A TM59 assessment is a highly detailed assessment of a domestic dwelling, used to assess the risk of residential overheating, and show compliance with Approved Document O of building regulations..
Contrary to a "Simplified Overheating assessment", a TM59 assessment looks at many of the finer details of a buildings design, and uses dynamic thermal modelling to assess the risk of overheating. For example, as well as the physical orientation of the building and the size and number of windows, a TM59 assessment also accounts for things like surrounding buildings providing shade, the type of glazing used and amount of sunlight which can enter the building through it, the building materials used, and their capacity to store heat in the day and release it later in the evening/night (thermal mass).
Including all of these fine details in an assessment might seem like an onerous task which doesn't provide much benefit; however, we often find that accounting for all these different factors, provides the extra design flexibility required in the case of a failure. Where a simplified overheating assessment may simply fail, a TM59 allows us to factor in low-G glazing, or a powerful mechanical ventilation system, or a range of other strategies which allow it to pass. A passing TM59 report shows compliance with part O, so the simplified assessment is no longer required!
There are also some situations where a building may have restrictions on windows being fully openable, for example, because of noise or security issues. In these cases a TM59 is the appropriate choice for assessment methodology, since natural ventilation from the windows is restricted so another cooling strategy should be tested.
No, a TM59 assessment is not always mandatory. Part O compliance can be demonstrated using either a simplified overheating assessment or a TM59 assessment, depending on the project.
Showing compliance with part O is mandatory for all new build dwellings, and a TM59 overheating assessment is one way to do this. However, compliance with part O can also be shown via a simplified overheating assessment.
Deciding between TM59 overheating assessment and a simplified overheating assessment normally depends on the building design. In our experience, simple houses with plenty of openable windows for ventilation stand a chance of passing the more budget friendly simplified overheating assessment. However, when there are complications such as restrictions on opening windows (due to noise or security issues), then a TM59 assessment offers the flexibility required to incorporate alternative cooling strategies into the equation.
Simply put, a TM59 is required for a new build dwelling, when it cannot pass a simplified overheating assessment.
If you need to show compliance with Part O, and you're not sure if you need a TM59 assessment or not, get in touch and we will be happy to take a look at your specific project, and discuss the options with you.
Ideally, a TM59 assessment should be carried out during the planning or early design stage, before key design decisions become difficult or expensive to change.
A TM59 overheating assessment evaluates how a dwelling is likely to perform during warm weather based on factors such as glazing areas, orientation, shading, ventilation strategy and building fabric. Carrying out the assessment early allows any overheating risks to be identified while the design is still flexible.
Although a TM59 assessment can be completed later in the project, making changes to the design becomes progressively more difficult as planning approval, detailed design and construction stages are reached.
In our experience, projects that are assessed early are often easier and cheaper to bring into compliance than projects assessed at the last minute. If overheating issues are identified early, the design team can usually consider a range of solutions such as adjusting glazing areas, incorporating shading features, improving ventilation or refining the building layout.
If you're unsure whether your project is at the right stage for a TM59 assessment, get in touch and we'll be happy to review your drawings and advise.
To obtain a TM59 report, you first need to have a TM59 overheating assessment carried out by an experienced assessor.
You provide information about the building, and the assessor inputs this into specialised dynamic thermal modelling software. An overheating simulation is then carried out and the results reviewed.
If the dwelling complies with the TM59 criteria, the assessor will issue a TM59 PDF report suitable for submission to Building Control as evidence of Part O compliance.
A TM59 report is one of the accepted methods of demonstrating compliance with Approved Document O.
If the assessment does not comply on the first attempt, an experienced assessor will often be able to identify practical improvements and reassess the design.
At Approved Building Consulting, we include up to two minor design revisions free of charge with our TM59 assessments. If you'd like a quotation for your project, try our instant quoting tool or get in touch.
To pass a TM59 assessment, the dwelling must demonstrate that internal temperatures remain within the limits defined by CIBSE TM59.
