How the Home Energy Model will be different from SAP
The Home Energy Model, or HEM, is to be introduced in the UK to replace the Standard Assessment Procedure (SAP), which is currently used to demonstrate that new homes comply with Part L of the Building Regulations and to produce Energy Performance Certificates (EPC's). HEM is in many respects a much more accurate and detailed modelling tool and has been under development with continuing industry consultations for a long time. It is intended to be implemented alongside the Future Homes Standard (FHS) in 2025.
The Future Homes Standard (FHS) applies more stringent efficiency targets to new homes to reduce carbon emissions. The FHS will aim to improve heating and hot water systems, reduce heat waste, require new homes to be more energy efficient than those built under 2013 standards, reduce carbon emissions from new homes by 75-80%, make new homes "zero-carbon ready", and use low-carbon heating and other renewable energy sources. In a similar way to other tools it will look at the dwelling primary energy rate, fabric efficiency and emissions rate. It is in effect the next iteration of Part L of the Building Regulations, and also impact other parts of the Approved Documents such as Part F (ventilation).
The HEM, along with SAP (10.2) are building energy models which are tools to help model, with reference to building physics, how buildings perform under a set of variables, usually modelling energy balances and thus giving estimates or indications of potential energy consumption. Other building energy models, include PHPP which is specifically used to comply with the Passivhaus standard and ESP-r, which was a research based open source building modelling tool. In some ways these have been considered more specialist tools, whilst SAP is the standard tool required to evidence compliance with Building Regulations for dwellings.
SAP and its non-domestic partner SBEM might be considered as simpler tools, and perhaps intended initially to be more accessible. However its replacement which is still under consultation and development (HEM) is in many ways aiming to achieve some of the high accuracy functions of tools such as ESP-r, whilst retaining some simplicity through the use of wrappers. These different tools have a wide variation in capability, complexity and function, and as part of the consultation and validation process in the development of HEM the Department for Energy, Security and Net-Zero carried out an "Inter-Model Comparison Summary Report", the full report can be found here. The extracted table below shows a brief comparison of the various tools.
To manage the broad range of data points or inputs that will be covered by the HEM, it is likely that the model will have to use a number of assumptions rather than inputting individual data for each project. These assumptions may vary in relation to the purpose of the assessment, the occupancy patterns, localised weather patterns, links to other building standards, white goods, air tightness, ventilation, and so on. It is intended that these data points and assumptions, which may also expand with time, are simplified and included via assessment wrappers. The model is intended to be flexible enough to account for and allow for adjustments based on specific buildings while being simple enough to be workable.
| ESP-r | PHPP | SAP 10.2 | HEM | |
| Thermal zones | Multiple, unrestricted | Single zone | Multiple – restricted to living and non-living zones | Multiple, unrestricted. however the FHS assessment will be restricted to a living and non-living zone only. |
| Simulation type | Dynamic | Steady state | Steady state | Steady state |
| Simulation timestep | High resolution (i.e. half-hourly or less). | Monthly | Monthly | High resolution (down to half-hourly). |
| Simulation areas | Building physics, system performance of PV array (but not heating systems etc). | Building physics and system performance of various services. | Building physics and system performance of various services. | Building physics and system performance of various services. |
| Validation | Demonstrated to have good accuracy vs field test data. Comparable to other detailed simulations tools when subject to the Building Energy Simulation Test (BESTEST) and other empirical trials. | Has demonstrated accurate simulation space heating demand and annual energy use of high- performance dwellings in field trials. |
The current calculation method for Part L compliance and EPC ratings. It is not regarded to provide accurate simulation of dwellings in the field. It is |
N/A – new model |
| Role | Comparator model for dynamic calculations of zonal operative temperature, solar energy, and elemental energy flux. | Comparator model for monthly space heating demand, elemental energy flux and annual energy use and demand. | Reference model to demonstrate the step change expected for compliance modelling under the FHS. | N/A |
Table 1 Summary of the modelling packages applied in this inter-model validation study. (Extract from "Inter-Model Comparison Summary Report - Home Energy Model Validation" December 2023.
[edit] Related articles on Designing buildings
- Air permeability testing.
- Air tightness.
- Applying Fabric First principles: Complying with UK energy efficiency requirements FB 80.
- Approved building energy calculation software.
- Approved documents.
- Building Regulations.
- Building Research Establishment.
- Changes in SAP 10.
- Co-heating test.
- Conventions for U-value calculations (2006 edition) BR 443.
- Dynamic Simulation Model.
- Emission rates.
- Energy efficiency of traditional buildings.
- Energy Performance Certificates.
- Energy Performance of Buildings Directive.
- The Home Energy Model and Future Homes Standard assessment wrapper.
- IHBC responds to Energy Company Obligation ECO4 and PAS 2035.
- National Calculation Method.
- Passivhaus vs SAP.
- Standard Assessment Procedure SAP.
- Simplified Building Energy Model.
- Target fabric energy efficiency rate.
- Thermal dynamic analysis.
- U-value conventions in practice: Worked examples using BR 443.
[edit] External links
Featured articles and news
The Home Energy Model and its wrappers
From SAP to HEM, EPC for MEES and FHS assessment wrappers.
Future Homes Standard Essentials launched
Future Homes Hub launches new campaign to help the homebuilding sector prepare for the implementation of new building standards.
Building Safety recap February, 2026
Our regular run-down of key building safety related events of the month.
Planning reform: draft NPPF and industry responses.
Last chance to comment on proposed changes to the NPPF.
A Regency palace of colour and sensation. Book review.
Delayed, derailed and devalued
How the UK’s planning crisis is undermining British manufacturing.
How much does it cost to build a house?
A brief run down of key considerations from a London based practice.
The need for a National construction careers campaign
Highlighted by CIOB to cut unemployment, reduce skills gap and deliver on housing and infrastructure ambitions.
AI-Driven automation; reducing time, enhancing compliance
Sustainability; not just compliance but rethinking design, material selection, and the supply chains to support them.
Climate Resilience and Adaptation In the Built Environment
New CIOB Technical Information Sheet by Colin Booth, Professor of Smart and Sustainable Infrastructure.
Turning Enquiries into Profitable Construction Projects
Founder of Develop Coaching and author of Building Your Future; Greg Wilkes shares his insights.
IHBC Signpost: Poetry from concrete
Scotland’s fascinating historic concrete and brutalist architecture with the Engine Shed.
Demonstrating that apprenticeships work for business, people and Scotland’s economy.
Scottish parents prioritise construction and apprenticeships
CIOB data released for Scottish Apprenticeship Week shows construction as top potential career path.
From a Green to a White Paper and the proposal of a General Safety Requirement for construction products.
Creativity, conservation and craft at Barley Studio. Book review.
The challenge as PFI agreements come to an end
How construction deals with inherited assets built under long-term contracts.
Skills plan for engineering and building services
Comprehensive industry report highlights persistent skills challenges across the sector.
Choosing the right design team for a D&B Contract
An architect explains the nature and needs of working within this common procurement route.
Statement from the Interim Chief Construction Advisor
Thouria Istephan; Architect and inquiry panel member outlines ongoing work, priorities and next steps.