Designing to prevent homes overheating TM59 is the way to go

17 January 2025 by Carlin Osborne

Overheating kills and it can kill quickly. The number of days where temperatures climb over 25°C is projected to increase between 40% and 100% by 2040 in New Zealand. Overheating is an especially significant risk for vulnerable groups like children, the elderly and those with pre-existing health conditions.

It’s much harder to ensure a building stays cool in summer compared to warm enough in winter. Overheating often occurs over just part of a day and in parts of a building. Plus each household is unique because of individual circumstances: how people live in their home, whether and when (or if) windows are opened plus individual preferences about temperature and how well they adapt as it gets hotter. People’s perception of their comfort (or otherwise) matters a lot and simple blanket targets don’t take that into account.

Building designers need to either design to prevent overheating to an acceptable level for the client or provide active cooling or do both. It’s more complicated when clients do not want to install active cooling or occupants will not use cooling because of cost, even if it’s installed. 

Ideally you want to determine during the design stage if overheating can be prevented by only opening windows or if active cooling is required. Some well-intentioned attempts to reduce overheating risk can really produce some perverse outcomes—more on that toward the end. But let’s start with what we do recommend for designing homes that are comfortable temperatures year round. 

TM59: a standardised way to design

We think TM59 is the best way to specify and design homes that counter overheating risks. It’s a design methodology developed by CIBSE for the assessment of overheating risk in homes. It provides a standardised approach to overheating assessments and addresses the complex ways residential buildings respond to external temperatures. This methodology is widely used in the UK. Here in Aotearoa, Kāinga Ora uses TM59 to assess overheating across its portfolio and its use achieves Homestar points (HC2: Summer Comfort).

We need assessment frameworks that reflect how people behave in real life. This is what adaptive comfort models seek to do. They come out of ISO 7730, which defines thermal comfort based on environmental variables, activity levels and clothing insulation. TM59 takes an adaptive comfort approach but also specifies some temperature thresholds. This provides clarity and consistency between overheating assessments. If it fails the TM59 criteria you need to put in active cooling (or improve your design).

TM59 is robust, clear, works at design stage, is proven in application and it is standardised. It can be used for single family homes as well as complex apartment buildings. It uses hourly climate data to examine room-by-room hourly temperatures, wind flow and natural ventilation through the dwelling, rather than the monthly data used for PHPP energy modelling. Sustainable Engineering Ltd can undertake these more detailed TM59 assessments. Tip: apartments and developments with openings and windows on only one or two sides of the building are particularly important to model.

How does TM59 work?

To ensure residential buildings are thermally comfortable while also minimising the risk of overheating, TM59 sets out two main criteria.

  1. In living areas, indoor temperatures can only exceed adaptive comfort thresholds for a set number of hours.
  2. Bedroom temperatures must remain below 26°C during sleeping hours (10pm-7am) for less than 26 hours over the whole year.

TM59 assessments rely on Design Summer Year (DSY1) climate files to simulate realistic warm-weather scenarios and MBIE has released these for all the NZ climate zones. We have them here

We recommend the following options for TM59 overheating assessments. 

  • Use DSY1 M2 (2050, SSP2-4.5) for pass/fail evaluations. 
  • Conduct stress testing using DSY1 M3 (2070, SSP3-7.0) to test resilience under more extreme climate scenarios. (This aligns with Kāinga Ora’s approach and ensures that building designs are robust enough to handle future climatic conditions.)
  • Bathrooms are best modelled as corridors in TM59 assessments even though this is not explicitly required. This ensures the evaluation of overheating risks is conservative and provides some kind of oversight on bathrooms that might get really hot (such as the ones with big west-facing windows).

DSY1 weather files represent a year with a moderately warm summer (warmer than typical for the location, with a return period of about seven years). M2 or M3 is a morphed version of this file made hotter to account for different emissions predictions. This is in contrast to a Typical Meteorological Year (TMY3), which is an average year (that is, neither warmer or colder than average). That is used to predict overall building energy consumption etc, different entirely from occupant thermal comfort. 

Additionally TM59 analysis defines conservative internal heat gain loads, window opening schedules and occupancy in a standard way to assess the thermal comfort for each space in the model and for each hour of the year.

TM59 reduces overheating risk-it relies on more accurate DSY data

TM59 reduces overheating risk-it relies on more accurate DSY data

TM59 reduces overheating risk-it relies on more accurate DSY data

The difference in air temperature between the DSY_M2 and TMY file can be significant as warmer summers can be significantly warmer than average. In these graphs you can see the DSY temperatures are up to 10C higher! All graphs © 2025 Sustainable Engineering Ltd

A note about social housing

Our team is pleased to see some housing providers start to address overheating risks in their requirements but there are right and wrong ways to go about this. We’re aware now of two different city councils which have specified that measured post-construction internal temperatures not exceed 25C for longer than four hours at a time or active cooling needs to be retrofitted. The motivation is good but this is not a good way of achieving the outcome they want. This just pushes risk to the end of the project or active cooling is installed every time instead of designing the building to achieve what you want. It has multiple unintended negative consequences for everyone involved: tenants, developers and property owners. 

We are preparing a white paper for housing providers that explains the problems such a requirement causes and offers TM59 as an alternative that will lead to much better outcomes. We’ll be looking to our network to help get this information in front of the decision makers at councils and community housing providers etc. Follow Sustainable Engineering Ltd on LinkedIn to find out how you can help.