When planning a retrofit in 2026, a robust TM52 Overheating assessment is no longer optional for many commercial projects. Planning authorities across the UK, particularly in London, now expect dynamic thermal modelling evidence to demonstrate that refurbished offices, schools, and healthcare facilities will remain habitable during hotter summers. For existing buildings, the challenge is different from new construction. You are not starting with a blank digital model; you are working backwards from a physical structure with known flaws, measured dimensions, and often patchy documentation. This guide explains how to use IES Virtual Environment to conduct a CIBSE TM52 assessment specifically for existing non-domestic buildings, from gathering survey data through to interpreting results and testing retrofit solutions.
Table of Contents
What is TM52 and Why Does It Matter in 2026?
CIBSE TM52, published in October 2013, provides the methodology for assessing overheating risk in free-running buildings. The term “free-running” is critical here: the standard applies only to buildings that rely on natural ventilation and passive measures for temperature control, not those with active mechanical cooling or air conditioning. At 24 pages, the document is concise but technically dense, setting out an adaptive comfort model that links acceptable indoor temperatures to the running mean outdoor temperature.
In 2026, TM52 compliance carries more weight than ever. BREEAM UK New Construction and Refurbishment schemes continue to tighten their thermal comfort credit requirements, with assessors scrutinising overheating evidence more closely. The Greater London Authority’s London Plan, first strengthened in 2015, now makes explicit reference to TM52 and TM49 in energy assessments, requiring dynamic modelling for developments across the capital. Beyond regulation, climate resilience has moved from a forward-planning concern to a present-day liability. Insurers, funders, and commercial tenants increasingly ask for evidence that buildings will perform adequately under hotter conditions. For existing stock, the risk is acute: buildings designed for cooler decades can become uninhabitable during heatwaves without intervention.
The assessment rests on three compliance criteria. A building passes if it fails no more than one of them. Fail two or more, and the building is deemed to be at unacceptable risk of overheating, requiring remedial design changes.
The Three Criteria Explained (Pass/Fail Logic)
Criterion 1, Hours of Exceedance, measures the proportion of occupied hours during which the indoor operative temperature exceeds the adaptive comfort threshold. For a building to pass, the total hours of exceedance must be no more than three per cent of occupied hours across the assessment period, which runs from 1 May to 30 September. This criterion catches chronic, low-level overheating that might not seem dramatic day to day but accumulates over the summer months.
Criterion 2, Daily Weighted Exceedance, looks at severity rather than frequency. On any single day, the weighted exceedance must not surpass six degree-hours. Weighting means that larger temperature overshoots count proportionally more than small ones, so a day with a brief but extreme spike can fail even if the rest of the day is comfortable. This criterion prevents the assessment from being gamed by averaging out a few very bad days across a mostly acceptable season.
Criterion 3, Upper Limit Temperature, sets an absolute ceiling. The indoor operative temperature must never exceed the maximum acceptable temperature by any margin. The maximum acceptable temperature is defined as Tmax plus four degrees Celsius, where Tmax is the upper boundary of the comfort band for the relevant building category. This criterion addresses acute risk: even a single hour above this limit represents a potential health hazard for occupants.
The pass/fail rule is straightforward. A building that passes all three criteria is fully compliant. A building that fails one criterion requires attention but is not deemed unacceptable. A building that fails two or three criteria has an unacceptable overheating risk and must be redesigned or retrofitted with mitigation measures.
Preparing Your Existing Building Model in IES-VE
Modelling an existing building for TM52 assessment demands a different mindset from designing a new one. With a new build, you work from architectural drawings and specifications, adjusting assumptions as the design evolves. With an existing building, you must reconstruct reality from evidence, and the quality of that evidence directly determines the credibility of your results.
Start with building geometry. If you have original construction drawings, use them as a base but verify critical dimensions on site. For older buildings where drawings are missing or unreliable, a measured survey using laser scanning or manual techniques is essential. Pay particular attention to floor-to-ceiling heights, window sizes, and the depth of any external shading features such as overhangs or recessed glazing. Small errors in glazing area can produce large errors in solar gain calculations.
Fabric data comes next. For a Category III existing building assessment, you can use assumed U-values from CIBSE Guide A tables if you know the construction type, but site inspection will improve accuracy. Check wall and roof constructions where accessible, note the presence or absence of cavity insulation, and record glazing types. Single glazing with metal frames performs very differently from double glazing with thermal breaks, and the difference can be the margin between pass and fail on Criterion 1. Infiltration rates are notoriously difficult to estimate without a blower door test, but a realistic assessment based on visible gaps, window condition, and building age is better than accepting software defaults.
