CIBSE TM59 overheating assessment
Walker Energy Analysis carries out CIBSE TM59 overheating assessments for residential schemes across the UK, using dynamic thermal modelling in IES Virtual Environment. TM59 is the standard methodology for predicting overheating risk in homes, and it’s the evidence base that sits behind several different requirements — Part O, GLA and London Plan energy assessments, BREEAM HEA04, and voluntary risk assessment for care homes, student accommodation, and other vulnerable-occupant buildings.
Rather than treating each of these as a separate piece of work, we build the TM59 assessment once and apply it to whichever requirement your project actually needs to satisfy.
How the TM59 Methodology Works
TM59 tests units from the scheme, zoned room by room, and models them against a full year of weather data using the building’s actual construction, glazing, shading and ventilation strategy. Depending on the project, we model the whole building or a representative sample – we’ll agree the right approach with you based on the scheme’s size and risk profile.
Homes that are predominantly naturally ventilated are tested against two criteria: total hours of overheating across living rooms, kitchens and bedrooms during May to September, and a separate night-time comfort check for bedrooms between 10pm and 7am. Both criteria have to pass. Homes that are predominantly mechanically ventilated – where opening windows isn’t a realistic option – are instead tested against a fixed temperature threshold, following CIBSE Guide A.
Where a scheme includes care homes or other vulnerable-occupant accommodation, the assessment applies adjusted occupancy assumptions to reflect that risk.
- Room-by-room zoning across the whole building or a representative sample, as appropriate to the scheme
- Dynamic thermal modelling in IES VE, using the appropriate CIBSE design summer year weather data
- Testing against both naturally ventilated and mechanically ventilated criteria, as applicable
- Adjusted assessment for care homes and vulnerable-occupant accommodation
- A report structured to support whichever requirement it’s being submitted against — Part O, planning, or BREEAM
One Assessment, Several Uses
TM59 is required for Part O compliance where the dynamic thermal modelling route is used instead of the simplified method. It’s also required at planning stage for major schemes in London – the GLA expects dynamic overheating modelling using TM59 and the associated London design summer year weather data regardless of which Part O route the scheme ultimately follows, so this is often the first assessment needed on a project, not the last. The same modelling underpins BREEAM’s HEA04 thermal comfort credit, and can be used as a standalone risk assessment for schemes outside London or outside Building Regulations scope where a client or funder wants overheating risk understood early.
If you know your project needs to satisfy more than one of these, tell us at the outset – the modelling can usually be structured to serve all of them without duplicating the work.
Why M&E Teams Work With Us
We’re a specialist energy consultancy – this is the core of what we do, not a side line. Every assessment runs through IES VE as standard, we turn changes around quickly, and results come back in a format your team, building control, or a planning officer can use immediately.
FAQs
Q: Is TM59 the same thing as Part O?
A: No – TM59 is the underlying modelling methodology. Part O is the Building Regulations requirement, which can be satisfied either through a simplified method or through dynamic thermal modelling using TM59.
Q: Do I need TM59 for a London planning application even if my scheme will pass Part O’s simplified method?
A: Often, yes. The GLA generally expects dynamic overheating modelling using TM59 at planning stage regardless of which Part O route the scheme later follows.
Q: What weather data is used?
A: We use CIBSE weather files local to the site, based on the 2020 DSY1 data set, along with DSY2 and DSY3 as recommended by TM59 to test against a short intense heatwave and a longer, less intense warm spell.
Q: Can the same assessment cover Part O and a BREEAM HEA04 credit?
A: Often, yes – the underlying thermal modelling is closely related. We check this against your specific project early so the assessment can be structured to support both where possible.
Get Your Overheating Risk Assessed Early
Talk to Walker Energy Analysis before your glazing and ventilation strategy is fixed, whether you need TM59 for Part O, planning, BREEAM, or all three.
Does your project need something else?