CIBSE TM59 inputs and the criteria a scheme fails on

TM59 overheating criteria are two tests, not one, and a residential scheme has to pass both. The first is an adaptive test applied to living rooms, kitchens and bedrooms across the summer. The second applies to bedrooms at night and uses a fixed temperature. Reviewers who check only the first miss the failure mode that matters most, because a bedroom can pass the adaptive test comfortably and still be too warm to sleep in.

Everything else about the method is input selection, and that is where most of the arguments happen.

What TM59 asks of a residential design

TM59 sets out a dynamic thermal modelling methodology for assessing overheating risk in homes. It specifies the modelling approach, the occupancy profiles, the internal gains, the window opening logic and the criteria, which is what makes results comparable between consultants.

For predominantly naturally ventilated homes the two criteria are:

• An adaptive criterion applied to living rooms, kitchens and bedrooms, limiting the hours during which the operative temperature exceeds the adaptive comfort threshold by one degree or more across the summer period to a small percentage of occupied hours.
• A fixed criterion applied to bedrooms during night hours, limiting the time the operative temperature exceeds 26 degrees to 1 per cent of annual hours.

For predominantly mechanically ventilated homes a single fixed criterion applies across occupied rooms, limiting hours above 26 degrees to a small percentage of annual occupied hours.

Those percentages translate into a specific number of hours, which depends on the period and the occupancy profile. State the hour count in the report rather than the percentage alone, because a reviewer cannot otherwise check the arithmetic.

One distinction worth making early. A TM59 assessment for a planning submission and an overheating assessment for Building Regulations compliance under Approved Document O are not the same exercise. Part O adopts a modelling route based on TM59 with its own modifications, including the weather data to be used and the treatment of openings, so a model built for one does not automatically satisfy the other.

Weather file choice and future scenarios

The weather file is the single most consequential input, and it is the one most often left undocumented.

The assessment uses a Design Summer Year file for the nearest appropriate location, which represents a moderately warm summer rather than an average or an extreme one. Files exist for several locations and for future time periods under emissions scenarios, and some authorities require a future scenario to be tested alongside the current one.

Three things to state explicitly: the location of the file, the time period it represents, and which of the available summer year variants was used. Where more extreme variants exist, running them as a sensitivity test is good practice and tells the client something useful about resilience, even where the criteria are assessed against the standard file.

A model run on an average weather year rather than a design summer year will pass almost anything. That is the first thing to check when reviewing somebody else's results.

Occupancy profiles and internal gains

TM59 prescribes the profiles so that results are comparable, which means departures have to be justified rather than quietly adopted.

The prescribed approach sets bedrooms as occupied continuously through the night and partially during the day, with living areas occupied through the day and evening, and it specifies the occupancy per room type based on the number of bedrooms. Internal gains cover people, lighting, equipment and cooking, with values set by the methodology.

Two inputs deserve particular attention in a review.

Heat from building services. Uninsulated or poorly insulated communal heating pipework in risers and corridors is a substantial gain, and it is frequently omitted or understated. On a scheme served by a heat network, the distribution losses have to be modelled with the actual pipe insulation specification, and where the network connection is being safeguarded for the future the riser design still needs to reflect it, a point we cover in safeguarding a future heat network connection.

Window opening logic. The methodology opens windows when the room is occupied and the internal temperature rises above a threshold. Where openings are restricted, either for fall protection, for security at ground level, or because of external noise, the restriction has to be modelled as it will be built. A model that opens a window to its full extent where a restrictor will limit it to 100mm is not describing the building.

The two criteria and typical failures

Four failure patterns account for most results in practice.

The night criterion is the one that catches schemes that look acceptable. A bedroom with a modest glazing area can pass the adaptive test and then fail at night because the fabric has stored heat during the day and the room cannot be purged while the occupant is in it.

Where noise forces windows closed, the overheating assessment and the acoustic assessment have to be run together with the same assumptions. The combination of a closed window acoustic specification and an open window overheating model is the single most common inconsistency in a residential submission, and reviewers should ask for both reports side by side.

Roof level and mansard units deserve separate attention, and we look at their particular issues in overheating in roof level and mansard flats.

Single aspect flats and corridor temperatures

Single aspect units are not prohibited and they are harder to resolve. The measures that work are reducing the glazing area, improving the shading, increasing the openable free area, and reducing internal gains, in roughly that order of effectiveness.

Corridors matter more than their floor area suggests. An internal corridor with heating pipework, lighting and no external openings becomes a heat reservoir, and a flat ventilated through a door onto that corridor cannot be purged. TM59 recommends assessing communal corridor temperatures against their own threshold, and a corridor result above it undermines any flat relying on cross ventilation to the corridor.

Three design responses are worth testing before mechanical cooling is proposed: insulating or relocating the distribution pipework, ventilating the corridor to outside at each end, and designing the flats so purge ventilation does not depend on the corridor at all.

Mechanical cooling as a last resort

The cooling hierarchy in policy is sequential, and an assessment that proposes active cooling without working through the earlier steps will be sent back.

The sequence is to minimise internal heat generation, then reduce the heat entering the building in summer, then manage the heat within the structure, then use passive ventilation, then mechanical ventilation, and only then active cooling. The submission should show the steps as iterations of the model rather than as a list of intentions, with the result after each step.

Where active cooling is unavoidable, three consequences follow. Energy use rises, which affects the primary energy position in the Energy Statement. Plant has to go somewhere, which affects the roof layout and the noise assessment. And the refrigerant choice becomes a planning matter in its own right on some schemes.

Presenting results so they can be audited

A reviewer needs to be able to reproduce the result, and that is a document design problem as much as a modelling one.

An auditable Overheating Risk Assessment contains the weather file named in full with its location, period and variant, the software and version, a room schedule listing every assessed room with its orientation, floor level, glazing area, openable free area and result against each criterion, the sample justification where not every unit has been modelled, the window opening and restrictor assumptions with drawing references, the internal gains schedule, the distribution loss assumptions with the pipe insulation specification, and the hours of exceedance as absolute numbers alongside the percentages.

Add the worst-case rooms as a short table at the front. Officers and reviewers go looking for them, and a report that volunteers them reads as honest. Councils normally expect this work from a suitably qualified author, and professional review of any draft is strongly advised before it is submitted.

And where a result sits marginally within a criterion, say so. A bedroom at 95 per cent of the permitted exceedance hours is a design risk, not a pass to be celebrated, and the client is better told at design stage than after handover.

Enter your postcode on PlanScope to see which energy and comfort reports your scheme is likely to need.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.