In IES-VE, you must set the correct building category before running the adaptive comfort calculations. For existing buildings, use Category III, which provides a four Kelvin comfort band around the running mean outdoor temperature. This is wider than Category I (two Kelvin, for sensitive occupants) and Category II (three Kelvin, the CIBSE recommendation for new buildings). Selecting Category II for an existing building will produce unrealistically stringent results and may lead you to specify unnecessary retrofit work.
Define occupied hours carefully. The TM52 assessment only considers hours when the building is occupied, so your schedule must reflect real usage patterns. For an office, this might be 08:00 to 18:00 on weekdays. For a school, it might include after-hours community use. The assessment period is fixed at 1 May to 30 September, the non-heating season, and this cannot be changed.
Sourcing and Applying Weather Data
IES-VE uses EnergyPlus Weather files in .epw format. For TM52 assessments, you need a CIBSE Design Summer Year, not a Test Reference Year. A TRY represents average conditions and will underestimate overheating risk. A DSY represents a moderately warm summer and is specifically designed for overheating analysis.
The standard choice is DSY1, which CIBSE provides for 14 UK locations. For London projects, the GLA requires the use of TM49 weather data, which includes specific London DSY files that account for the urban heat island effect. Using a non-London weather file for a central London project will produce misleadingly optimistic results.
In 2026, running a future climate scenario alongside the current DSY is strongly recommended. CIBSE provides future weather files for the 2050s and 2080s under different emissions scenarios. The 2050s High Emissions scenario is a practical choice for stress-testing: it represents conditions that buildings refurbished today will almost certainly experience within their service life. If your retrofit design passes TM52 under current weather but fails under the 2050s file, you have an early warning that additional measures may be needed sooner than expected.
Running the TM52 Simulation in IES-VE (Step-by-Step)
The simulation workflow in IES-VE follows a logical sequence: build the model, define the ventilation strategy, run the thermal simulation, and analyse the results. Each stage requires attention to detail.
Begin in ModelIT, the geometry module. Import your floor plans or build the model from scratch, ensuring that rooms are correctly assigned to thermal zones. For existing buildings, zone grouping matters: a south-facing office with large glazing will behave very differently from a north-facing corridor, and lumping them together will mask local overheating problems. Define constructions in ApacheConstructions, assigning the U-values, thermal mass, and glazing properties you gathered during the survey phase.
Ventilation setup is where many assessments go wrong. TM52 applies to free-running buildings, so your model must represent natural ventilation realistically. In MacroFlo, define window openings, trickle vents, and any purpose-provided ventilation openings. Set opening schedules that reflect occupant behaviour: windows are not permanently open, and in many buildings, night-time security concerns prevent the night-time purge ventilation that would otherwise reduce overheating. If your building has mechanical ventilation without cooling, model this in ApacheHVAC, but be aware that adding mechanical systems changes the building’s classification and may take it outside the scope of TM52.
In VistaPro, the results analysis module, select the adaptive comfort model and choose Category III for existing buildings. IES-VE will calculate the running mean outdoor temperature from the weather file and apply the appropriate comfort band. Run the simulation for the full assessment period and generate the TM52 output report. This report automatically calculates all three criteria and presents them in a standard format suitable for submission to planning authorities or BREEAM assessors.
Check the key output values immediately. Criterion 1 appears as a fraction of occupied hours exceeding the comfort threshold; it must be 0.03 or lower. Criterion 2 shows the daily weighted exceedance for each day in the assessment period; no single day should exceed six degree-hours. Criterion 3 reports the maximum temperature reached in each zone; it must not exceed Tmax plus four degrees Celsius. If any value looks suspicious, verify your inputs before accepting the result.
Interpreting Results and Identifying Failure Points
A failed TM52 assessment is not the end of the process; it is diagnostic information that tells you where to focus your retrofit efforts. Suppose your model shows Criterion 1 at 4.5 per cent exceedance and Criterion 2 at seven degree-hours on multiple days. The building fails two criteria and is at unacceptable risk. The question is why.
Use VistaPro to drill down into the data. Identify which zones are driving the exceedances. Sort the daily weighted exceedance results to find the worst days and examine the temperature profiles for those days. Often, the pattern is clear: a sharp temperature rise in the late afternoon on west-facing zones, or a steady climb through the morning on east-facing spaces. Correlate these spikes with solar gain data to confirm that glazing orientation is the primary driver.
Common failure points in existing buildings follow predictable patterns. Large areas of east or west-facing glazing without external shading are the most frequent culprit. Morning sun on east facades and afternoon sun on west facades strike at low angles that penetrate deep into the building, and internal blinds are far less effective than external shading at blocking this heat. High internal gains are another common issue: densely occupied offices, server rooms, or spaces with large amounts of IT equipment can overheat even on relatively mild days if ventilation is inadequate. Poor natural ventilation paths, often caused by internal partitions blocking cross-flow or windows that occupants cannot easily open, compound the problem.
