Overheating Calculations
for Part O Compliance
Ensuring your building meets Building Regulations Part O
requirements for overheating
Expert assessments across the UK
What are Overheating Calculations?
Overheating calculations assess the risk of overheating within buildings during warmer months, helping to ensure a building is a comfortable and healthy environment for occupants.
The overheating calculation process evaluates several key factors including:
- Solar gain: the amount of heat a building absorbs from sunlight
- Internal heat gains: come from everyday activities and the use of electrical devices
- Ventilation rate: how well heat in a building can escape
- Thermal mass: how a building’s materials store and release heat
Do I Need an Overheating Calculation?
If you are designing or constructing a new residential building in the UK, you will need to carry out overheating calculations to comply with Part O Building Regulations.
Part O (Approved Document O) applies specifically to new residential developments such as houses, flats, care homes, and student accommodation.
It sets out the legal requirements to limit unwanted heat gain and provide adequate ventilation for occupants.
You do not need overheating calculations if you are adding an extension to an existing home or if a building is undergoing a material change of use, as Part O does not apply in these cases.
For non-residential buildings, Part O is not applicable, but overheating risk is still addressed through other guidance, such as Part L of the Building Regulations and CIBSE TM52, which focus on thermal comfort and energy efficiency.
Overheating Assessment Methods
There are two recognised ways to demonstrate compliance with Part O of the
Building Regulations – the Simplified Method and Dynamic Thermal Modelling.
The Simplified Method
Simplified method calculations are a fast way to assess the risk of overheating in a building.
This approach is based on the principle that overheating is driven by excess solar radiation and insufficient ventilation.
The simplified method uses standard assumptions and generalised data to estimate overheating risk. Although less detailed than dynamic thermal modelling, it provides a useful early-stage indicator of whether a design is likely to comply.
It can be used for buildings outside the postcode areas listed in Appendix C of Part O, where the risk of overheating is considered lower. In these cases, the simplified method is usually sufficient to demonstrate compliance with the Building Regulations.
Dynamic Thermal Modelling
Dynamic thermal modelling is a more advanced and accurate method for assessing overheating.
It uses computer-based simulations to model the thermal behaviour of a building in detail, considering solar gain, internal heat sources, thermal mass and ventilation rates.
Dynamic thermal modelling follows the methodology developed by CIBSE in TM59, as referenced in Approved Document O. This provides a highly accurate picture of the risk of overheating of a building under real weather conditions and occupancy patterns.
It is required for buildings located within the high-risk postcode areas listed in Appendix C of Part O, where the simplified method isn’t sufficient due to increased overheating risk.
What is the Difference Between The Simplified Method and Dynamic Thermal Modelling?
Although both methods assess overheating risk under Part O of the Building
Regulations, they differ in scope, accuracy and when each should be used.
The table below highlights the key differences between the Simplified Method
and Dynamic Thermal Modelling.
| The Simplified Method | Dynamic Thermal Modelling | |
|---|---|---|
| Purpose | Basic screening to check risk of overheating using general assumptions | Detailed analysis of building temperatures under realistic summer conditions. |
| Complexity | Low – quick calculations using standard input data. | High – advanced 3D simulation with variable data and hourly results. |
| Accuracy | Moderate – suitable for low-risk sites and simple designs. | High – reflects actual design features, materials and occupancy patterns. |
| Applicability | Buildings outside Appendix C (low-risk postcodes). | Buildings within Appendix C (high-risk postcodes) or complex designs. |
| Inputs Considered | Limited – uses standard assumptions for solar gain and ventilation. | Comprehensive – includes orientation, glazing type, shading, occupancy, thermal mass and ventilation rate. |
| Output | Pass/fail indication of likely overheating risk. | Detailed temperature profile and compliance summary for each room. |
| When to Use | Early design checks or straightforward residential project. | Required for developments in high-risk zones or when simplified method fails. |
How do I ensure I pass my Overheating Assessment?
