The World Health Organization recommends a minimum indoor temperature of 18 °C, and 20–21 °C for more vulnerable occupants, such as older people and young children [1
]. Previous studies show that the minimum threshold of indoor temperature required for limiting respiratory infections is 16 °C, meaning that there is an increased risk of respiratory infections when indoor temperatures are below 16 °C. A growing body of epidemiological evidence shows links between indoor temperatures and excess winter mortality and morbidity in various European regions, although difficulties of demonstrating direct causality exist. To date, studies relating to cold homes and health effects have been largely carried out in the United Kingdom, Ireland and New Zealand. However, epidemiological research has shown that the problem of cold indoor temperatures is replicated in other countries. Where buildings are designed primarily to cope with extreme summer temperatures, houses may not effectively protect against low temperatures during the relatively brief but cold winter season [3
]. Multidisciplinary perspectives have been applied, and the current research findings on cold, ill health, and energy use and the impact of low indoor temperature on occupants’ health include a range of brief reviews, case studies and policy analyses. Indoor temperatures below 12 °C can cause short-term increases in blood pressure and blood viscosity, which may increase winter morbidity and mortality due to heart attacks and strokes. When elderly people are exposed to indoor temperatures of 9 °C or below for two or more hours, their deep body temperature can start decreasing [5
]. An extremely low indoor temperature not only negatively impacts occupants’ thermal comfort, but also occupants’ health. The indoor environment of school classrooms is mainly focused on thermal comfort, chemical air pollutants and microbiological stressors, which can impact and potentially affect students’ health [8
]. The quality of the indoor thermal environment is very important for students’ health and performance, and the classroom should provide a conducive environment to promote teaching and learning [9
There are a number of studies related to the thermal comfort of primary and secondary school classrooms in countries with different climates. Some include a literature review on thermal comfort aspects within schools, focusing on the effects of thermal quality on the students’ learning performance [13
]. The thermal comfort and thermal comfort parameters for children in primary school classrooms in three different schools in the Netherlands have also been investigated [14
]. Results from thermal comfort surveys done in eight primary schools in the West Midlands, UK, showed a direct link between the attainment of children at school and the thermal conditions in classrooms and suggested that simply designing to a threshold comfort temperature was not enough to ensure that the most effective learning environments are delivered [15
]. Findings from thermal comfort surveys and measurements of indoor environmental variables in naturally ventilated classrooms in Hampshire, England, suggest that children are more sensitive to higher temperatures than adults, with comfort temperatures being about 4 °C and 2 °C lower than the PMV and the EN 15251 adaptive comfort model predictions, respectively [16
]. Indoor environmental conditions examined in seven primary schools near Venice confirmed that studying in a comfortable environment enhances students’ well-being and satisfaction and, therefore, their productivity and learning [17
]. A study investigating the effects of building envelope energy regulations on the thermal comfort level in naturally ventilated classrooms in primary and secondary schools in Taiwan confirmed that building envelope design has a remarkable impact on indoor thermal conditions in naturally ventilated spaces [18
]. The results of a field study about indoor thermal comfort, based on investigations in Italian classrooms show a trend characterized by a gradual change in the thermal preference from the heating season to the mid and warm season [19
]. Examination of the seasonal performance, occupants’ comfort and cold stress in cross-laminated timber school buildings in the USA specifically showed the impact of lower temperatures in different school spaces [20
]. The existing knowledge in the field includes an investigation of the thermal comfort in non-air-conditioned schools, and proposes an expectancy factor value for the Mediterranean climate [21
]. Moreover, the results of a field study of indoor thermal comfort, based on investigations in Portuguese secondary schools’ classrooms, consisted of measuring the environmental parameters: air temperature (Ta), air relative humidity (RH), and CO2
]. A field study was conducted in a secondary school building in Cyprus to assess the indoor thermal conditions during the students’ lesson hours (school hours); air temperature (AT) and relative humidity (RH) were monitored using indoor and outdoor sensors simultaneously throughout the four seasons of the year, and data analysis compared the results with international standards, ASHRAE Standard 55, ISO Standard 7730, etc. [23
]. These previous studies provided valuable insight and informed this research about new cross-disciplinary approaches that have been developed, starting from existing standards, best practices, policies, guidelines, techniques, procedures, and tools at the international level. In all these previous studies, there seems to be no controversy about the importance of the thermal performance of school buildings. However, most of these studies have been concerned with energy efficiency rather than the relationship between the building envelope and indoor health. This study specifically considers this relationship and provides results on how different building envelopes impact the indoor health of school buildings. Information on the correlations between indoor environments, health and educational outcomes are sorely limited in New Zealand [24
]. There are limited data and studies on measurements of the New Zealand school environment, especially for indoor thermal conditions and indoor air quality [25
]. Based on field-study data of indoor thermal environments, this study investigates the impacts of classrooms with different building envelopes on the winter indoor air temperature, not only related to students’ thermal comfort but also to their health.
