TM52 Overheating Assessments

TM52 Overheating Assessments

CIBSE TM52 is the standard method for checking whether a non-domestic building will overheat in summer. We build a dynamic thermal model of the building, run it hour by hour against summer weather data, and test each occupied space against the TM52 criteria. It is used for offices, schools, nurseries, healthcare buildings, shops and the communal areas of residential schemes.

When you need one

• Major planning applications in London. London Plan Policy SI 4 asks major developments to show how they reduce the risk of overheating and follow the cooling hierarchy. Councils expect dynamic modelling: TM59 for homes and TM52 for non-domestic spaces.
• Schools. BB101 uses the TM52 method for classrooms that are not mechanically cooled.
• BREEAM. Thermal comfort credits, where the assessor asks for TM52 modelling.
• Mixed-use, care and student schemes. TM59 for the flats and bedrooms, TM52 for offices, shops, lounges and other non-residential areas.
• Before you commit to air conditioning. To find out whether a building will stay comfortable without it.

Part O of the Building Regulations covers new residential buildings only. Offices, schools and other non-domestic buildings are not tested under Part O, so TM52 is how their overheating risk is shown when planning, BREEAM or the client asks for it.

How TM52 judges overheating

TM52 uses adaptive comfort: the acceptable indoor temperature rises as the weather gets warmer, because people adapt to it. The limit is set from the running mean of the outdoor temperature. For new buildings the normal expectation is Category II, which allows 3°C above the comfort temperature.

Each space is tested during occupied hours from 1 May to 30 September, against three criteria:

• Hours of exceedance. The space can be 1°C or more over the limit for no more than 3% of occupied hours.
• Daily weighted exceedance. A daily score for how far and for how long the space is over the limit. It must be 6 or less on every day.
• Upper limit. The space must never be 4°C or more above the limit.

A space that fails two of the three is classed as overheating. Schools follow BB101, which uses the same method but sets the first criterion at fewer than 40 hours.

TM52 is for spaces without mechanical cooling. Cooled spaces are assessed against fixed design temperatures instead.

Weather files

We use the CIBSE Design Summer Year weather file for the nearest location. In London, councils ask for the TM49 London files: the 2020s DSY1 year has to pass, and the hotter DSY2 and DSY3 years and the 2050s and 2080s files are run to show how far the design and its mitigation go. We confirm with you which files your council or BREEAM assessor wants before we start.

The cooling hierarchy

The London Plan expects passive measures before air conditioning, in this order:

1. Reduce the heat entering the building: orientation, shading, light-coloured materials, glazing, insulation and green roofs or walls.
2. Reduce the heat generated inside, through energy efficient design.
3. Manage heat inside the building with exposed thermal mass and high ceilings.
4. Passive ventilation.
5. Mechanical ventilation.
6. Active cooling.

Our report shows which steps were used, and why the later ones are needed if they are.

What you get

A report for planning, building control or your BREEAM assessor with the model inputs, the result for every space against each criterion, and a clear pass or fail table. Where a space fails, we test the fixes and show which ones work: solar control glass, external shading, larger opening areas, night ventilation, ceiling fans or mechanical ventilation. Cooling comes last.

What we need from you

• Floor plans, elevations and sections
• Window and door schedule with sizes and how each one opens
• Construction build-ups and glazing specification
• How each space will be used and its occupancy hours
• Ventilation strategy, and any acoustic or air quality report

Turnaround: 5 working days once we have everything we need. The price depends on the size of the building and the number of spaces. Send your drawings for a quote.

Questions we get asked

Do we need TM52 or TM59? TM59 for homes, including flats, care home bedrooms and student rooms. TM52 for non-domestic spaces. Mixed schemes often need both, and we run them in the same model.

Our building will have air conditioning. Do we still need TM52? Not for the cooled spaces. In London you still need to show the cooling hierarchy was followed and why cooling is needed.

What if a space fails? We test the fixes and tell you which costs least. Glazing, shading and opening areas usually come first.

Can you do the SBEM and energy statement too? Yes. We use the same drawings for the SBEM calculation, the BRUKL report and the London Plan energy statement, so the figures line up.

More: TM59 overheating assessments, Part O overheating assessments, SBEM calculations and BRUKL reports, energy statements, energy assessments in London

Email your drawings or WhatsApp us for a quote.