Building Regulations Part O: Overheating Assessments

Walker Energy Analysis provides Part O overheating assessments for residential schemes, using CIBSE’s TM59 methodology and dynamic thermal modelling in IES Virtual Environment. We give you clear, defensible pass evidence for building control, and if the design doesn’t pass first time, we tell you what to change and why.
 

Part O has been a mandatory requirement since June 2022 for new residential buildings in England. Building control won’t sign off without it, and getting caught out late in the programme, after glazing and layouts are fixed, is an expensive place to discover an overheating problem.

Simplified Method or Dynamic Modelling: We Cover Both Routes

Approved Document O sets out two ways to demonstrate compliance, and which one suits your project depends on the design.

The simplified method is a set of fixed rules on glazing area and free area for ventilation. It’s quick to apply, but it’s conservative. Schemes with larger glazing areas, exposed sites, high levels of shading, or high insulation and airtightness standards can fail it even when the building would perform perfectly well in reality.

Dynamic thermal modelling, following the CIBSE TM59 methodology, tests how the building actually behaves, hour by hour, room by room, against a full year’s weather data. We model using CIBSE weather files local to the site, based on the 2020 DSY1 data set, along with DSY2 and DSY3 as recommended by TM59. It’s the route that gives you the most design freedom, and it’s often the only realistic option once a scheme is beyond early massing and the glazing strategy is set.

We’ll tell you early which route fits your project, and run the assessment accordingly, including a mix of both across a single site where that’s the more efficient approach.

Every TM59 assessment includes:

  • Room-by-room zoning of the whole building or a representative sample, agreed with you based on the scheme’s size and risk
  • Testing against both TM59 pass criteria: living areas and bedrooms
  • A completed Part O compliance report, matching Appendix B’s checklist requirements
  • Clear identification of which design features are doing the work, so the strategy holds if the design changes later
  • Practical mitigation recommendations where a unit doesn’t pass first time: shading, opening areas, g-values, or mechanical cooling where passive measures aren’t enough

Mixed-Use Schemes

Part O applies only to new residential buildings, so extensions, conservatories added to existing homes, and buildings undergoing a change of use fall outside its scope. Where it does still apply is mixed-use schemes: residential over commercial, for example, need Part O applied to the residential element, alongside whatever compliance route the non-domestic space follows separately. We’ll confirm early which parts of a mixed-use scheme are actually in scope, so the assessment is targeted correctly from the outset.

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 and building control can use immediately.
 

FAQs

Q: Do all residential buildings need a Part O assessment?

A: Yes. Part O applies to new residential buildings in England, including dwellings, flats, and institutional and other residential accommodation such as student housing and care homes.

Q: Which method should I use, simplified or dynamic modelling?
A: It depends on the design. If your scheme has generous glazing, high airtightness, unusual shading, or sits on an exposed or high-risk site, dynamic modelling gives a much more realistic result and more design flexibility than the simplified method.

Q: What happens if a unit fails the TM59 assessment?
A: We identify what’s driving the risk and recommend practical changes, adjusting openable areas, ventilation, shading, glazing specification, or, where passive measures aren’t enough, mechanical cooling, then re-test to confirm the fix works.

Q: Do you assess the whole building, or a sample?
A: It depends on the scheme, but usually the whole building. We agree the right approach with you based on the building’s size and overheating risk profile. Sometimes that’s the whole building, sometimes a representative sample in line with CIBSE guidance.

Get Your Part O Evidence in Place

Talk to Walker Energy Analysis before your strategy is fixed, and avoid a late-stage overheating problem.
 
Approved Document Part O

Follow this link to the full guidance on Approved Document Part O.

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