Auckland has a temperate climate with a comfortable warm, dry summer and mild, wet winter. Most of the factors that adversely affect health, such as bacteria, viruses, fungi, mites, respiratory rhinitis and asthma, and chemical interactions, increase in conditions of high indoor relative humidity. Maintaining the indoor relative humidity at between 40% and 60% can minimise indirect health effects [26
]. New Zealand has some of the highest levels of indoor dust-mite allergens in the world [27
]. The indoor relative humidity required by dust mites to thrive is 75–80% or higher, and dust mites prefer temperatures of around 18–25 °C [26
]. Maintaining indoor relative humidity below 50% can reduce indoor dust mites and their allergens, and mite populations are almost eliminated in winter when indoor relative humidity is maintained within 40 to 50% [31
]. Recent studies show that, in order to maintain indoor dust-mite allergens at an acceptable level, winter indoor mean relative humidity adjacent to the floor must be maintained below the threshold for dust mites to thrive [35
]. According to international and national standards, indoor relative humidity should be lower than 60% for optimum indoor air quality [37
Visible mould growth on indoor surfaces is a common problem in over 30% of New Zealand houses [40
]. The threshold of indoor relative humidity for mould survival and growth conditions is 60%. Mould growth is likely on almost any building material if equilibrium relative humidity of the material exceeds 75–80% [41
]. Mould germination requires not only high relative humidity (80%), but also time (30 days) [44
]. One option to prevent mould growth on indoor surfaces is to control the indoor relative humidity to a level below the threshold (80%) of mould germination [9
In New Zealand, students normally stay in school for about six hours a day, and 90% of classrooms are designed for natural ventilation using windows [46
]. In winter, when windows of school buildings are typically closed, the air change rate in classrooms is much lower than the requirement of New Zealand standards [47
]; consequently, indoor CO2
concentration often exceeds the guideline value [46
]. Indoor air pollutants in a classroom can have serious and long-lasting negative impacts on the students. There are significant studies on health-related exposures in New Zealand and overseas schools. Some of the studies analysed the concentration and sources of air pollution in a school in New Zealand to understand the factors critical for assessment and to develop strategies for controlling and reducing exposure to indoor air pollution [51
]. Similarly, research with an analysis and assessment of the indoor air quality of 27 primary schools in Belgium aimed to obtain correlations between the various pollutant levels concerning the comfort and health of the students [52
]. Other studies identified that, while respiratory health effects of damp housing are well recognised, less is known about the impact of dampness and water damage in schools, when developing a study on the correlation between school dampness, levels of fungal and bacterial markers, respiratory symptoms and lung function, in children in schools in Spain, the Netherlands and Finland [53
]. To assess school buildings’ characteristics in relation to the students’ comfort and health, existing literature examined the concurrent exposures of young children to past-use and current-use pesticides in their everyday environments [54
]. Similarly, some of the research on the relation between exposure to ambient total fungal spores and a reduction of childhood lung function extended to determine what specific fungal spores were responsible for observed changes in lung function in schoolchildren in Taiwan [55
]. Others conducted a health-risk assessment for chronic toxic effects and cancer and measured concentrations of volatile organic compounds (VOCs) in the classrooms, kindergartens, and outdoor playgrounds of three primary schools in the spring, winter, and fall terms in Turkey [56
]. Indoor air pollutants and health effects in New Zealand schools have not been extensively investigated [24
]. Auckland has a humid winter, and major indoor air pollutants and health effects such as bacteria, viruses, fungi, mites, respiratory rhinitis and asthma, and chemical interactions are closely related to indoor high relative humidity [26
]. This study also investigates the impact of classrooms with different envelopes on winter indoor relative humidity levels, which can impact major local indirect health effects of relative humidity, such as mould and dust-mites.
