into the ISO 52016-1 heat balance to determine how much solar energy enters through glazing and how much is absorbed by opaque elements. SAP, by contrast, used monthly radiation values applied only to windows, a far coarser approach that could not capture the daily and seasonal variation in solar gains.

BS EN ISO 6946:2017: Fabric Thermal Performance

BS EN ISO 6946:2017 specifies the methods for calculating the thermal resistance and thermal transmittance (U-value) of building components, excluding doors, windows, and components through which air is designed to permeate. It covers elements made up of thermally homogeneous layers and provides correction methods for air gaps, mechanical fasteners, and precipitation on inverted roofs.

HEM uses this standard as the basis for fabric heat loss calculations documented in HEM-TP-05. Every opaque building element (wall, floor, and roof) has its U-value calculated according to ISO 6946, which then feeds into the ISO 52016-1 thermal network at each timestep. The standard also provides the surface resistance values used for the internal and external boundary conditions of the node-based model.

BS EN 16798-7:2017: Ventilation Calculations

BS EN 16798-7:2017 defines calculation methods for determining air flow rates in buildings, including methods for natural ventilation, mechanical ventilation, and infiltration through the building fabric. It provides a pressure-driven model that accounts for wind pressure on the building envelope and stack effect (buoyancy-driven airflow due to indoor/outdoor temperature differences).

This standard is implemented in HEM through HEM-TP-06, which models air movement through specific paths: purpose-provided ventilation openings, extract fans, MVHR systems, and infiltration through the fabric. At each timestep, HEM calculates the pressure balance across the building envelope and determines the resulting air flow through each path. This is critical for accurately modelling airtight homes with MVHR (Mechanical Ventilation with Heat Recovery), where the interaction between controlled ventilation and residual infiltration significantly affects energy performance.

SAP's ventilation model, by comparison, used simplified wind and shelter factors that could not capture these dynamic pressure-driven effects. HEM's approach properly credits well-installed MVHR systems and accurately penalises poor airtightness, reflecting the real-world energy impact of ventilation design decisions. This is directly relevant to Part F compliance under the Future Homes Standard.

Heat Pump Standards: EN 14825, EN 15316-4-2, and EN 16147

Three European standards govern how heat pump performance is characterised and calculated within HEM. Together, they enable the dynamic, temperature-dependent modelling of heat pump efficiency that is one of HEM's most significant improvements over SAP.

EN 14825: Seasonal Performance Testing

EN 14825 specifies the test conditions and calculation method for determining the seasonal coefficient of performance (SCOP) and seasonal energy efficiency ratio (SEER) of heat pumps. It defines performance data at multiple part-load conditions and outdoor temperatures, providing the characteristic performance curves that HEM uses to interpolate the COP at any given operating point.

Within HEM, the EN 14825 test data feeds into the HEM-TP-12 heat pump methodology. At each half-hourly timestep, HEM looks up the heat pump's performance at the current source temperature (outdoor air for ASHPs, ground for GSHPs) and sink temperature (flow temperature required by emitters), then calculates the actual COP under those specific conditions. This means a well-designed system with large, low-temperature emitters will show demonstrably better performance than one with undersized radiators requiring higher flow temperatures.

EN 15316-4-2: Performance Calculation

EN 15316-4-2 provides the calculation methodology for determining the energy consumption of heat pump systems for space heating. It specifies how to account for part-load operation, cycling losses, auxiliary energy consumption (pumps, fans, controls), and back-up heating. HEM implements this methodology within HEM-TP-12 to determine the total electrical energy consumed by the heat pump system at each timestep: not just the compressor energy, but the full system draw including circulation pumps, fans and crankcase heaters. Defrost is not calculated separately; it is carried implicitly by the low-temperature EN 14825 test points.

EN 16147: Water Heating Performance

EN 16147 defines the test method for heat pump water heaters, specifying standardised tapping cycles that represent typical domestic hot water use patterns. HEM-TP-12 uses it for one case only: heat pumps that provide water heating alone. Load profile M is required, with profile L used as well where the manufacturer has submitted it. A heat pump that also provides space heating is calculated from its EN 14825 data instead, with the cylinder as the sink. Either way, hot water production requires higher flow temperatures than space heating, which reduces the heat pump's COP during those periods.