To ensure your building passes the overheating calculation and complies with Part O of the Building Regulations, it must be designed to minimise the risk of excessive internal heat build-up.
Good design choices at the planning stage make compliance easier and help to maintain comfort for occupants.
This can include measures such as:
- Using high-performance insulation – prevents heat transfer and maintains stable internal temperatures.
- Shading windows to reduce solar gain – add external shading devices, overhangs or solar-control glazing to limit sunlight entering the space.
- Incorporating natural ventilation – enable cross-flow through openable windows, vents or rooflights to remove excess heat.
- Considering building orientation and surroundings – position and size windows to balance daylight with solar exposure, and use landscaping or adjacent structures for additional shading.
At JosTec, our assessors provide clear recommendations based on your
overheating calculations results, helping you understand where improvements
can be made to achieve Part O compliance efficiently.
Why Choose JosTec?
With 20 years of experience in planning and building regulations compliance across the UK, we understand what Building Control requires and how to help your project meet Part O overheating regulations efficiently.
- 10,000 overheating calculations delivered
- Simplified method and dynamic thermal modelling trained expert assessors
- Advice before and after testing to help you achieve compliance
Frequently Asked Questions
What is Part O of the Building Regulations?
Part O (Approved Document O) sets out requirements to prevent overheating in new residential buildings. It ensures unwanted heat from solar gain and internal sources is limited and that excess heat can be safely removed through shading, ventilation, or cooling strategies.
What is an overheating assessment?
An overheating assessment evaluates whether a building will become too hot during summer. It considers factors such as solar radiation, window size and orientation, ventilation rates, and thermal mass to determine compliance with Part O.
Are overheating calculations mandatory?
Overheating calculations are mandatory for all new residential buildings submitted for Building Control approval under Part O of the Building Regulations. They are not required for extensions added to residential dwellings after they are built or to buildings undergoing a material change of use.
Do I need an overheating assessment for an EPC?
No. Overheating calculations and EPCs are separate requirements. EPCs (from Part L assessments) measure energy efficiency, while Part O focuses on limiting internal heat. However, both use similar data – such as insulation and glazing – so carrying them out together can save time and ensure consistent results.
When Should I Book Overheating Calculations?
It’s best to book overheating calculations at design stage, before construction begins. This allows time to make any small design adjustments (for example, shading or window changes) if needed to meet Part O compliance.
What do I need to do before Overheating Calculations are carried out?
You’ll need to provide information about the building’s location, design, and specifications, including site plans, floor plans, materials, window and doors, heating and cooling and ventilation details. It’s also important to confirm whether your site is located within a high-risk zone under Approved Document O (Appendix C), as this determines which assessment method applies. You can check if your building is in a high-risk zone here.
How are Overheating Calculations Carried Out?
Qualified assessors use your building’s drawings and specifications – such as floor plans, U-values, glazing and ventilation details – to model how it performs in warm weather. Depending on the risk zone and project type, they’ll apply either the Simplified Method or Dynamic Thermal Modelling. The simplified method uses fixed design limits for glazing and ventilation, while dynamic thermal modelling runs detailed simulations using local climate data to predict internal temperatures and solar gain. The results show whether your building meets the overheating limits set in Approved Document O and if any design changes are needed for compliance.
What is a TM59 assessment?
A TM59 assessment is a dynamic thermal modelling study carried out in accordance with the methodology developed by CIBSE (TM59). It simulates hourly internal temperatures using climate, glazing, ventilation and thermal parameters, then checks whether the building complies with the overheating limits set out in Approved Document O.
What Happens if I Fail my Overheating Assessment?
If a design doesn’t pass, your report will outline where and why overheating occurs. At JosTec, our assessors will then recommend targeted design or ventilation changes (such as adding shading or adjusting glazing) to help your project achieve Part O compliance.
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Explore our other services to help you navigate building regulations and achieve compliance.