TM59 Overheating Assessments

TM59 overheating assessments for residential design

IN2 uses dynamic thermal modelling to assess overheating risk in homes and residential accommodation, including new developments and major refurbishments. We test the proposed form, fabric, glazing, shading and ventilation against the appropriate assessment criteria and weather data, then translate the results into practical design actions and a clear technical report.

The regulatory route depends on the project’s jurisdiction within the UK. Approved Document O applies to new residential buildings in England; Scotland, Wales and Northern Ireland have separate building regulation frameworks. IN2 confirms the applicable route at the start of each assessment.

What is a TM59 overheating assessment?

CIBSE Technical Memorandum 59 (TM59), Overheating risk in homes, provides a consistent methodology for assessing the likelihood of uncomfortable or excessive internal temperatures in residential buildings. It uses dynamic thermal modelling to simulate how rooms respond to solar gains, internal heat gains, ventilation and outdoor weather over time.

The assessment helps the project team understand whether a design is likely to overheat, where the risk is concentrated, and which combination of passive and building-services measures will address it. It supports design development, planning submissions, building-control discussions and client requirements.

CIBSE published an updated TM59 methodology in 2026. CIBSE’s current guidance states that TM59 (2017) and the 2016 CIBSE weather files remain required for compliance with Part O in England, while TM59 (2026) and the 2025 weather files are recommended for current industry best practice. The edition, weather files and reporting route are recorded at the start of every assessment.

When should overheating risk be assessed?

Overheating risk should be considered before the building form, facade and ventilation strategy are fixed. An early assessment can identify risks while there is still time to change orientation, glazing, shading, layouts, opening sizes or services without major redesign.

A TM59 assessment may be commissioned at:

• Concept or feasibility stage, to compare massing, orientation, facade and dwelling-layout options.
• Planning stage, where an overheating statement or technical assessment is requested by the planning team, local policy or the project brief.
• Detailed design, to test coordinated construction, solar-control, opening and ventilation assumptions.
• Building-control stage in England, where the dynamic thermal method is used to demonstrate the overheating requirements of Approved Document O.
• Major refurbishment or retrofit, where changes to airtightness, insulation, glazing, shading or ventilation could alter summer performance.
• Resilience or client-led reviews, where future climate performance or the needs of vulnerable occupants require consideration beyond the minimum regulatory route.

The project team should confirm the applicable building regulations, planning policy, certification requirements and assessment methodology for the project location before work begins.

What standards and regulations apply?

CIBSE TM59: technical guidance, not universal legislation

TM59 is published by the Chartered Institution of Building Services Engineers (CIBSE). It standardises the modelling assumptions, comfort criteria, weather inputs and reporting needed to assess overheating risk. CIBSE describes it as applicable to new dwellings and major residential refurbishments, and also identifies applications including sheltered accommodation, care homes and student accommodation.

TM59 does not automatically become a legal requirement because it is named in a project brief, and a TM59 assessment does not by itself prove compliance with every planning or building-regulation requirement. IN2 establishes the purpose of the assessment, the applicable edition and the approving or reviewing body before defining the modelling scope.

Approved Document O: England

Approved Document O provides statutory guidance for the overheating requirements in Part O of the Building Regulations in England. It applies to new residential buildings and is not a UK-wide regulation.

Part O offers a simplified method and a dynamic thermal modelling method. Where the dynamic method is used, the building-control body receives a report demonstrating that the residential building passes the relevant CIBSE TM59 assessment, together with the Approved Document O Appendix B compliance checklist. The report should follow the reporting requirements of the TM59 edition and Part O route being used.

CIBSE currently notes an important transition: TM59 (2017) with 2016 weather data remains required for Part O compliance in England, while TM59 (2026) and 2025 weather data represent CIBSE’s recommended current best practice. A design team may choose to run both a regulatory assessment and additional future-climate testing; the submission strategy should be agreed with the building-control and project teams.

Scotland, Wales and Northern Ireland

Scotland, Wales and Northern Ireland have separate regulatory and planning arrangements for building energy performance and thermal comfort. Approved Document O and the England-specific Part O route do not apply in these nations. IN2 can confirm the assessment question and coordinate the evidence needed for the project’s specific jurisdiction.

How does dynamic thermal modelling work?

Dynamic thermal modelling is a computer simulation that calculates changing indoor conditions at hourly or shorter intervals, rather than judging a room from a single design temperature. The model combines the building’s geometry and construction with weather, solar radiation, internal heat gains, ventilation and occupancy assumptions to estimate how spaces perform over a full assessment period.

For overheating work, the model can test the effect of:

• orientation, exposure, storey and surrounding context;
• room geometry, thermal mass, fabric build-ups and insulation;
• window area, glazing solar properties, doors and rooflights;
• external shading, internal blinds and control assumptions;
• occupancy, equipment and lighting heat-gain profiles;
• window-opening schedules, free areas, cross-ventilation and night purge;
• infiltration, mechanical ventilation, heat recovery, fans and cooling systems;
• noise, security, air-quality and fall-protection constraints that limit ventilation; and
• current or future climate weather files selected for the assessment route.

The result is an evidence-based picture of which rooms, dwelling types, orientations or time periods present the greatest risk. It also allows the team to compare mitigation options before selecting a coordinated design solution.

What are Design Summer Years (DSYs)?

A Design Summer Year (DSY) is a CIBSE weather file containing a year with a defined type of hot-weather event. DSYs are used as climate inputs for overheating-risk assessments; a Test Reference Year (TRY) represents a typical year and is generally used for annual energy assessments.

