If you're designing homes in the UK, this is not just another acronym to ignore. The HEM Calculation (short for Home Energy Model) is quietly but fundamentally transforming how we evaluate the energy performance of dwellings. And if you’re an architect, consultant, or developer, this matters more than you might realise.
With roots in detailed thermal simulation and a mandate from future regulation, HEM is set to replace SAP as the compliance method underpinning Part L and the Future Homes Standard. The jump is comparable to going from a rough sketch to a high-definition 3D model that updates every 30 minutes.
Moving Beyond Assumptions
In the UK’s next generation of energy compliance, dwellings will no longer be treated as simplified shells. The Home Energy Model, or HEM, introduces a form of simulation that captures the real behaviour of homes. Each property is divided into thermal zones. Every 30 minutes, the model calculates how heat moves within these zones, between walls and air, through glazing, across floors and ceilings, into and out of storage systems. It simulates what happens in response to weather, shading, control schedules and user behaviour. This isn’t theoretical abstraction. It is a complete shift in how we understand building performance.
If a designer specifies a south-facing bedroom with minimal shading, HEM will model what the internal temperature looks like on a hot day, how it changes hour by hour, and whether it crosses thermal comfort thresholds. If a heat pump is oversized or a battery undersized, the model reflects those inefficiencies. It applies realistic losses to distribution pipework, calculates the thermal benefit of shading devices, and considers solar gains not just by orientation but by time of day and season. Airtightness values, thermal mass, control schedules and hot water pipe layouts are all treated as core variables, not estimations, not placeholders.
This changes the foundation of energy compliance. SAP provided average outcomes based on typical conditions and notional systems. HEM discards that method. It replaces assumptions with physics. That has implications across every professional role.
A Shift in How Buildings Are Designed Under The Home Energy Model
Architects will feel the shift immediately. Glazing ratios, façade orientations, materials and nearby buildings affect both energy demand and comfort. Thermal mass becomes more than a passive benefit. The use of blinds, frame types or curtain walling is no longer a matter of visual or cost preference. These details alter performance outcomes in measurable ways. Architects will need to collaborate earlier with assessors and services engineers, and make design choices with energy performance explicitly in mind.
For MEP consultants, the model sets a new bar for input precision. Heating systems are not defined by static efficiencies. Cycling penalties are calculated. Hot water systems must be defined through their pipework layouts, distribution losses, and operational schedules. Thermostat behaviour and temperature setbacks are no longer theoretical. If a system turns off too late, or cycles inefficiently, HEM will show it. Controls must be explained, not assumed.
Developers face both risk and opportunity. The data burden is higher. Products require performance documentation. Time allowances for modelling and compliance will increase. But HEM also removes ambiguity. It reduces the risk of speculative assumptions. It rewards specification discipline. Most significantly, it makes a clear distinction between well-designed systems and poorly integrated ones. For developers who invest in quality, this is a competitive advantage.
Real Conditions, Real Data, Real Outcomes
Weather is no longer a backdrop. It becomes an active agent in every timestep. External temperature, wind speed and solar radiation are imported from hourly weather files. They affect heating and cooling demand, infiltration, ventilation systems and PV generation. The model tracks solar position, recalculates shading, and adjusts building responses in real time.
Heat absorbed in a concrete slab is released gradually over the day or into the night. Internal temperatures are not reset with each new day. They evolve based on building physics. Past events affect future performance. The same is true for storage systems. Batteries maintain a state of charge and discharge. Losses accumulate.
The result is a model that rewards accountability. Where data is lacking, HEM applies conservative defaults. These defaults are not forgiving. They are there to protect regulatory integrity. The energy assessor’s role becomes more than computational. It becomes interpretative, diagnostic and strategic.
There are practical challenges. Modelling takes longer. Software tools are still evolving. And transitioning from SAP to HEM will involve a learning curve. But the benefits are substantial. HEM brings clarity to what was once opaque. It gives design teams real feedback, before construction begins. It turns energy compliance from an end-of-project box-tick into a design tool.
For the first time, the UK’s energy model will reflect the way buildings actually work. That is not a regulatory burden. It is a professional opportunity.
Energy Digest Is Ready to Support Your Home Energy Model
As The Home Energy Model becomes the new benchmark, you’ll need an assessment partner who understands how to translate complex building data into reliable, actionable models. Energy Digest is ready to deliver exactly that. We provide full HEM-compliant modelling with a seven working day turnaround and the most competitive pricing in the market. Whether you're a developer preparing for the Future Homes Standard or a design team looking to optimise early-stage layouts, we can support your project from start to sign-off, with speed, clarity and technical confidence.