For naturally ventilated homes, living rooms, kitchens and bedrooms must not exceed the adaptive comfort temperature by more than 1°C for more than 3% of occupied hours during the summer period. In addition, bedrooms must not exceed 26°C for more than 1% of annual hours between 22:00 and 07:00.
For homes that are predominantly mechanically ventilated, the criteria are slightly different. In these cases, occupied rooms should not exceed 26°C for more than 3% of occupied hours annually.
The purpose of these limits is to ensure homes remain comfortable and safe to occupy during periods of warm weather.
Excessive overheating can affect comfort, wellbeing and quality of life. Demonstrating compliance with TM59 helps ensure the building has been designed with overheating risk in mind.
If you have any questions about TM59 compliance, get in touch and we'll be happy to help.
The best way to pass a TM59 assessment is to consider overheating mitigation measures early in the design process.
TM59 assessments evaluate overheating risk by modelling internal temperatures throughout the year. The assessment considers glazing, solar gains, orientation, shading, ventilation strategy, occupancy patterns and local weather data.
In our experience, the most common causes of TM59 failures are excessive glazing, restricted ventilation and insufficient shading.
Addressing these issues during planning or early design stages is usually straightforward and cost-effective. Leaving the assessment until later can result in delays, redesign costs and fewer available mitigation options.
There is no single feature that guarantees compliance. A good TM59 assessor will use the assessment as a design tool, identifying practical opportunities for improvement where necessary.
At Approved Building Consulting, we include up to two minor design revisions free of charge with our TM59 assessments to help projects achieve compliance where possible.
Most TM59 failures are not catastrophic. In our experience, the issue is often excessive solar gains through glazing or insufficient ventilation rather than a fundamental flaw in the design.
A TM59 assessment identifies whether a dwelling is at risk of overheating during warm weather. If the assessment shows that internal temperatures exceed the allowable limits, the causes can usually be identified and addressed.
Relatively small design changes are often enough to achieve compliance. Common solutions include improving natural ventilation, introducing shading measures or adjusting glazing specifications.
The most effective solution depends on the specific building design, orientation, surrounding environment and any planning or operational constraints.
At Approved Building Consulting, we don't simply report a failure. We review the results, identify the causes of overheating and recommend practical mitigation measures that can help achieve compliance.
To carry out a TM59 assessment, we typically require architectural drawings, glazing specifications, ventilation information and the project location.
A TM59 overheating assessment uses dynamic thermal modelling to predict how hot a dwelling is likely to become during warm weather. To create an accurate model, the assessor requires information about the building's layout, construction, glazing and ventilation strategy.
Providing complete information helps improve assessment accuracy and reduces the need for assumptions.
If you're unsure whether you have enough information available, simply send us your drawings and we'll confirm what's needed.
The cost of a TM59 assessment depends on the size, complexity and number of dwellings involved in the project.
Factors affecting cost include the number of units, the complexity of the building geometry, the ventilation strategy, and whether multiple design iterations are required.
Simple single-dwelling projects are generally quicker to assess than large apartment blocks or developments with multiple dwelling types.
Obtaining a quotation early in the design process can help ensure overheating risks are identified before they become costly to address.
For accurate pricing, use our instant quoting tool or get in touch and we'll be happy to discuss your project.
| Misconception | Response |
|---|---|
| "A TM59 assessment is always mandatory." | No. Many projects can demonstrate Part O compliance using the simplified overheating assessment. TM59 is typically required where the simplified method cannot be used or does not achieve compliance. |
| "Large windows automatically mean a TM59 failure." | Not necessarily. Glazing size is only one factor. Orientation, shading, glazing specification, ventilation strategy and thermal mass can all influence the result. |
| "If a TM59 assessment fails, the design must be completely redesigned." | Usually not. In many cases, relatively modest changes such as improving shading or ventilation are enough to achieve compliance. |
| "It's cheaper to leave the TM59 assessment until the end of the project." | No. The assessment itself is often a similar amount of work, but carrying it out early gives much greater flexibility to resolve any issues before they become expensive. |