The new study, commissioned by climate solutions company Zehnder Group UK in collaboration with CIBSE Journal, found that while a third (34%) of respondents claimed to know Part O inside out, the majority had a limited understanding. And, concerningly, 19% of respondents admitted they simply don’t understand it at all. 

These findings form part of a new report released today that surveyed architects, consultants, developers and contractors, to gain a better insight into the building industry’s understanding of overheating.

The issue of overheating

When residential properties exceed a temperature threshold of 26°C for extended periods it can seriously affect occupant thermal comfort, health and wellbeing as well as productivity. Currently, the Climate Change Committee states that nearly one fifth of UK homes already overheat, even during cool summers. It is reported that 90% of existing UK homes will be at risk of overheating if worldwide temperatures rise to 2°C above pre-industrial levels by 2050, if global warming continues its current trajectory.

The research found that despite this evidence of overheating risks, the topic ranked relatively low among the respondents’ list of priorities, with design, quality, building regulations, and energy efficiency being top in new building projects. Modelling against the risk of overheating ranked eighth. 

Jason Bennett, an indoor air quality expert at Zehnder Group UK, said: “The issue of overheating isn’t going to go away; it’s only going to get worse as climate change has more of an impact, and we continue to create highly energy-efficient homes that are effectively turning into ‘hot boxes’. 

“While improved energy efficiency must continue to be a priority, the future has to be about cooling solutions to ensure comfort and improved health for building occupants.” 

High-rise apartments were identified by 65% of respondents as the most at-risk buildings for overheating. However, a significant proportion raised concerns about schools (40%), care homes (39%), and hospitals (30%). All these building types cater for children and the elderly, who are among the most vulnerable in society. 

Part O in practice

Dr Anastasia Mylona, Technical Director of CIBSE, said: “Many building service consultants and engineers do not encounter overheating due to the specialised nature of their roles. If you’re not doing overheating assessments, then you will not know a lot about Part O and how to comply with it.

“Engineers and consultants working in smaller teams are more likely to carry out Part O compliance calculations. It must be looked at in the context of the much lower level of awareness about overheating just a few years ago. Five or 10 years ago, nobody thought about it. That is a massive improvement that Part O has brought.”

Yet Zehnder believes this lack of understanding is “a worry”. Jason counters that the 19% may include contractors who tend not to get involved with specifying overheating measures, but it will include others that do. “I would expect all the consultants, developers, specifiers and architects to know about Part O. [It] suggests that overheating should be more firmly embedded within the sector’s education curriculum.”

Knowledge of the causes of overheating in buildings was higher among respondents. The majority of respondents (78%) pointed to solar gain through increasing glazing as a primary cause, followed by unsuitable ventilation (45%), global warming (40%), inability to open windows (40%), and excessive insulation (39%). 

When it comes to tackling overheating in buildings, 78% of respondents said that they actively prioritised passive cooling options, such as solar shading, optimised glazing and openable windows, over mechanical ones, with just 14% opting for the latter, reflecting the approach laid out in Part O.  

The survey findings underscore the need for a holistic and joined-up approach to buildings that incorporate robust strategies to mitigate overheating risks. This includes promoting better indoor air quality alongside cooling.

Jason said: “Mechanical cooling can be achieved with air conditioning, but this method is costly to run and recirculates air rather than replaces it with filtered air – and you would still need to ventilate the property. A better solution is to incorporate air temperation into an effective ventilation strategy, which is also required in line with Part F. The difference is fresh, clean air supply all year round with heat recovery in the winter and cooling in the hotter months – promoting better, healthy indoor air quality.”