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The Martin Centre for Architectural and Urban Studies

- the research arm of the Department of Architecture

Studying at Cambridge

Victoria Lee

Heat stress in dwellings: Assessing thermal vulnerability and accounting for exposure duration

Increasing occurrences of record high temperatures, hot spells, and heat waves in recent years have warranted a growing global concern with heat-related deaths and illnesses. As most people live and work indoors, buildings are on the front line of heat-related health impacts, where good thermal performance may attenuate the challenging outside environment and unsatisfactory thermal performance may fail to protect the occupants from and even expose them to further oppressive conditions. It follows that buildings’ role in thermal health will only become ever more important with climate change and the growing urban and ageing populations in the developed and developing countries alike. However, current building research into occupants’ indoor thermal experience does not account for health; neither does epidemiological research into heat-related health impacts consider the mediating effect of buildings. 

In response to this interdisciplinary disconnect, this research sets out to develop a new methodological framework to quantify overheating and heat stress inside buildings in ways that may help inform the relative health risks of the occupants. The overall objective is to improve the assessment of potentially oppressive indoor thermal environments in order to draw inferences from and to link up with existing knowledge regarding thermal health.

The new approach was established in three phases. First, an extensive survey of existing literature in the fields of building research, health sciences, and biometeorology was conducted to gain an understanding of how the health impacts of thermal environments are currently assessed and quantified. This investigation concluded that the state of being overheated (or more precisely, the exposure to heat stress) can be characterised in several different ways: occurrence, intensity, and duration. Epidemiological studies have suggested that each characteristic has distinct and unequal contributions to health impacts. In particular, there are increasing recognition of the importance of heat exposure duration and the cumulative effect of heat it embodies. However, this is the one aspect of the thermal environment that has not been considered in existing building research.

The second phase of this research addresses this gap by proposing a suite of new parameters based on the concept of continuously overheated intervals (COI) to quantify heat exposure duration and to encapsulate the manner with which an oppressive thermal environment can unfold. This part of the research concludes that the data required to conduct existing overheating assessments can be explored further to render previously hidden information that better illustrates the state of being overheated as a process beyond a binary judgement.

Using the idea of structural vulnerability from earthquake engineering as a paradigm, the third phase of this research introduces the concept of heat vulnerability to quantitatively express a building’s susceptibility or predisposition to incur a potentially oppressive indoor environment – as characterised in terms of COI – for a given level of outdoor heat. In essence, heat vulnerability relates in probabilistic terms a building’s thermal response to the external environment (characterised as heat hazard) that triggers that specific response. In so doing, it is able to predict the best possible story of the indoor heat stress situation that can be expected on average for a given outdoor condition. As a proof of concept, this research concludes that not only is heat vulnerability a viable approach to assess indoor heat stress, but it is also a more complete characterisation of a building’s general thermal behaviour.

Three simulation case studies were carried out to illustrate how the concept of heat exposure duration and heat vulnerability may be applied to investigate indoor heat stress at different scales, in different climatic and socioeconomic contexts, and for different practical needs. In particular, the Pakistan case study demonstrates how the new approach can be used to reveal the previously hidden overheating behaviours due to thermal mass and ventilation for a collection of theoretical dwelling archetypes and their variants. An evaluation of the typical semi-detached terraced dwellings in London shows how the additional information gleaned from the new approach can help augment the existing ways of assessing overheating and potentially alter the conclusions drawn regarding the impact of insulation under future climate scenarios. Finally, the Chicago case study illustrates how the new approach can provide an informative way to compare the retrofit trade-offs between winter energy savings and summertime overheating for one detached single family home.