When assessing whether a residential building will overheat, CIBSE Guide TM59 should be used. This is not only considered a robust design methodology step, but is a requirement for London Plan and the new Part L 2021.

What is TM59

So what is it? TM59 takes the first Criterion from CIBSE TM52, the overheating methodology for non-domestic buildings. This is adaptive in that a daily running mean temperature is used to calculate what the maximum temperature can be on an hourly basis. This is important as it gets hotter, so will people’s ability to adjust to these hotter temperatures. Therefore, when assessing under future weather files, it can be a robust method for ensuring a building can still be comfortable.

The issue with TM59

The issue with TM59 is centred around the bedroom metric, TM59 states that between 10pm – 7am, the temperature cannot exceed 26°C for more than 32 hours. We are finding that well designed buildings, ventilation louvres and thermal mass are struggling to achieve this when assessing using a future, more intensive 2050 or 2080 weather file. This is due to the requirement for doors to be shut, limiting the potential for cross ventilation and the thermal mass releasing heat overnight.

The big problem it that this metric is not adaptive and takes no account of people’s ability to adapt to higher overnight temperatures that may be prevalent in 30 years’ time. On one building we have looked at, if the threshold was raised by just 1°C the building could pass.

Whilst this part of TM59 is a robust metric for assessing buildings just now using the current weather files, we believe the metric needs to be adapted slightly when assessing under a future weather file.