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Tackling overheating – design tools to address the risk

Overheating in buildings is a hot topic at Ecobuild this year. We know the key elements that need to be tweaked in the design process to help prevent new homes from overheating, but why have these not already been integrated into standard industry practice here in the UK? Dr Anastasia Mylona, an expert in thermal comfort, and the Research Manager for CIBSE, explains what is possible…

Engineers have the knowledge and technical information to assess building performance against summer as well as winter temperatures, and projected changes in climate until the end of the century: so why are clients such as housing associations, private landlords, housing developers, self-builders and those professionals that use the outputs of models to inform design of homes, for instance, architects, not requesting it?

Ever increasing winter energy efficiency measures and increasing external temperatures as a result of intense urbanisation and climate change will increase the risk of overheating in buildings. The risk is likely to be greatest in homes that primarily depend on passive measures to achieve year round internal comfort.

Occupant comfort, health and well-being should be a priority in the design of buildings. In order to achieve this, a holistic approach to performance analysis needs to be implemented where internal thermal comfort is investigated throughout the year.

Overheating and design

A detailed review of all design tools and methodologies for assessing overheating risk, as well as all aspects of our current knowledge of the overheating risk in homes, has been published by the Zero Carbon Hub. The review reveals that the design tools and models (described below) to assess overheating risk are not perfect, there are known issues with all of them. For example:

  • The design tools are not accurate but sensitive to the users’ ability – small changes in the description of the building and its occupancy can produce very different results.
  • The adaptive thermal comfort criteria are mainly based on office-based field studies, so arguably not applicable to the design of homes.
  • The weather files are selected based on temperature only, while we know that humidity and solar radiation could affect thermal comfort too.

The truth is that there will always be uncertainty in the design process. The design elements outlined above can give you an indication of your building’s vulnerability but cannot predict its exact performance. That should not prevent the industry from addressing the issue of overheating.

The design and assessment processes in place for the energy performance of buildings are equally, if not more, uncertain. There are well known gaps in predicted and actual energy performance of buildings to prove that. The only difference is that energy assessment is compulsory. Building regulations demand that certain tests need to be followed in order to show that the use of energy within the building has been addressed. There is no such provision for summer thermal comfort of occupants. All the necessary elements exist to develop contractual or regulatory ‘sticks’ in order to start addressing a current problem that could potentially become very serious in the future.

Technical detail: modelling of overheating risk

The key elements in a design methodology that assess overheating risk are;

  • a calculation algorithm (a design tool)
  • a description of the building and its occupancy
  • a description of external environmental conditions (weather data); and
  • a definition of overheating with pass/fail criteria.

All elements are available to industry to use, but the motivation seems to be missing.

The thermal performance of a building is a dynamic phenomenon, constantly changing based on the interaction between building characteristics and external environment. The most appropriate design tools to provide a realistic thermal performance are the Dynamic Simulation Models (DSM). These allow for the detailed description of the building, its occupancy, and the description of external environment to be considered. As an output, among other things, they offer hourly thermal performance profiles.

Assessment of overheating risk within the design process

But how can one assess overheating within the design process? The professional institute responsible for building services engineering (or alternatively ‘building systems’), the Chartered Institute of Building Service Engineers (CIBSE), has provided the industry with guidance and technical advice that can be applied to assess the vulnerability of a building design to overheating.  At a European level, there is a standard to assess overheating – BS EN 15251: 2007. CIBSE has recently published for its members a new guidance document which defines thermal comfort and describes design overheating pass/fail criteria (TM52) based on the adaptive thermal comfort model.

The adaptive thermal comfort model explained:

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. Contrary to the use of static comfort temperatures, the periods of discomfort and potential energy demand for active cooling, are not overestimated.

Furthermore, CIBSE has available a series of weather files (the Design Summer Years – DSYs) appropriate for overheating analysis which are available for 14 UK locations.
The DSYs are hourly weather files, a description of a near extreme hot year, selected based on a twenty-year baseline of observations. Future DSYs are also available based on climate change projections for the periods of 2020s, 2050s and 2080s.

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