In New Zealand there are 14,637 school buildings built from the pre-1940s to the 1990s (see Figure 1
). In accordance with the current building codes [38
], there could be a significant number of New Zealand school classrooms without sufficient insulation in their envelopes and with single-glazed windows, which can negatively impact the indoor thermal environment. There were about 425 schools built in Auckland before 2010. An Auckland school commonly comprises a number of low-rise, isolated buildings spread over a large site. Most Auckland school buildings, with one to four classrooms in a row, have a large external surface area which includes two or three sides consisting of external walls and roof surface areas. For this type of school building, the building envelope becomes the most important factor determining the winter indoor thermal environment. Based on the field study, this study contributed physical data on the winter indoor thermal environment of three classrooms with different insulation and with or without thermal mass in their building envelopes in Auckland. This study also compared and evaluated thermal performance of school buildings with different envelopes under a temperate climate with mild and wet winter, which can be used in the processes of thermo-modernization of school buildings in New Zealand or overseas under similar climatic conditions.
The redevelopment of Avondale College, in West Auckland, from 2010 to 2014, represented one of the biggest school rebuilding programs in New Zealand’s history. The project included 92 new and refurbished teaching and resource spaces. Refurbished buildings in Avondale College are of conventional lightweight timber frame construction (timber structure) with internal insulation and lightweight external cladding. In the retrofitted school buildings at Avondale College, the original timber structures were retained, and the new building envelopes included lightweight walls and roofs with sufficient insulation and double-glazed windows. For the retrofitted school building, the project mainly focused on increasing the R-value in building envelopes without considering the thermal mass effect (i.e., without adding thermal mass in the building envelope). For some new school buildings in the Avondale College redevelopment project, insulated precast panels (with thermal mass) were used for the main structure and building envelope. This was the first time that insulated precast concrete panels had been used as the main structure and building envelope for new secondary-school buildings in New Zealand. It was also the first time for a school building with thermal mass in its envelopes to be used for a field study of the winter indoor thermal environment in New Zealand. This study compared and investigated differences in impact of school buildings with different building envelopes on the winter indoor thermal environment in a temperate climate with a mild and humid winter.
Three classrooms at Avondale College, with different R-values and with or without thermal mass in their building envelopes, were selected for the field study of winter indoor thermal environments. Classroom 1 is in an old one-storey prefab school building (built in the 1990s) without thermal mass and without sufficient insulation in its envelope (demolished after the field study) and with north orientation. Classroom 2 is in the middle of a newly retrofitted one-storey building (built in the 1990s and retrofitted in 2011) with sufficient insulation, double-glazed windows and without thermal mass in its envelope and with north orientation (see Figure 2
), and has roof, north wall and south wall as its external envelope and a north orientation. The adjacent indoor spaces of Classroom 2 have the same space heating. Classroom 3 is in the middle and second floor of a new two-storey building (built in 2011) with sufficient insulation, double-glazed windows and thermal mass (precast insulated concrete panel wall and concrete structure) in its envelope and with north orientation (see Figure 3
), and it has roof, north wall and south wall as its external envelope and a north orientation. The adjacent indoor spaces of Classroom 3 have the same space heating. Table 1
shows the construction elements of the three classrooms. The design data in Table 1
are derived from the building plans provided by Jasmax, the project’s architects. The study compares and evaluates winter indoor thermal conditions not only related to students’ thermal comfort but also to their health.
The field study of the winter indoor thermal environment of three classrooms with different R-values and with or without thermal mass in their building envelopes was carried out by the author at Avondale College during the winter months from 13 June to 22 September 2013. Air temperatures and relative humidity adjacent to the ceiling and the floor, and in the shaded outdoor spaces under the roof eaves of the three classrooms, were continuously measured at 15-min intervals, 24 h a day by a Lascar EL-USB-2 USB Humidity Data Logger. The measuring points adjacent to the ceiling and the floor were located close to the south internal walls and the measuring points under the roof eaves were on the south side of classrooms, which can minimize the impact of solar gain during the daytime.