SAP Standards Basis vs HEM Standards Basis

One of the most important changes in HEM is the shift from SAP's older, simplified standards to the modern ISO 52000 family and updated EN series. The table below summarises this transition.

The shift to these modern standards is not merely academic. It directly enables HEM's ability to model the dynamic behaviour of low-carbon technologies, particularly heat pumps, which SAP's simplified seasonal factors could not properly represent. For a non-technical explanation of why this matters, see our SAP vs HEM comparison.

UK Building Regulations Context

The international standards underpinning HEM do not exist in isolation. They serve as the scientific basis for demonstrating compliance with UK Building Regulations. Three Approved Documents are directly relevant.

Part L: Conservation of Fuel and Power

Part L sets the energy performance requirements for new dwellings. The Future Homes Standard represents the next major update to Part L, requiring at least a 75% reduction in carbon emissions compared to 2013 standards. HEM, using the ISO 52016-1 dynamic simulation, will be the primary calculation tool for demonstrating Part L compliance under the FHS. During the transition period, SAP 10.3 will also be available as a compliance route.

Part F: Ventilation

Part F sets the requirements for adequate ventilation and indoor air quality, expressed as minimum extract, whole-dwelling and purge rates. Compliance is demonstrated against those rates, not through the energy calculation. What HEM's implementation of BS EN 16798-7 adds is the energy consequence of running the chosen system: the ventilation heat loss, the heat recovered, and the fan electricity, resolved at each timestep. A dwelling can pass the HEM energy assessment and still fail Part F.

Part O: Overheating

Part O addresses the risk of overheating in new dwellings, a growing concern as buildings become more insulated and airtight, and as summers become warmer. It has two compliance routes: the simplified method in Approved Document O section 1, which sets maximum glazing areas and minimum free areas by category of residential building, and dynamic thermal modelling to CIBSE TM59 under section 2. HEM is not the compliance vehicle for either, and no overheating assessment forms part of the Future Homes Standard calculation. What HEM's half-hourly internal temperatures give designers is a view of the same physics during design, before a TM59 model is commissioned.

Key Organisations

Several organisations play critical roles in the development, governance, and application of the standards that underpin HEM.

• MHCLG (Ministry of Housing, Communities and Local Government): owns the Building Regulations framework and operates the ECaaS platform that delivers HEM as a centralised calculation service
• DESNZ (Department for Energy Security and Net Zero): commissioned the development of HEM and owns the energy performance methodology policy
• BRE (Building Research Establishment): one of the organisations named in the technical papers as contributing to HEM's development, and the body that runs the PCDB. It does not host the code: the Python reference implementation is published on Azure DevOps and the Rust implementation on MHCLG's GitHub
• Etude: leads the independent quality assurance consortium, whose members also include Levitt Bernstein, People Powered Retrofit, the University of Strathclyde's Energy Systems Research Unit, Julie Godefroy Sustainability and UCL
• BSR (Building Safety Regulator, under HSE): provides building control oversight and enforcement of the Building Regulations that these standards support
• CIBSE (Chartered Institution of Building Services Engineers): publishes technical guidance, weather data files used by HEM, and professional standards for building services engineering
• Future Homes Hub: provides industry guidance and resources for the transition to the Future Homes Standard, including practical interpretation of how these standards affect design and construction

Government Documents and Further Reading

The following government publications provide authoritative detail on how these standards are applied within HEM and the wider regulatory framework.

• HEM Technical Documentation: the complete set of HEM-TP technical papers documenting each calculation module and its standards basis
• HEM Consultation: the original consultation on replacing SAP with HEM, including the October 2025 government response
• Future Homes Standard 2023 Consultation: the regulatory framework that HEM will underpin for new-build compliance
• ECaaS Guidance: how to access the HEM calculation engine through the centralised cloud API
• HEM Rust Implementation (GitHub): the open-source performance implementation of HEM maintained by MHCLG

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