Why Thermal Modelling is a Game-Changer for Low-Energy, Comfortable Buildings

There’s a growing recognition in the UK that “energy efficient” doesn’t always mean “comfortable”. During a number of recent hot summers, even well-insulated new homes can get uncomfortably warm. It’s not that insulation is bad; it’s that we’ve become very good at keeping heat in, and sometimes forget to let it out.

That’s where thermal modelling comes in. It’s now one of the most useful tools for designing buildings that feel comfortable to live or work in.

Understanding what thermal modelling really does

Thermal modelling is a computer simulation that shows how heat moves through a building over time. It considers how sunlight hits the windows, how warm materials get, how air flows through rooms, and how people use the space. The software runs different scenarios – larger glazing areas, added shading, improved ventilation – to find a good balance between comfort and energy use.

There are two main ways to assess thermal performance. The simplified method uses fixed rules, which can work for small or straightforward homes. For anything more complex, we use dynamic thermal modelling. This method, based on guidance like CIBSE TM59, looks at conditions hour by hour using a full year of weather data. It’s detailed and realistic. It gives design teams a clearer view of how a building will perform.

Lessons from another hot summer

Each summer shows us more about how buildings work in the real world. Overheating is becoming more common, especially in new flats with lots of glazing. South-facing rooms that look great on paper can feel like greenhouses by July. This is partly because UK design has focused on keeping heat in, not getting rid of it. Add the effects of urban heat islands and warmer nights, and even modern homes can feel stifling.

Studies from the Passivhaus Trust show that small decisions – like window size, orientation, and ventilation – can make a big difference to summer comfort. The goal isn’t to lose efficiency, but to design it more carefully.

How Part O changed the outlook

Approved Document O (Part O) came in during 2022 to address overheating in new residential buildings. It has two routes for compliance: the simplified method, which uses set limits for glazing and ventilation, and the dynamic thermal modelling route, which uses tools like CIBSE TM59 to predict real-world conditions.

This change has pushed overheating higher up the design agenda. Design teams now consider daylight, shading, and ventilation early on, rather than leaving them as final checks. That’s improved the quality of decision-making at the start of projects. Thermal modelling can support compliance or provide evidence for other parts of Building Regulations, including Part L, as well as planning and carbon targets. It can also support up to 3 credits under the BREEAM HEA 04 Thermal Comfort criteria (depending on the scheme version and building type), making it useful for projects aiming to improve BREEAM scores.

At enevo, we find that using thermal modelling early in a project can avoid later delays and extra cost. Once construction begins, changes are harder and more expensive. At concept stage, models can be run, tested, and adjusted before anything is built.

Looking at the bigger picture

Thermal modelling shows the invisible side of building performance. It highlights where overheating is likely, how much energy systems may use, and where comfort and cost meet, and it’s not just about summer, good models also show how to keep buildings warm in winter without overusing heating systems. While the dynamic modelling process is primarily used to assess overheating risk, the same tools can also provide insight into year‑round thermal behaviour, including winter demand, something that matters significantly for both efficiency and occupant wellbeing.

We often model different layouts, window options, and shading for residential blocks. In some cases, we can lower predicted cooling demand by up to 20% without adding active systems. Small changes to ventilation schedules or internal layouts can fix problems before they happen. This kind of testing helps us make better design decisions from the start.

The road ahead

Thermal modelling is becoming more integrated with lifecycle carbon assessments and planning documents. It already supports Energy & Sustainability Planning Statements. That link will only strengthen as councils ask for stronger proof that buildings can handle rising temperatures.

The point is simple: comfort can be planned for. We can test it. We can prove it. And that means fewer surprises later on.

What matters most is not just arbitrarily ticking a box, but aiming to create places that feel good to live in. Comfortable in August. Warm in January. Enjoyable year-round.

If you’re starting a new project and want to get comfort right from the beginning, enevo can support you with thermal modelling and overheating assessments. We’ll help you make clear, tested design decisions before the build begins.