During the school hours, each sample classroom could be used for different courses, which could accommodate about 25 students. As the occupancy and use time for each classroom during the field study could be different, it is difficult to monitor or account for the heat gain of students for this study. The windows of the three classrooms were closed during the field study, and ratios of glazing surface to external wall area of Classroom 2 (retrofit) and Classroom 3 (thermal mass) are 0.4 and 0.41, respectively. (Classroom 1 was a very old building without a sufficient building plan and was demolished soon after the field study). There was no mechanical ventilation in the three classrooms during school hours at the time of the field study. The school buildings only used space heating during the school hours (from 8:30 a.m. to 3:30 p.m.), which was provided by the gas-boiler central-heating system. During the school hours, the indoor thermal environments of the sample classrooms are mainly impacted and controlled by space heating. This study not only investigated and compared the indoor thermal environments of school buildings with or without thermal mass during the school hours with the impact of space heating, but also during the night-time without the impact of space heating. During the school hours under the same space heating method, the field study data of the indoor thermal environments can be used to compare how the different building envelopes respond to space heating, especially for building envelopes with or without thermal mass. After school hours, especially during the night-time without the impact of space heating and the heat gain of students, the indoor temperatures could be significantly lower than during school hours and indoor relative humidity could be significantly higher than during school hours, which mainly depends on building thermal performance. As the local major indirect health effects of relative humidity such as mould and dust-mites increase in conditions of high indoor relative humidity, this study not only investigated and compared school buildings’ thermal performance and the indoor thermal environments of the three classrooms during the school hours and during the whole winter, but also during the night-time when the indoor thermal environment was not impacted by space heating and the heat gain of students.
The field-study data on air temperatures and relative humidity of indoor and outdoor environments have been converted into percentages of time related to different ranges of indoor air temperature and relative humidity throughout winter (24 h per day), winter night (from 7:00 p.m. to 7:00 a.m.) and winter school hours (from 8:30 a.m. to 3:30 p.m.), which can be used to compare the indoor thermal environment related to students’ thermal comfort and health. In accordance with WHO [1
] and previous studies [3
], the study used percentages of time when indoor air temperatures were greater than or equal to 16 °C, 18 °C, 20 °C and 22 °C to compare the indoor thermal environments related to students’ thermal comfort, and used percentages of time when indoor air temperatures were lower than 16 °C, 12 °C, 10 °C and 9 °C to compare indoor thermal conditions related to students’ health. The study also used indoor mean relative humidity and percentages of time when indoor relative humidity was in the optimal range of 40% to 60% [26
] to compare indoor relative humidity levels related to students’ health.
All field-study data have been converted into hourly mean temperature and relative humidity, which can be used to represent a mean variation of indoor temperature and relative humidity throughout the winter. The hourly mean temperature or relative humidity was derived from averaging all temperature or relative humidity data within a particular hour (e.g., at 1 a.m., 1:15 a.m., 1:30 a.m. and 1:45 a.m.) for all winter days. The hourly mean temperature and relative humidity data used in this study are the averages of the hundreds of temperature and relative humidity measurements within a particular hour on all winter days. As the temperature and relative humidity at a particular testing time on different winter days could be significantly different, the hourly mean temperatures and the hourly mean relative humidity for the whole winter may not precisely follow the correlation between air temperature and relative humidity (relative humidity decreases or increases in association with increasing or decreasing temperature), but ranges of their variations can still be identified and used to compare the indoor thermal environments.
Based on the field-study data of Classroom 1 (prefab) in Table 2
, Table 3
and Table 6
, an old, conventional New Zealand school building with a timber structure, lightweight envelope and insufficient insulation can have very low winter indoor mean temperature (14.9 °C in Table 2
) and quite high mean relative humidity (70% in Table 6
), and a large fluctuation in indoor air temperature and relative humidity during the winter. The indoor mean minimum temperature can go down to 6 °C and the indoor mean maximum relative humidity can go up to 85%. During school hours, there were only 51%, 27% and 7% of time when indoor air temperatures were greater than or equal to 16 °C (the minimum threshold of indoor temperature required for limiting respiratory infections [3
]), 18 °C and 20 °C (the minimum indoor air temperature required by WHO [1
]), respectively. During the winter there was only 2.7% of time when indoor relative humidity was in the range of 40% to 60% (the optimal range of relative humidity for indoor air quality and minimising the indirect health effects of relative humidity such as bacteria, viruses, fungi, mites, respiratory rhinitis and asthma, and chemical interactions [26
]). Classroom 1 (prefab) had Classroom 1 (prefab) with insufficient insulation in its envelope had very poor indoor thermal environment related to students’ thermal comfort and health. In New Zealand, those old school buildings should be fully retrofitted with sufficient insulation according to the current building codes [38
] to improve the winter indoor thermal environment related to students’ thermal comfort and health.
