The importance of the new CIBSE TM59 and how is it different from CIBSE TM52?

What are CIBSE TM52 and TM59?

TM52 and TM59 were both developed by CIBSE to assess and address the risk of overheating in buildings. Whereas TM52 can be used to assess any type of building, TM59 has been tailored to specifically target overheating risk in homes

TM59 “Design methodology for the assessment of overheating risk in homes” aims to address the complex way dwellings respond to external temperatures and present the building industry with a standardised methodology to assess overheating risk.

Why do we need to address overheating?

A home that overheats can have a significant impact on residents. During a heatwave there is a huge risk to the occupant’s health and, with the Committee on Climate Change’s predictions that there will be increasingly more heatwaves in the future, they have estimated that deaths arising from overheating could rise from 2,000 per year in 2015 to 7,000 per year by the 2050s.

There are many factors that contribute to the overheating risk in both new and refurbished homes – and a lot of these are down to design trends such as high proportions of glazing, thermally insulating and single-aspect designs. Community heating systems and inadequate natural ventilation strategies can also contribute towards overheating. Any building that cannot dissipate heat gains is at risk.

What is ‘too hot’?

When assessing temperatures within a home what is considered too hot is subjective – everyone has a different level of comfort. It is dependent on human and environmental factors such as the occupant’s health and their level of fitness.

However, when temperatures exceed 35 degrees Celsius, it can result in stress, anxiety, sleep deprivation and even early death, especially for vulnerable occupants such as young babies, the less mobile and the elderly. A high bedroom temperatures of greater than 26 degrees can lead to sleep deprivation.

How does CIBSE TM59 address the issue?

TM59 sets out a methodology for assessing the risk of a home overheating, broken down into criteria for homes that are predominantly naturally ventilated, those that are predominantly mechanically ventilated and communal corridors.

The first compliance test of TM59 is based on the principals of TM52, “The limits of thermal comfort: avoiding overheating in European buildings” which lays out the percentage of hours that cannot exceed the target temperature, based on the running mean.

The second test is based on CIBSE Guide A’s number of hours exceeding 26oC in bedrooms at night (the temperature above which sleep patterns are likely to be disturbed).

Naturally Ventilated Homes

The two overheating criteria set out by TM59 are:

  1. For living rooms, kitchens and bedrooms: the number of hours during which DT is greater than or equal to one degree (K) during the period May to September inclusive shall not be more than 3 per cent of occupied hours.

  2. For bedrooms only: to guarantee comfort during the sleeping hours the operative temperature in the bedroom from 10 pm to 7 am shall not exceed 26 °C for more than 1% of annual hours.

Mechanically Ventilated Homes

For homes with restricted window openings, the CIBSE fixed temperature test must be followed to assess whether any of the occupied rooms exceed an operative temperature of 26 ̊C for more than 3% of the annual occupied annual hours.

Communal Corridors

The overheating test for corridors is based on the number of annual hours for which an operative temperature of 28°C is exceeded. Whilst there is no mandatory target, if an operative temperature of 28°C is exceeded for more than 3% of total annual hours, this would be flagged as a significant risk.

When should these assessments take place?

As with all design assessments, the earlier the overheating analysis is carried out, the more chance there is of passing without any additional incurred costs, delays or issues.

Professional help and guidance with TM59

At Energytest we can provide fast and accurate CIBSE TM52 and TM59 thermal comfort analysis, helping you to arrive at the most economic and feasible solution to achieve compliance. Our experienced consultants can guide you through the process, from concept to completion.