Heat Waves and their Impact on Indoor Environments: An Assessment of Human Bioclimate using the UTCI

Because of the fact that people in industrialized countries spend on average 90% of the day in confined spaces and can be negatively affected by unfavourable thermal conditions, the assessment of indoor heat stress is an important issue, regarding adaptation. The study aims to assess indoor heat stress with the focus on differences between building structures (year of construction, percentage of window surfaces) and differences within the buildings regarding floor level and orientation of the rooms. Additionally, the nighttime situation was investigated separately, paying attention to the influence of heat stress on the recovery phase of the human body.

The Universal Thermal Climate Index (UTCI) was used to assess the indoor environment in 26 rooms within 5 different buildings in Berlin. The buildings differ in their usage between two office buildings, two residential care homes for the elderly and one public school. In each building the considered rooms are south-west oriented, located on different floor levels and have the same size. The presented results were derived through a detailed measurement campaign from the 1st of June to 31st of August 2013. During this period three heat waves were recorded from the German weather service. In the two office buildings air temperature, relative humidity, air velocity and mean radiant temperature were measured in 5min time steps and UTCI values were calculated. The three remaining buildings were equipped with air temperature and relative humidity sensors. To measure air temperature and relative humidity, each room was equipped with two Testo 174H loggers. Air velocity was derived by one hot wire anemometer per room (PCE-009) and mean radiant temperature through the use of one black globe thermometer per room (150 mm in diameter; 0.4 mm thickness). The sensors were fixed at a height of approximately 1.1 m above the ground, corresponding to the average height of the center of gravity for adults. For the UTCI calculations, mean radiant temperature was set to air temperature, if not measured separately, and air velocity was set to a constant air flow of 0.3 m/s. A metabolic heat production of 135 W/m2 was assumed for all UTCI calculations.

Within the study period, the mean UTCI values ranged from 23.1 ± 1.2 °C to 29.9 ± 3.2 °C. Whereas differences within the school and the two residential care homes for the elderly were just around 1 K, the two office buildings showed noticeable differences of 4-7 K. Maximum UTCI values in all buildings were recorded from 25.6 °C up to 37.7 °C. All rooms showed moderate heat stress and rooms within three buildings strong heat stress during the heat waves. At two days, one room exceeded the 38 °C threshold for very strong heat stress with UTCI maximum values of about 39 °C. Additionally, two buildings showed discrepancies from the assumption that heat stress increases with increasing floor levels, as stated in previous literature. The night time analysis was conducted using UTCI minimum values. Data were recorded from 20.1 °C up to 24.5 °C. The internal difference of two buildings was approximately 1 K, whereas three buildings showed internal differences between 1.9 K, 2.7 K and 4.4 K. All rooms showed moderate heat stress and two rooms within one building strong heat stress during nighttime.

The variations between the buildings can be explained by differences in building material as well as by the percentage of window surfaces. The two office buildings presented the highest heat stress levels and have concurrently the highest ratio of window surfaces. The year of construction is no determining factor, because the two latest buildings in this study showed diverse heat stress patterns. One office building constructed in 2003 showed the highest heat stress values, whereas one residential care home for the elderly (2004) experienced the lowest values within this study. Differences within the buildings can be traced back to floor level, orientation and user behavior. Especially the decrease of heat stress with increasing floor levels within two buildings is caused by the influence of users. The rooms at the lower floors are occupied by people with no or limited knowledge about possibilities to reduce heat stress, whereas the users at the higher floors took measures to reduce heat stress. The same effect was analyzed within one residential care home for the elderly due to bedridden people at the lower floors and mobile residents at the higher floors. In summary, the results indicate that indoor heat stress is a prevailing threat during heat waves throughout the day. People within the buildings are likely affected by heat conditions regarding thermal comfort and health issues, especially when they have no possibilities or knowledge about adaptation measures.