Based on a comparison of the field-study data between Classroom 1 (prefab) and Classroom 2 (retrofitted) in Table 3
and Table 6
, the indoor mean temperature of Classroom 2 (retrofit) was 3.6 °C higher than that of Classroom 1 (prefab). Classroom 2 (retrofit) had 34%, 43% and 41% more time than Classroom 1 (prefab) at indoor air temperatures greater than or equal to 16 °C, 18 °C and 20 °C, respectively, during winter school hours. During the winter, Classroom 2 (retrofit) had 42.8% more time than Classroom 1 (prefab) when indoor relative humidity was in the optimal range of 40% to 60%. Retrofitting an old school with sufficient insulation according to the current building codes [38
] can significantly improve winter indoor thermal environment related to students’ thermal comfort and health. Based on the field-study data of Classroom 2 (retrofitted), this study identifies that a disadvantage of the conventional school building, with a timber structure, lightweight building envelope and sufficient insulation in the local climate with a mild and humid winter, is that the sufficient insulation in the building envelope can increase winter indoor mean air temperature and decrease indoor mean relative humidity, but cannot reduce the large fluctuation of winter indoor air temperatures and relative humidity, which still results in very low indoor air temperatures, e.g., the indoor mean minimum temperature can go down to 8.5 °C (see Table 3
) and very high relative humidity, e.g., the indoor mean maximum relative humidity can go up to 84% (see Table 6
). Winter daily indoor minimum air temperatures occur during the early morning, just before school hours. The very low indoor air temperature of Classroom 2 (retrofitted) is a challenge for maintaining indoor thermal comfort in the morning of school hours, which takes time to heat the space up to the comfort level temperature, e.g., 18 °C. According to winter indoor hourly mean air temperature of Classroom 2 (retrofit) in Figure 10
, it took over four hours to rise indoor hourly mean air temperature from 14.1 °C (at 6 a.m.) to 18 °C (at 10 a.m.), which not only negatively impacts students’ thermal comfort and health but also potentially costs more space-heating energy. Increasing insulation in the building envelope without thermal mass can increase indoor mean air temperature and decrease indoor relative humidity but cannot reduce the fluctuation of indoor air temperature and relative humidity during the winter.
Based on the winter field-study data of Classroom 3 (thermal mass) and Classroom 2 (retrofitted) in Table 2
, Classroom 3 with thermal mass has 31%, 34% and 9% more time than Classroom 2 without thermal mass when indoor air temperatures were greater than or equal to 16 °C (the minimum threshold of indoor temperature required for limiting respiratory infections [3
]), 18 °C and 20 °C (the minimum indoor air temperature required by WHO [1
]), respectively. For Classroom 3 (thermal mass), there was only 6% of time in winter when indoor air temperatures were lower than 16 °C. For Classroom 2 (retrofit), there was 37%, 15% and 3% of time in winter when indoor air temperatures were lower than 16 °C, 14 °C and 12 °C respectively. These very low indoor air temperatures can negatively impact occupants’ health [3
]. Classroom 3 (thermal mass) with thermal mass has 21.4% more time than Classroom 2 (retrofit) without thermal mass when indoor relative humidity was in the optimal range of 40% to 60%. Classroom 3 (thermal mass) had 17%, 2% and 1% less time than Classroom 2 (retrofit) at indoor relative humidity greater than or equal to 60%, 75% and 80%, respectively (60% of relative humidity is the threshold of mould survival and growth conditions, 75–80% of relative humidity are required for dust mites to thrive and 80% of relative humidity is the threshold of mould germination [41
]). According to winter hourly mean temperatures and relative humidity of Classroom 3 (thermal mass) in Figure 10
and Figure 11
, winter indoor hourly mean temperatures of Classroom 3 (thermal mass) are more stable and significantly higher than those of Classroom 2 (retrofit) during the late afternoon, evening, night and early morning, and winter hourly mean temperature of Classroom 3 (thermal mass) is always higher than 18 °C or close to 18 °C (the minimum winter hourly mean temperature is 17.8 °C). Winter indoor hourly mean relative humidity of Classroom 3 (thermal Mass) was more stable and lower than Classroom 2 (retrofit) and was always in the healthy range 50% and 60% (see Figure 11
). Classroom 3 with thermal mass has a significantly better winter indoor thermal environment related to students’ thermal comfort and health than Classroom 2 without thermal mass. Adding thermal mass in the building envelope can not only increase indoor mean air temperature and decrease indoor mean relative humidity, but also reduce fluctuation of indoor air temperature and relative humidity. This can reduce the incidence of very low indoor air temperatures and very high indoor relative humidity, and significantly improve the winter indoor thermal environment related to students’ thermal comfort and health. Adding thermal mass in the local school building should be considered as a strategy or a guideline to improve the winter indoor thermal environment related to students’ thermal comfort and health for future school design and development in a temperate climate with mild and humid winter.