The effects of thermal mass and air-conditioning on summer temperature thermal comfort and occupant behaviour in homes

Abstract

The rapid increase of global mean temperature and the regular occurrence of “once in a century” events such as heatwaves, highlights the necessity to heat proofing buildings. In this respect, thermal comfort
in residential buildings is of primary importance because 80% of a people life is spent in their home.
High indoor temperatures can be prevented by increasing the fabric’s thermal mass and using airconditioning. A high thermal mass passively reduces temperature fluctuations and air-conditioning reduces the discomfort probability providing any desirable thermal condition. In practice both strategies have their limitations: thermal mass cooling efficacy depends on climate and occupant behaviour; airconditioning is energy and carbon intensive and reduces the thermal adaptability of occupants.
Scientific literature lacks empirical observations of cooling extent of high thermal mass buildings compared/or combined with AC, of the interference of these heat-proofing with people’s thermal adaptive processes and behaviours in homes located in Csa climate. Therefore, this PhD thesis addresses this gap in the literature through a field study on indoor temperature, thermal comfort and
occupant behaviour in homes characterized by different levels of thermal mass and air-conditioning presences/absence.
100 rooms and 72 residents in 32 homes in Catania (Csa), Italy, were monitored from June to September2019, during which time 5 heat health warnings and 2 heatwaves occurred [1]. The rooms were categorized into four groups, labelled: ACTM2, ACTM1, NVTM2, NVTM1. The TM2 and TM1 refer to respectively high and medium thermal mass (areal heat capacities of 436.86 and 162.4 JK-1m-2
, response factors of 4.32 and 1.79). The main difference is due to the load-bearing internal/external volcanic stone walls in TM2. The NV and AC indicate naturally ventilated or air-conditioning retrofitted rooms respectively. The methodology is divided into three main stages: longitudinal monitoring and survey campaign of indoor temperature, thermal comfort, and occupant’s behaviour; statistical modelling of thermal comfort and window, shading and AC use with environmental parameters; comparative analysis of the parameters grouped by the thermal mass levels and AC presence/absence in the rooms. The significance of TM2 and TM1 temperature difference was assessed
by a year-long monitoring campaign of an unoccupied sub-sample of rooms; and - prior to this PhD -XVII by dynamic simulation modelling of a room whose external wall was changed from TM2 and TM1thermal mass levels, and simple schedule-based occupant behaviour scenarios were applied.
The results of this thesis showed that the difference in thermal mass and air-conditioning presence/absence have a small effect on thermal conditions limited in time. The ACTM2 rooms did result in the lowest indoor air temperature during the first heatwave in July and the TM2 room temperatures were observed being relatively the highest across room-groups at the end of the summer.
In the hottest months, the thermal comfort analysis showed that: the people in NVTM1 rooms have the lowest thermal sensitivity to the rise of operative temperature; and regardless of AC presence/absence, the people in TM2 rooms are the least accepting of predicted mean vote (PMV) values higher than 1.
At high outdoor temperatures, the occupant behaviour analysis showed that: the windows are most likely closed in AC and open in NV, and in particular those windows in NVTM1 have the highest probability of being open; and also the shadings are likely to be closed as the outdoor temperature rises,
regardless of room group. Finally, the air-conditioning use was found to be affected by thermal mass difference: the TM2 air-conditioners were used for shorter duration at lower set-points in June than ACs in TM1; while, towards the end of the summer, the ACs in TM2 increased were used for longer and
switched off at higher indoor air temperatures.
The overall findings show the limitations of the heat-proofing strategies performing during exceptionally hot summers. Their long-term effects on occupant thermal perceptions and heat acceptability should not be minimized since they are the driving factors to use AC in homes and they are responsible for heat related health risks. In the view of indoor heat prevention without heavily airconditioned indoor spaces, occupant heat adaptation and education to use passive heat-proofing strategies is the priority.

Date of Award	1 Nov 2021
Original language	English
Awarding Institution	University of Bath
Supervisor	David Coley (Supervisor), Sukumar Natarajan (Supervisor) & Marika Vellei (Supervisor)