Make comfort part of the design
Understand overheating risk while you can still act on it.
Glazing, orientation, shading and ventilation all affect how a home performs in warm weather. An early assessment helps you understand the design choices that matter.
We provide simplified-method calculations and TM59 dynamic thermal modelling, with clear reporting and practical recommendations where improvements are needed.
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Part O and CIBSE TM59
What is a TM59 overheating assessment?
A TM59 assessment uses dynamic thermal modelling to predict overheating risk in a home. It considers how the building’s layout, glazing, shading, occupancy and ventilation affect indoor temperatures under the modelled conditions.
For new residential buildings in England, Approved Document O describes a simplified method and a dynamic thermal modelling route. Where TM59 is used for Part O, the assessment must also apply the requirements set out in Approved Document O. See the official Part O guidance.
We review your project before recommending a route. Window openings need to be considered alongside practical constraints such as security, noise and safe use, so the assessment reflects a ventilation strategy that can be used in practice.
Your assessment is supported by William Simpson, an Elmhurst-accredited overheating assessor.
What you receive
The right route and clear next steps.
Choose an assessment route
We review your drawings and project details to establish whether simplified calculations or dynamic thermal modelling are appropriate.
Assess the design
We complete the agreed calculations or model, considering the glazing, shading, ventilation and window-opening information.
Understand the findings
A report explaining the results, assumptions and recommended design measures, with practical guidance where changes are needed.
Feedback on the Barlings service
“A fantastic, professional company to work with. Great communication and turnaround time.”Chris Henderson · Barlings KWA client
Two routes, one clear starting point
You don’t need to choose the method alone.
Simplified calculations
We assess whether the simplified method is suitable for the dwelling and review the relevant glazing, shading and ventilation information.
TM59 dynamic modelling
A detailed thermal model evaluates overheating risk, allowing us to assess the design and consider changes where needed.
Practical design decisions
Explore the measures that suit your home.
Glazing and solar gain
Review glazing areas, solar-control glass and g-values as part of the assessment.
Shading
Consider how external shading can help limit the sunlight entering the building.
Ventilation
Assess window-opening strategies, night-purge ventilation and relevant mechanical ventilation options.
The appropriate measures depend on the project and the assessment results. We explain the assumptions and options so you and your designer can make informed choices.
A straightforward process
From project details to practical recommendations.
Share your design
Send drawings, the site location and available glazing, shading and ventilation details.
Assess the options
We confirm the scope, carry out the agreed assessment and discuss findings that may require design changes.
Receive your report
We provide the results, assumptions and recommended measures for your project team to review.
Project example · Nettleham, Lincolnshire
Overheating assessment for a replacement bungalow.
Barlings carried out a TM59 assessment for a bungalow project in Nettleham, alongside the project’s SAP, EPC and S7 energy work. The overheating assessment examined how the proposed home would perform using DesignBuilder dynamic thermal modelling with the EnergyPlus simulation engine.
Designing for night-time ventilation
The model treated windows as closed overnight. The strategy combined daytime window opening with mechanical ventilation with heat recovery (MVHR), summer bypass and specified bedroom boost ventilation controlled by local switches.
Results linked to the specification
The report recorded passes for the kitchen/dining room, lounge and both bedrooms under the assessment criterion used. These results depended on the glazing, opening arrangements and ventilation settings described in the report.
This project shows why an overheating assessment needs to consider how ventilation will work when occupants are asleep. The report gave the project team a defined strategy to follow, rather than relying on windows being left open overnight.
We can coordinate overheating work with your SAP assessment and EPC requirements. Each service and its fee will be identified in your quotation.
Useful answers before you enquire
Your overheating assessment questions.
Do I need TM59 modelling or simplified calculations?
We can help establish the appropriate route after reviewing your project. Send the drawings and any requirements already raised by your designer or Building Control.
When should overheating be assessed?
Early in the design process is helpful, while there is still scope to consider glazing, shading and ventilation choices.
What information should I send?
Start with the drawings and site location. Glazing specifications, window-opening details, shading and ventilation proposals will also help. We’ll confirm any missing information.
What if the assessment identifies overheating risk?
We discuss practical design measures, such as solar-control glazing, shading or ventilation changes, and explain what further assessment is needed.
Is this the same as a SAP assessment?
No. SAP concerns dwelling energy performance; an overheating assessment considers the risk of excessive indoor temperatures. We can help with both services.
Can you support a planning requirement?
Send us the wording of the planning requirement along with your drawings so we can confirm the assessment scope and reporting needed.
An initial guide
Explore your Part O assessment route.
Our free Part O overheating route finder helps you explore the assessment route using your project information. It is a starting point for discussion, not a replacement for an assessment. Send us the drawings and any requirements from your project team so we can confirm the scope.
If you also need energy calculations, see our SAP assessment service.
Your project, your quotation
Let’s review your overheating assessment needs.
How much does an overheating assessment cost?
Simplified-method assessment: £216–£300 including VAT (£180–£250 plus VAT at 20%). We’ll check whether this assessment route is suitable for your project.
TM59 dynamic thermal modelling: £540–£780 including VAT (£450–£650 plus VAT at 20%).
These guide prices cover the overheating assessment only. SAP calculations, EPCs and S7 energy work are quoted separately. We’ll review the size, complexity and scope of your project, confirm the appropriate assessment route and provide a quotation. Tell us if you need several dwellings assessed or want alternative design options modelled.
Tell William and Kerry about the dwelling, its location and the stage of design. We’ll confirm the information needed and quote for the appropriate assessment.
Useful details: site location · drawings · glazing and window-opening details · shading · ventilation · deadline