## 1 Introduction
### Occupants Vulnerable to the Internal Environment  
The issue of people being more vulnerable to thermal conditions appeared as soon as universal overheating problems in buildings were revealed. Generally, elderly people seem to perceive thermal conditions differently than the young due to a combination of physical ageing and behavioural differences [14]. Older adults may have, among others, reduced muscle strength, work capacity, sweating capacity, ability to transport heat from their core body to skin, hydration levels, and lower cardiovascular stability. On average,the body's response to heat is impaired with age due to changes in sweating, blood flow to the skin, and cardiovascular function [15]. These decrease the body's ability to keep the core temperature at safe levels, especially during long exposure to heat or physical activity in the heat. Older adults may also have a reduced capacity to sense and adapt to dehydration [16]. Although the thermal environment may be a danger to their health or well-being, they may not realise it before it is too late to respond [17]. Even though many studies found differences in the thermal comfort of people varying by age, the results were not consistent. For example, the study in the environmental chamber discovered no significant variations in many test conditions between the thermal sensation, comfort, and acceptability of old and young people [16]. Some publications stated that individual diversities were too large to draw clear conclusions about the requirements of older adults regarding their preferred thermal environment [14; 15]. Nevertheless, statistical evidence from heat waves in American, Japanese, and English cities showed that mortality increased with age when the temperature increased and was the highest among people over 75 [15; 18]. In the age group 0-64, a smaller increase in mortality was seen, which might reflect a greater vulnerability in children and infants, who were also endangered by heat-related deaths [18]. In the age group 65-74, the increase was small (3%); however, according to [18], vulnerable households included one or more occupants older than 65, together with disabled or with long-term illnesses. Factors that may contribute to the poor response by vulnerable occupants to high indoor temperatures were reduced physical mobility, social isolation, and security concerns [19]. These caused a tendency to rarely open windows or have windows less widely open compared to non-vulnerable households, lowering the air quality. Older people tend to be more sedentary than younger people and spend most of their time at home, which makes them more susceptible to higher temperatures within buildings [20]. On the other hand, according to [21; 22], the elderly prefer a higher temperature in comparison with young adults. Monitoring in two London care homes showed a disparity between how the indoor environment was perceived by elderly residents as compared to the care home staff. The majority of residents found their thermal comfort neutral, while staff members were twice as likely as residents to rate their thermal sensation as warm or hot [23]. Also, the research concerning nursing homes in Australia showed that residents were more tolerant than non-residents to temperature variations in their environment [24]. Applying adaptive comfort criteria also raised some questions about the actual possibilities of adjusting to changing temperatures among older people. For instance, residents of elderly care centres may have restricted options to open windows or change clothing [25]. In contrast, some surveys showed adapting processes and opportunities among elderly people living independently in their residences [26]. The first thing the elderly did to feel more comfortable when they were hot in the summer season was window opening [26], followed by the use of electric or hand fans and the use of air conditioning [27].