Abstract
The UK is committed to meeting a Zero-Carbon policy by 2050 to tackle climate change. Whilst decarbonisation of the built environment through improving energy efficiency is key, there is some potential for unintended consequences related to the deterioration of the Indoor Environment Quality (IEQ). Considering that people spend up to 90% of their time in indoor spaces, including office buildings, within industrialised countries, the impact of the indoor environment on health and wellbeing can be significant. An important aspect which is affected by a poor indoor environment is productivity. This has a significant impact on personal wellbeing and the wider economy. However, building design policies have mainly focused on comfort criteria and the reduction of energy consumption rather than occupants’ wellbeing and productivity. This can impose a range for temperature and Relative Humidity (RH) which does not necessarily comply with the range for optimum productivity. The aim of this PhD, therefore, is to analyse the impact of variations in both temperature and RH on productivity.Although several previous studies have considered the impact of mild thermal stress (a few degrees above or below an optimum comfort temperature) on occupants’ productivity, the results are not consistent. There are two conflicting theoretical models for the relationship between temperature and productivity; the `Extended-U theory', which proposes a range of temperatures in which productivity is not affected significantly, and the `Inverted-U theory', which identifies one optimum temperature for the best productivity and any deviation from that temperature results in a productivity decrease. Not only there is no consensus about the effect of temperature on productivity, but the impact of RH and its combined effect with temperature is poorly appreciated and often ignored in IEQ studies. Furthermore, research to date has tended to view `productivity' as a single construct and has not asked whether the various cognitive components underpinning productivity are affected differently by realistic changes in the environment.
In this study, 56 participants undertook a range of cognitive tests – each investigating different underlying cognitive abilities – within an environmental chamber under different temperatures and RH conditions. At the same time, they responded to questions about their self-perceived productivity and workload, their thermal comfort level, and questions about health and wellbeing. In total, participants were subjected to 12 different hygrothermal conditions; combinations of five temperatures (15 °C, 17 °C, 21 °C, 25 °C and 27 °C), and three RH levels (30%, 50% and 70%). The experiments were carried out in two phases; In Phase 1, 24 participants were tested in nine combinations of 17 °C, 21 °C and 25 °C and 30%, 50%, and 70% RH , while, in Phase 2, 32 participants were subject to more extreme temperatures of 15 °C and 27 °C in combination with RH of 30% and 70% in addition to a baseline condition of 21 °C and 50% RH, to enable comparison between the two phases. In phase 2, people were exposed to cognitive tests and questionnaires twice, once 15 minutes after the beginning of the session and once at the end of the one-hour session. This allows assessing the impact of exposure time on the results.
The relationship between temperature and objective productivity (cognitive productivity) was found to align with the `Extended-U theory'. This suggests that productivity is not affected significantly within the range - 17 °C to 25 °C in UK climate, regardless of RH level. When subject to temperatures outside the thermal comfort range (i.e. 15 °C and 27 °C, a productivity loss was observed. Cognitive functions and, more specifically, productivity in different cognitive tasks are found to be different in different hygrothermal conditions. It is shown that working memory and executive function tasks are the most affected functions of productivity; while tasks such as ‘Visual Search’ which measure visual attention, showed no significant degradation.
Interestingly, some cognitive tasks results suggest an `Inverted-W shape' relationship between temperature and productivity such that a few degrees difference with the most comfortable temperature increased productivity. As such, the productivity at 17 °C and 25 °C are slightly higher than that of 21 °C. It is observed that optimum productivity and optimum thermal comfort do not necessarily coincide. As a result, designing based on comfort does not necessarily lead to maximum productivity. At constant temperatures, no effect of RH is observed on productivity while there are specific permutations of temperature and RH that affect productivity of some cognitive tests compared to the base condition of 21 °C and 50% RH.
Self-reported productivity on the other hand fits the `Inverted-U theory', such that, even within the thermal comfort range, unlike the measures of cognitive productivity, people’s perception was that productivity was affected significantly when temperature varied from 21 °C. This correlates with their perception of thermal comfort and wellbeing and the prevalence of reported SBS symptoms within this thermal comfort range. The combined impact of temperature and RH is also observed in the subjective results such that at higher temperatures, a lower RH decreases productivity loss, while, at lower temperatures, a higher RH decreases productivity loss. As a result of variation in RH, self-reported productivity and cognitive productivity results can vary significantly, especially when temperatures are within thermal comfort range. This may explain the discrepancies in the existing literature. However, it is shown that by increasing the length of time subjects are exposed to the condition, they tend to have a better estimation of their real productivity.
Mild fatigue and Workload were also evaluated to assess if they are affected by different hygrothermal conditions. Mild fatigue, which was measured using blink rates and via a self-reported fatigue question (as part of the SBS questionnaire), is found to increase with time in more extreme conditions while, at 21°C and 50% RH, there is no/less increase in fatigue level. The self-reported mental workload - the mental construct imposed on a worker doing a task due to environmental or operational conditions- was measured using NASA-TLX questionnaire in each condition and was also found to be significantly affected by specific permutations of temperature and RH compared to the base condition, with more extreme conditions causing an increase in workload.
The outcome of this PhD shows demonstrable variation in both the effectiveness of different tests in assessing wellbeing and productivity, and the impact the indoor environment has on them. These methodological differences in productivity measurement against changes in temperature and RH are shown to contribute to the existing discrepancies in the literature. The holistic methodology developed in this thesis will enable the future design of buildings to be informed based on productivity rather than comfort alone. Addressing the impact of IEQ on productivity within office buildings will impact both productivity and energy use in offices. Significantly, this has environmental and financial implications on building operations and staff.
| Date of Award | 29 Mar 2023 |
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| Original language | English |
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| Supervisor | Dan Maskell (Supervisor), Antony Darby (Supervisor) & Ian Walker (Supervisor) |