Following our recent blog discussing the implications of the Future Homes and Buildings Standards, we are continuing our series by delving into the details of the emerging Home Energy Model (HEM). The online assessment tool is set to be implemented in 2025, replacing the current Standard Assessment Procedure (SAP) methodology for domestic energy modelling, which is utilised to demonstrate compliance and generate Energy Performance Certificates (EPCs). We explore the main changes below:

Methodology transparency:

While HEM and SAP share similarities in their interfaces, HEM incorporates significant methodological changes. Notably, the source code for the HEM methodology has been published to enhance transparency and facilitate better understanding.

Level of detail:

The most notable change lies in the increased level of detail available for input. This enhanced model enables more accurate simulation of energy performance, utilising a half-hour timestep compared to SAP’s monthly timestep. Additionally, the new tool requires the selection of a local weather file, improving the accuracy of climate impact predictions on the site.

Users are also required to provide more specific information such as the number of bedrooms, energy ratings of household appliances and measurements of living spaces. Also required are detailed inputs regarding dwelling systems, including hot water distribution, insulation specifications, envelope area, dwelling volume, airtightness type and test results, and ductwork dimensions and insulation properties. These extra details significantly enhance the accuracy of the model’s energy calculations.

Heat loss accuracy:

The new tool introduces a more precise heat loss calculation methodology by allowing users to divide the building into thermal zones. This requires precise inputs for each element in the building fabric section of the tool. Users must specify the thermal resistance and thermal transmittance (U-value) of elements, along with solar absorption coefficients, mass distribution class and other relevant measurements. These details increase the granularity of the model’s energy calculations.

Areas for improvement:

Regarding lighting, users are only required to input the efficiency rating, while the heating section offers a limited selection of heating sources and systems. Currently, there is no accessible database for heating and ventilation equipment in the HEM tool, which remains in its beta version during its consultation stage (open until 6 March 2024), and will undergo further development before its official release.. Additional options for heating and ventilation sections will be incorporated as the tool progresses towards its final release.

Overall, this update aims to produce more accurate estimations of new the energy performance of new homes, narrowing the performance gap between predictions and actual operation. However, this refined method requires a substantial amount of additional information to generate meaningful results. Consequently, the project’s design team and building services consultants will have to consider how to best approach this additional layer of design during RIBA Stages 1 and 2.

Further information

Contact George Kfory and Dan Jestico