TM59 Assessment
Detailed overheating analysis using dynamic thermal modelling, helping demonstrate Part O compliance, identify high-risk rooms and refine glazing, shading and ventilation strategies before construction.
What Is a TM59 Assessment?
A TM59 assessment is a detailed overheating analysis for residential buildings. It uses dynamic thermal modelling to simulate how internal temperatures are likely to change throughout the year under defined weather, occupancy, ventilation and solar-gain conditions.
The assessment is commonly used where the simplified Part O method is unsuitable or where a project fails the simplified route. It provides a more detailed understanding of which rooms are at greatest risk and which design changes are most effective.
One Stop Consult coordinates TM59 assessments through experienced specialists who can model the proposed development, review the results and test practical mitigation measures before issuing the final report.
TM59 and the Building Regulations
Part O was introduced to reduce excessive internal temperatures in new residential buildings by limiting unwanted solar gains and providing an effective means of removing excess heat.
New Dwellings
New houses and flats may require evidence that the design limits overheating risk under the applicable Building Regulations.
Student Accommodation
New student residences can require assessment of bedrooms, studios and relevant communal areas.
Buildings Where People Sleep
Certain institutional and multi-use residential buildings can fall within the scope of Part O.
Live/Work Units
Buildings containing both residential accommodation and work space may require review where they fall within the relevant scope.
When Is a TM59 Assessment Required?
Dynamic thermal modelling is generally used where the simplified route cannot adequately demonstrate compliance.
Failure of the Simplified Method
Where the glazing, orientation or ventilation strategy falls outside the simplified limits, a detailed assessment may be required.
Complex Building Designs
Irregular layouts, mixed orientations, rooflights, large glazed areas and varied dwelling types can require more detailed modelling.
Single-Aspect Apartments
Flats with openings on only one elevation may have limited cross-ventilation and a higher risk of overheating.
Noise-Constrained Sites
Where external noise prevents windows being left open, the model may need to assess an alternative ventilation strategy.
Planning or Client Requirements
A TM59 report may be requested through planning, sustainability commitments, funding requirements or employer specifications.
Design Optimisation
Developers may commission modelling early to test different façade, glazing, shading and ventilation options before technical design.
What Is Included in the Dynamic Thermal Model?
The building is recreated within specialist simulation software so that room temperatures can be assessed over time rather than through a single fixed calculation.
The accuracy of the result depends on the quality of the architectural, construction and building-services information supplied.
Weather Data and Site Location
Overheating risk varies significantly by location. Dynamic modelling uses suitable weather data to represent external temperature, solar radiation and other climatic conditions affecting the development.
The project address, local exposure and urban context are therefore important inputs. A highly glazed scheme in a warmer or dense urban location may perform differently from the same design elsewhere.
How TM59 Results Are Assessed
The model tests internal temperatures against recognised overheating criteria for occupied rooms.
Living Spaces
Living rooms, kitchens and combined living spaces are assessed for the frequency and severity of elevated temperatures.
Bedrooms
Bedrooms receive particular attention because high night-time temperatures can affect sleep and occupant health.
Occupied Hours
The methodology considers the periods when spaces are expected to be occupied rather than treating every hour identically.
Representative Units
Larger schemes may be assessed through carefully selected dwellings and rooms that represent the highest-risk orientations and layouts.
What Can Cause a TM59 Failure?
Failures are often caused by a combination of high solar gains and insufficient heat removal.
Excessive Glazing
Large windows can create substantial solar gains, particularly on south- and west-facing façades.
Limited Window Opening
Small openable areas, restrictors, security requirements or façade design can reduce natural ventilation.
Lack of Cross-Ventilation
Single-aspect rooms can struggle to achieve the airflow available to dual-aspect dwellings.
High Internal Gains
Occupants, appliances, lighting and communal systems can add heat throughout the day and evening.
Top-Floor Exposure
Roof-level rooms and units beneath rooflights may be exposed to additional solar gain.
Acoustic Constraints
A strategy that relies on open windows may be unsuitable where the external environment is too noisy.
How Can a TM59 Failure Be Resolved?