A building that fails two criteria is officially at unacceptable risk, but even a single criterion failure warrants investigation. Criterion 3 failures are particularly serious because they indicate temperatures that could pose immediate health risks to vulnerable occupants.
Mitigation Strategies to Achieve Compliance
The most effective retrofit interventions address the root causes identified in your failed simulation. External shading is almost always the highest-impact measure for solar-driven overheating. Brise-soleil, fixed louvres, or retractable external blinds on east and west facades can reduce peak solar gain dramatically. In IES-VE, you can model these as local shade elements and re-run the simulation to quantify the improvement.
Solar control glazing with a low g-value is an alternative where external shading is impractical, such as on listed buildings or where planning restrictions apply. Modern solar control coatings can reduce solar gain by half compared to standard double glazing while maintaining reasonable visible light transmission. The trade-off is reduced winter solar gain, so check that heating energy use does not increase unacceptably.
Enhanced natural ventilation often provides the second-largest improvement. Increasing window opening areas, adding trickle vents to fixed glazing, and creating secure night-time purge ventilation paths can all help. Night-time cooling is particularly effective in buildings with exposed thermal mass: cool night air flushes heat from the structure, giving it capacity to absorb heat the following day. In IES-VE, you can test this by adjusting the MacroFlo opening schedules to allow night-time ventilation during the summer months.
Increasing thermal mass is a deeper retrofit option. Exposing concrete soffits, replacing carpet with hard flooring over screed, or adding phase-change materials can all increase a building’s capacity to absorb heat without rising in temperature. This measure works best in combination with night-time ventilation, as the mass needs a daily cooling cycle to remain effective.
Test each mitigation option iteratively in IES-VE. Start with the most cost-effective measures and run the simulation after each change. Often, a combination of modest interventions (external shading plus improved ventilation) will achieve compliance where a single large intervention would be disproportionate. If you find that natural ventilation measures are insufficient and mechanical cooling is required, the assessment framework changes: the building is no longer free-running, and TM52 no longer applies. In that case, you would move to a different standard such as TM59 or a fixed-temperature assessment.
Common Pitfalls When Using IES-VE for TM52
Several errors recur in TM52 assessments, and most are avoidable with careful checking. The most frequent mistake is selecting the wrong building category. Using Category II for an existing building produces results that are too stringent, potentially flagging a building as failing when it would pass under the correct Category III assessment. Conversely, using Category III for a new build where Category II is required by planning conditions will not be accepted by the approving authority.
Weather file selection is another common error. Using a Test Reference Year instead of a Design Summer Year will underestimate overheating risk because the TRY represents average rather than warm conditions. For London projects, using a generic UK weather file instead of the TM49 London-specific files ignores the urban heat island effect, which can add several degrees to peak temperatures.
Occupant behaviour assumptions can skew results significantly. Assuming windows are always open during occupied hours will overestimate ventilation and underestimate overheating. Assuming blinds are always closed will underestimate solar gain. Use realistic schedules based on observed behaviour or published guidance. For offices, CIBSE TM52 itself provides example window-opening schedules that reflect typical occupant behaviour.
Underestimating internal gains is a particular risk in existing building models. Equipment densities have increased in most commercial buildings over the past decade, and lighting loads may be higher than assumed if older fluorescent fittings are still in place. Measure or estimate these gains realistically, and include all significant sources: computers, servers, printers, kitchen equipment, and occupants themselves.
Why TM52 Compliance is a 2026 Priority
TM52 compliance is not a box-ticking exercise for a planning submission. It is a direct measure of whether a building will remain safe and usable during the hotter summers that the UK is already experiencing. Overheating affects occupant health, cognitive performance, and productivity. For building owners and operators, it also affects asset value: a building with a documented overheating problem is harder to let, harder to sell, and increasingly difficult to insure.
IES-VE remains the industry-standard tool for conducting these assessments, but the software is only as good as the data and judgement applied to it. Success depends on accurate survey information, correct category and weather file selection, realistic ventilation and occupancy schedules, and a methodical approach to testing mitigation options. For existing buildings, the process is more demanding than for new builds, but the payoff is greater: a retrofit that solves overheating is a retrofit that extends the building’s useful life and protects its occupants.
If you are planning a commercial retrofit and need a professional TM52 assessment, or if you want to discuss how overheating risk might affect your project, contact CCA Environmental. Our team provides end-to-end assessment services, from initial surveys through to mitigation design and compliance reporting.