Ventilation and overheating in the UK dwellings

Even with mild climate to date the overheating in the UK homes is a well-documented phenomenon. The issue is a major challenge due the increasing severity of heat waves linked with climate change and heat island effect linked with dense urbanisation. It also has adverse impact on the inhabitants’ health with 2091 excess deaths in England caused by 2003 heatwave. A sharp rise in the use of air conditioning in the EU residential sector is predicted, resulting in 72% increase in the related energy consumption between 2010 and 2030. The underlying physical causes of overheating are well understood with published guidance for the public on how to identify and reduce the risk of overheating in homes. However the significant impact of severe overheating on inhabitant’s practices and the actual cooling effect of these practices is a much less explored area. One example of this is an inhabitant developing a habit of pulling out an in-built oven 10 cm out of its normal position due to an effect of draft associated with such position and observed by chance on the occasion of oven repair.

Our published socio-technical case study of 20 homes in a retrofitted urban apartment block in Leeds using in-depth enquiry has covered both key aspects of overheating in homes: the home systems as well as the inhabitant practices. The mean temperature inside the dwellings during August 2013 was 8^oC higher than the mean outside. The highest outside temperature recorded was 29^oC for one hour across the year whereas inside it reached 42^oC. We discovered that the overheating severity substantially varied across comparable dwellings and that practices were shaped significantly by occupancy patterns (e.g. working from home or leaving home for 10 hours every weekday) and security concerns.

Understanding ventilation systems

Importantly the inhabitants’ level of understanding of the relationship between their practices and overheating revealed major differences between households ranging from innovative best practice to misconceptions and complete lack of knowledge. We used the unique Usability Tool we had previously developed to focus on inhabitant interaction with mechanical and electrical systems controls which showed that ventilation was a problem area for the majority of the inhabitants. After over a year into occupancy the mechanical extract ventilation system was an unknown feature for 20% of all households equipped with the system. Interestingly, the Usability Tool also revealed that cooker hood was used intermittently as a ventilation extract while the actual extract fans were switched off and ignored by 40% households across all seasons. The implications of such practices, clearly missing the opportunity for effective ventilation therefore adding to the overheating severity, were discussed with the respondents directly after their filling in the questionnaires providing them with contextualised explanation of best practice approach. The combination of the ventilation being switched off with floor to ceiling glazing (often unshaded) in airtight single aspect dwellings facing east or west caused such severe overheating that 13 of 20 households examined wedged their front door open to allow cross ventilation despite security concerns. The latter were outweighed with the need for the cooling effect of purge ventilation, but only when the inhabitants were in the dwelling. Some left their front doors open late into the night but all locked the doors when leaving their apartment. As a result those living in west facing apartments and working full time returned home late in the afternoon to experience highest build-up of heat difficult to mitigate. Some of these inhabitants equipped their homes with air conditioning units as last resort. On the other hand there were inhabitants who developed highly contextualised and complex practices that effectively prevented overheating, e.g. some used mechanical extract ventilation continuously as designed, supplementing it when needed with portable free standing tower fans located in direct proximity to open windows to further increase of air change rates. Such strategy combined with black out curtains and wedging the front doors open at times proved efficient in overheating prevention of a top floor apartment, i.e. at highest overheating risk.

The impact of the project related to overheating prevention and mitigation resulted in three households out of 20 repairing broken fans, five households changing their ventilation habits. The change involved e.g. having the extract ventilation fans on when at work, i.e. ca. 10 hours per day. Such practice accommodated the inhabitants’ strong opinion about having the fans off when at home due to noise while allowing air change when it was possible without compromising their acoustic comfort. Other households improved their shading habits by installing curtains or making better use of these. One household found the combination of new practices effective enough to give up the use of air conditioning unit. Interestingly the knowledge accumulated by the 20 households directly involved in research was voluntarily disseminated across the development of over 400 apartments after the end of the project. It was established that five participating inhabitants contributed to discussions on ventilation and best strategies for preventing or coping with heat on a closed Facebook group for their housing development allowing nearly 500 group members to be exposed to their findings. Such use of social media could be encouraged and possibly supported by facility managing companies or residents associations in other developments, where the people live in comparable homes and are likely to be simultaneously at risk of overheating. Building on the inhabitants’ tacit knowledge and self-learning in terms of developing best-practice ventilation strategies for their homes to avoid or mitigate overheating has the potential to significantly strengthen the resilience of new and retrofit housing.

Overheating is a serious and growing issue in the UK. It is related in particular to retrofit and new high rise urban apartment blocks. In the UK the residential buildings higher than 6 storeys above ground contribute by over 1.5% to the overall housing stock. However the trend to build high rise residential buildings is on the rise: more tower blocks are to be built in the UK in 2018 alone than in the previous ten years. These new and retrofit buildings are often inhabited by people who previously lived in typically leaky UK dwellings where overheating was less of a problem. The findings indicate the important role inhabitants have in preventing or aggravating overheating in a UK domestic context. This calls for paying more attention to what the inhabitants really do when studying building performance and to inform and encourage design and technologies that enable these rich practices.