Dynamic modelling allows mitigation measures to be tested before they are committed to the final design.
Review Glazing Areas
Reduce or redistribute glazing where solar gain is excessive in relation to daylight and façade requirements.
Add External Shading
Balconies, fins, overhangs, shutters or other external measures can intercept solar radiation before it enters the building.
Use Solar-Control Glass
A revised glazing specification may reduce solar transmission while maintaining daylight and appearance.
Increase Openable Area
Larger or better-positioned openings can improve purge ventilation and increase airflow through occupied rooms.
Improve Mechanical Ventilation
Mechanical ventilation may support heat removal where natural ventilation is insufficient or constrained.
Coordinate MVHR Summer Bypass
Where MVHR is proposed, the summer-bypass arrangement should be properly considered so heat is not unnecessarily recovered.
TM59 Assessments on Noise-Constrained Sites
External noise can have a direct effect on the overheating strategy. A dwelling may technically pass when windows are assumed to remain open, but that may not represent a practical solution beside a busy road, railway, airport or commercial use.
The acoustic and overheating strategies should therefore be reviewed together. Where open windows are unsuitable, alternative ventilation or cooling measures may need to be modelled.
How the TM59 Assessment Process Works
Early modelling gives the design team more opportunity to resolve risk through proportionate changes rather than expensive late-stage redesign.
Project Review
Confirm the scope, building type, location and available technical information.
Model Setup
Recreate the relevant dwelling geometry, fabric, glazing and ventilation strategy in the simulation software.
Thermal Simulation
Run the model using the selected weather, occupancy and operating assumptions.
Design Review
Identify failures and test practical changes to glazing, shading or ventilation.
Final Report
Issue the completed assessment for the design team, client and Building Control.
Information Required for a Quotation
Provide the full project address, number and type of dwellings, current project stage and the latest architectural drawings.
Please also send available information on glazing, shading, window openings, construction build-ups, ventilation, occupancy assumptions and any acoustic or planning constraints affecting the design.
TM59 Assessments for Residential Developments
Nationwide TM59 Assessment Service
One Stop Consult coordinates TM59 dynamic thermal modelling for residential projects throughout mainland UK. Support is available for individual dwellings, apartment schemes, student residences, care facilities and larger multi-unit developments.
Other Services Your Project May Require
Important TM59 Assessment Information
The applicable assessment route depends on the building type, design, location, Building Regulations requirements and available project information. Dynamic thermal modelling relies on defined assumptions for occupancy, ventilation, weather and building operation. Changes to the glazing, façade, ventilation strategy or room layouts may affect the result and require the model to be updated. One Stop Consult provides consultancy and coordination support through its specialist assessor network.
TM59 Assessment FAQs
What is TM59?
TM59 is a recognised methodology for assessing overheating risk in homes using dynamic thermal modelling and defined criteria.
Is TM59 the same as Part O?
No. Part O is a Building Regulations requirement. TM59 is a dynamic modelling methodology that can be used within the detailed compliance route.
When is a TM59 assessment needed?
It is commonly required where the simplified Part O method is unsuitable or fails, or where detailed modelling is requested.
What buildings can be assessed?
TM59 is primarily used for residential developments such as houses, flats, student accommodation and similar sleeping accommodation.
What happens if the model fails?
The assessor can test mitigation measures such as reduced glazing, shading, solar-control glass, increased openable area or revised mechanical ventilation.
Can external noise affect the result?
Yes. Where windows cannot reasonably remain open because of noise, the model may need to use a different ventilation strategy.
When should TM59 modelling be completed?
It should be completed as early as possible, before glazing, façade and ventilation choices become difficult to change.
What information is required?
The assessor normally needs the architectural drawings, glazing data, window-opening details, fabric specification and ventilation strategy.
Do you provide TM59 assessments nationwide?
Yes. We coordinate suitable specialists for eligible projects throughout mainland UK.
Request Your Free TM59 Assessment Quote
Complete the form below with the project address, number of dwellings and current design stage. Upload the latest plans, elevations, sections, glazing information and ventilation strategy so our team can review the requirement and coordinate an appropriate quotation.