Summary of findings
This study analyzed the relationship between living room temperature in the winter season and the characteristics of residents based on a baseline (before insulation retrofitting) survey of 2,190 households. The cross-sectional analysis showed that 1) the average temperature in the living room, bedroom and changing room was 16.8 °C, 12.8 °C and 13.0 °C, respectively; 2) living room temperature was highest (19.8˚C) in Hokkaido, where outdoor temperature is lower than other areas, but lowest (13.1˚C) in Kagawa, which is considered to have a mild climate; 3) lower household income and single-person households were correlated with lower indoor temperatures; and 4) use of a kotatsu (traditional Japanese local heating device) and wearing larger amounts of clothes were correlated with lower indoor temperatures.
Housing Disparities Between Nations And Within Japan
The results of actual measurements of indoor temperature from 2,190 houses across Japan indicate that the average living room temperature in winter was 16.8 °C. Similarly, as described in the Introduction, the average living room temperature of 602 houses across Japan was 17 °C during winter. In contrast, a large-scale room temperature survey in the UK, “The comprehensive English House Condition Survey 1996,” reported that the average living room temperature in winter in the UK was 18.1 °C. In addition, the Energy Follow-Up Survey 2011 involving 823 dwellings in the UK revealed that the mean monthly temperature for the whole house/apartment was 18.1 °C in December. Furthermore, an investigation that targeted apartments in New York, USA, reported an average living room temperature in winter of 23.3 °C.
Currently, 39% of existing houses in Japan are not insulated, which means that a large proportion of houses have low insulation performance. In addition, energy consumption in houses in Japan is minimal compared to that of other countries, with energy used for heating only one-quarter of that used in European and American countries. This is because, while continuous heating of the entire building is the norm in Europe and the USA, in Japan, partial intermittent heating is used, and in the living room only. Due to these two factors - low insulation performance and difference in heating use - room temperature in Japan is considered to be of a lower standard than that in European and American countries.
The present analysis also showed that there is a major disparity in indoor temperatures even within Japan. Comparison of indoor temperature between prefectures revealed a maximum difference of 6.7 °C (average living room temperature in Hokkaido minus that in Kagawa). Furthermore, the results of multilevel analysis showed that the indoor temperature in area 2 (which includes Hokkaido) was significantly higher - by 3.7˚C - compared with that in area 6 (which accounts for the majority of Japan, including Tokyo). Although the insulation specification of house is determined based on the HDD in each area, there may be still scope for improvement in insulation standards within Japan.
WHO has emphasized the risk of low indoor temperature to health in its guideline. Consistent with this, concern has been raised that housing disparities between and within nations have the potential to cause health disparities.[24, 25] We expect that our present results will contribute to regulatory developments for thermal insulation standards in Japan and a reduction in health disparities.
Residents At High Risk Of Hypertension And Cardiovascular Disease
Our previous analyses indicated that indoor temperature is strongly related to blood pressure, and that hypertension occurs in low-room-temperature environments. Furthermore, hypertension during the winter season can lead to the development of cardiovascular diseases (CVDs), and has been known to cause excess winter deaths.[26, 27] In other words, residents living in low-room-temperature environments have a high risk of hypertension and CVDs.
Studies of the determinants of room temperature conducted in the UK[28, 29] suggest that resident attributes such as house composition and employment status significantly affect room temperature. Our present analysis showed that, in addition to low household income, single-person households, and lifestyle factors such as the use of heating and amount of clothes worn also affect the indoor temperature. Low household income may force residents to limit the use of heating or to live in housing with low performance insulation. A certain number of fuel-poverty households are unable to afford heating expenses and live in cold homes. Recent research has shown that fuel-poverty households also exist in Japan. Our data confirmed that the indoor temperature is lower in single-person households. A previous study suggested that people living alone have a higher risk of hypertension, and low room temperature is hypothesized to be a contributing factor. Households using kotatsu had lower indoor temperatures. Kotatsu is a form of local heating which does not allow heating of an entire room or house. Similarly, wearing more clothes was correlated with lower indoor temperatures, which suggests that some residents attempt to brave the cold by wearing more clothes.
The method used in this study allowed the identification of residents who live in cold homes. We expect that these results will be useful in the development of prevention strategies for these residents.
Strengths And Limitations
The strengths of this study are following three points. First, this is one of the largest surveys on the relationship between the characteristics of residents and indoor temperature in winter, involving 2,190 households in Japan. Second, participants were recruited throughout all 47 prefectures of Japan. The length of the Japanese archipelago stretches in a north to south direction, resulting in large regional differences in climate. We can evaluate housing disparities within Japan, taking differences in climate into account. Third, actual measurement on indoor temperature have been conducted for 2 weeks in real-world settings. This objective data for a certain period of time may reduce observational bias.
This study has the following limitations. First, the survey was conducted on households which had relatively low thermal insulation levels. In Japan, the majority of existing houses have low insulation performance, so the present results may be broadly applicable throughout the country. Second, we were unable to survey details of the design specifications of participants’ houses because few residents had retained design drawings of their house, and they were generally unfamiliar with the installed insulation. We took the age of the house into consideration by adding the duration of residence into the multilevel model as an independent variable. However, it is necessary to consider floor area, house structure and so on for more detailed analysis. We therefore suggest that future research should be done in cooperation with housing experts. Finally, because the present results were based on a cross sectional analysis, they are limited to showing only the actual conditions of houses in Japan and are unable to suggest how low indoor temperatures can be improved. Further studies are needed to examine an effective way to improve indoor temperature.