CIBSE defines three DSY event types:

• DSY1 – a moderately hot year with heat events having a seven-year return period;
• DSY2 – the year containing the most intense heat events; and
• DSY3 – the year containing the longest heat events.

CIBSE’s 2025 weather guidance also describes differing time periods, emissions scenarios and probability percentiles, including future 2030s, 2050s and 2080s datasets. CIBSE states that its 2025 files are recommended for design-stage building-performance analysis, but that the 2016 files remain required for Part O compliance in England at the time of writing.

DSY selection is not a universal instruction to use one file for every project. The appropriate file depends on the jurisdiction, TM59 edition, regulatory route, design brief and resilience question.

What does the assessment cover?

IN2 defines the assessment boundary and representative sample with the design team. Depending on the building and purpose of the study, the work covers:

1. Risk identification and sample selection – identifying representative and potentially high-risk dwellings, rooms, orientations, top floors, glazed facades, single-aspect units, vulnerable-occupant spaces and communal areas where relevant.
2. Model development – translating drawings, schedules and specifications into a thermal model with documented geometry, fabric, glazing, shading, thermal mass and building-services assumptions.
3. Internal gains and use profiles – applying the occupancy, equipment, lighting and operating assumptions required by the selected methodology, with project-specific changes recorded clearly.
4. Ventilation and openings – testing the intended natural or mechanical ventilation strategy, including opening restrictions and practical constraints such as security, noise and air quality.
5. Weather and assessment criteria – recording the weather files, climate period, emissions scenario, comfort criteria and any regulatory or client thresholds used to judge the results.
6. Option testing – comparing mitigation measures and identifying residual risks, trade-offs and items requiring coordination with architecture, facade, acoustics, fire, MEP and planning disciplines.

Which overheating mitigation measures should be considered first?

The most effective approach is to reduce heat gains and improve passive heat removal before relying on mechanical cooling. The final order depends on the site and brief, but the design review will typically consider:

• Prevent solar gains: orientate and shape the building carefully; control glazing areas; select suitable glazing solar properties; and provide external shading or well-designed internal shading with realistic controls.
• Remove heat naturally: provide usable secure openings, cross-ventilation and night-time purge where feasible, while checking acoustics, security, air quality, safety and the actual free area available.
• Use the building fabric: coordinate thermal mass and fabric design so that heat is delayed and dissipated without creating unintended winter or moisture problems.
• Improve low-energy comfort: consider ceiling fans or appropriately controlled mechanical ventilation where consistent with the selected methodology and operational strategy.
• Use active cooling only where justified: where passive measures and low-energy options cannot reasonably address the risk, test mechanical cooling with the associated energy, controls, maintenance and Part L implications.

An assessment is most valuable when these measures are tested as design options, rather than added as a late list of generic recommendations.

What will you receive?

The agreed deliverables can include:

• a TM59 overheating assessment report with the assessment purpose, edition, criteria, sample selection, model inputs and assumptions;
• a record of the location-appropriate weather files and climate scenarios used;
• room- or dwelling-level results identifying exceedances, risk patterns and the assumptions behind them;
• graphics, temperature profiles and comparison tables where they help the project team interpret the findings;
• practical mitigation recommendations prioritised by passive design, ventilation and systems implications;
• an updated or comparative assessment of agreed mitigation options; and
• planning, client or building-control information presented in a form suitable for the relevant submission or review process, including the Part O Appendix B checklist where applicable and agreed.

The report explains what the modelling does and does not demonstrate. It is not a guarantee of planning approval, building-regulation approval or future in-use performance; construction quality, commissioning, controls and occupant behaviour all affect the completed building.

How does IN2 integrate the work into design and approval?

IN2 brings building-physics analysis into the design process early, working with architects, planners and developers to identify constraints and opportunities. For overheating, this means using the model to inform decisions about building form, facade and glazing strategy, ventilation approach and the coordination of environmental performance objectives.

A typical workflow:

1. Confirm the question. Establish the project location, building type, stage, authority or client requirement, selected TM59 edition and required weather-data route.
2. Review the available design information. Check drawings, elevations, room data, window and shading schedules, facade specifications, ventilation strategy, thermal properties and relevant energy or compliance models.
3. Identify early risk. Agree the representative sample and test the elements most likely to drive overheating, such as highly glazed or single-aspect dwellings, top floors and constrained openings.
4. Coordinate mitigation. Feed findings back to the architectural, facade, acoustics, fire, MEP and planning teams so that measures are practical, safe and buildable.
5. Report and support decisions. Provide a transparent report, identify outstanding assumptions and explain the evidence needed for the relevant planning, certification or building-control review.

What information is needed to get started?

Send what is available; a complete set of information is not needed before an initial scope discussion. The most useful items are:

• project location, jurisdiction and planning or building-control stage;
• site plan, floor plans, sections, elevations and proposed massing;
• dwelling or room schedule, orientations and representative-unit strategy;
• window, glazing, shading and opening information;
• facade and construction specifications, including thermal mass where known;
• natural or mechanical ventilation strategy and any cooling proposals;
• acoustic, security, air-quality, safety or planning constraints; and
• the required TM59 edition, weather files, certification target or authority/client brief, if already specified.

IN2 can then help define the assessment boundary, identify information gaps and agree the next modelling step.

It all starts with a plan. Contact us today to chat about TM59 Overheating Assessments for your next project.