Thermal Modelling - overheating, operational energy & compliance

What is dynamic thermal modelling?

Dynamic thermal modelling uses simulation software to understand how a building behaves over time. It takes account of weather conditions, occupancy patterns, building fabric, glazing, internal gains and mechanical systems. Rather than a single snapshot, it produces hour-by-hour results, giving a more realistic picture of temperatures, energy use and system loads.

What software does KJ Tait use for thermal modelling?

We use IES VE for dynamic thermal modelling. It is widely used across the UK and recognised by planning authorities, BREEAM assessors and NABERS UK. It allows us to test design options in detail and produce outputs that align with industry standards.

What is an overheating assessment and when is it required?

An overheating assessment looks at whether a building is likely to experience uncomfortable or excessive temperatures during warmer periods. For residential schemes, this is typically assessed using CIBSE TM59. For non-residential buildings, TM52 is used. Many planning authorities require overheating analysis as part of an application, and it is also needed for BREEAM. In England, Part O of the Building Regulations introduced specific overheating requirements for new homes.

What is the difference between TM52 and TM59?

TM52 is used to assess overheating risk in non-residential buildings, while TM59 applies to residential developments. The difference reflects how buildings are used, particularly the importance of night-time comfort in homes. Both methods rely on dynamic simulation and assess how often temperatures exceed defined comfort thresholds.

What is TM54 operational energy modelling?

TM54 modelling is used to predict how much energy a building is likely to use once it is occupied. It goes beyond compliance calculations by including all energy uses, such as small power, lifts and equipment. This results in a more realistic estimate of operational energy. TM54 is commonly used for NABERS UK Design for Performance and aligns with the UK Net Zero Carbon Buildings Standard, which focuses on measured performance rather than theoretical targets.

How is thermal modelling used in planning applications?

Thermal modelling plays a key role in planning submissions. It underpins energy strategies and overheating assessments, helping demonstrate that a proposal meets local policy requirements. In London, for example, the GLA requires detailed energy assessments for major developments. Modelling outputs are used to show how a design performs, how overheating risk is managed and how carbon targets will be met.

At what stage of a project should thermal modelling begin?

The earlier it starts, the more useful it is. At concept stage, modelling can test key decisions such as orientation, glazing ratios, shading and thermal mass before they are fixed. Leaving it until later limits what can be changed and often leads to mechanical systems being used to compensate for earlier design choices.

Can thermal modelling help size heating and cooling plant?

Yes. Dynamic simulation provides accurate peak heating and cooling loads based on how the building will actually be used. This allows systems to be sized properly — avoiding oversizing, which increases cost and reduces efficiency, and undersizing, which can lead to comfort issues.