Overheating Position Statement

Summary of action

CIBSE encourages its Members and the wider industry to follow current CIBSE guidance, along with the other relevant industry guidance identified here, in all new designs and major refurbishments to assess the risk of overheating and mitigation options. CIBSE also recommends that, as well as energy performance of buildings, occupant safety, comfort, health and wellbeing are also considered at a design stage, in a holistic way where the energy and environmental performance of buildings is investigated throughout the year.

Summary of issue

Ever increasing winter energy efficiency measures and external temperatures as a result of intense urbanisation and climate change are increasing the risk of overheating in buildings especially in homes that primarily rely on passive measures to achieve year-round internal comfort. Passive design principles such as sufficient natural ventilation and shading can still achieve indoor thermal comfort when considered early in the design process and as part of the building’s year-round performance.

Key points

High indoor temperatures affect occupant thermal comfort, health and wellbeing, and productivity.
Productivity is affected by daytime high temperatures in the work environment or indirectly through sleep deprivation during night-time.
In heating dominated climates, such as that of the UK, winter morbidity and mortality are likely to remain a primary concern in the future. Nonetheless, heat-related deaths are expected to rise as a result of climate change induced increases in temperature as well as the frequency and severity of heatwave events (CCC, 2014, Managing climate risks to wellbeing and the economy)
Overheating in buildings is expected, if not addressed, to cause some 4,500 premature deaths per year by 2050 (CCC, 2019, Progress in preparing for climate change - 2019 Report to Parliament)
Environmental factors affecting thermal comfort include air temperature, radiant temperature, air speed and humidity; personal factors include age, gender, state of health, clothing and activity levels. Because of these multiple influences, it is difficult to set a single set of environmental conditions which would be comfortable for everyone (an environment which is comfortable for someone who is physically inactive may be too hot for someone engaged in sustained physical activity). This is an important reason why CIBSE advocate for adaptive comfort criteria where possible, and in many cases for a degree of user control to be provided over their environment.

CIBSE’s position

CIBSE considers that occupant safety, comfort, health and wellbeing should be a priority in the design of buildings and recommends a holistic approach to performance analysis at design stage where internal thermal comfort is investigated throughout the year. This requirement should be reflected in the regulatory framework which currently is not sufficiently addressing the potential risk of overheating in buildings. CIBSE expressed this position at the Environmental Audit Committee’s inquiry ‘Heatwaves: Adapting to Climate Change’ and a clear recommendation that building regulations should address overheating risk was included in the Committee’s subsequent published report.

Occupants of domestic buildings are especially vulnerable to overheating as homes often rely on passive measures to combat the risk. The behaviour of these occupants is much more unpredictable than in the non-domestic sector. These aspects make homes more prone to the risk of overheating and their design presents a greater challenge to professionals to make them resilient to hot weather events.

The most vulnerable occupants are elderly people, young children and those with pre-existing health problems who are likely to suffer most as a result of higher internal temperatures. Additionally, they tend to spend more time in their homes with possibly reduced capacity to adapt their circumstances and their environment in order to become more comfortable and to enhance their wellbeing.

If overheating risk in buildings is not urgently addressed by policymakers and industry, we anticipate a significant increase in the use of mechanical cooling systems such as air conditioning and fans. This could potentially reverse much of the progress that has been made in reducing the energy consumed in buildings, just when targets of reducing energy consumption are getting more ambitious.

Recommended guidance

CIBSE has adopted the adaptive thermal model to define thermal comfort and design overheating criteria (TM52). The adaptive thermal comfort model is based on the principle that an individual’s thermal expectations and preferences are determined by their experience of recent (outdoor) temperatures and a range of contextual factors, such as their access to environmental controls. The adaptive thermal comfort model allows for the natural adaptation of human physiology to extended periods of hot events. By using the adaptive thermal comfort model, instead of the use of static comfort temperatures, the periods of discomfort, and so the potential energy demand for active cooling, are not overestimated.
CIBSE has published TM59 Design methodology for the assessment of overheating risk in homes, to provide the industry with a consistent methodology for assessing thermal comfort at design stage. Among other features it provides design comfort criteria based on the adaptive comfort model, standardised heat gains and occupancy profiles.
CIBSE recommends the use of future weather files (TM48 and TM49), that capture future projections of changes in climate when assessing overheating risk and mitigation options at the design stage. The use of the 2020s (2011 – 2040) medium emission scenario Design Summer Year 1 (DSY1) was introduced as the minimum requirement in order to demonstrate compliance with the TM59 methodology as well as for compliance with DfE’s BB101 requirements in the design of schools. CIBSE recommends it is adopted as the minimum weather file used for all overheating assessments. Additional weather files such as DSY 2&3 for the 2020s, as well as future timelines (2050s and 2080s), are recommended to explore performance where there is particular concern, for example presence of vulnerable occupants, and/or where required in the client’s brief, or for testing the performance of mitigation options under more extreme events.
The effectiveness of natural ventilation as an overheating mitigation strategy should be evaluated in relation to any site constraints, such as exposure to outdoor pollutants, noise levels and security concerns that would impact its effectiveness. A new guidance document by the Association of Acoustics and Noise Consultants recommends an approach to acoustic assessments for new residential development that take under consideration the interdependence of acoustics, ventilation, and overheating (https://www.association-of-noise-consultants.co.uk/avo-guide/).
In addition to design stage performance assessment, CIBSE recommends the use of post-occupancy evaluation and in-use monitoring as diagnostic tools during the operational phase of buildings, to identify possible discrepancies between design intent and real-life performance, especially of high-risk buildings.
Clear communication of the purpose, operation principles and maintenance schedules of both passive (e.g. window opening and operation of shading) and active (HVAC) systems that are in place to tackle overheating risk in buildings is essential.

The latest draft of the London Plan (2019) requires the use of CIBSE resources to demonstrate the thermal performance and energy efficiency of new developments in London.

Further information

Other relevant CIBSE resources

Other useful resources

Contact

Anastasia Mylona, Head of Research
[email protected]

Date approved by Technology Committee: March 2020