dwellers have the freedom to behave upon their preferences, arising from their own residential context, and that those behaviours lead to personal and technological adaptations. Researchers9 have shown that contextual differences typically lead to a wider range of indoor temperatures in both mechanically and naturally conditioned homes, indicating that occupants at homes are more adaptive and tolerant of cooler and warmer temperature conditions than predicted by existing comfort models. A BROADER CONTEXT FOR RESIDENTIAL THERMAL COMFORT Residential buildings in many places of the world are not even equipped with mechanical heating and/or cooling devices. In those places, the building itself must provide sufficient protection against drastic outdoor temperature variations. For example, in Bogotá, Colombia, located 2,548 metres above sea level, ambient temperatures are mild, but below the adaptive comfort zone (Figure 2). However, mechanical heating is not even considered for any type of building in that city. Even though indoor temperatures may fluctuate between 16ºC and 25ºC, dwellers’ daily adaptive behavioural patterns include adjusting clothing, drinking warm beverages and opening/closing windows and blinds. Designing residential buildings for adaptive thermal comfort in hot and hot-humid climates is particularly challenging. A study of comfort in households in Japan during hot-humid seasons, focusing on living rooms and bedrooms, found that a large proportion of dwellers were well adapted and satisfied with the thermal environment in their free- running (naturally ventilated) houses. The mean comfort temperature in free running mode was 27°C in hot and humid season. Residents adapted to hot and humid environments by increasing the air movement usage through actions such as opening the windows and using fans.10 Buildings without mechanical cooling (Figure 3) abound in tropical towns. However, designing buildings without mechanical cooling would be unthinkable in large cities like Miami or Hong Kong, where occupants demand narrow indoor temperature bands for comfort. So, does the size of a city and the wealth of its residents equate to increased thermal comfort expectations and need for mechanical energy for thermal comfort? Figure 2. Outdoor temperatures, solar radiation, and adaptive comfort zone in Bogotá, Colombia FIGURE 2: OUTDOOR TEMPERATURES, SOLAR RADIATION AND ADAPTIVE COMFORT ZONE IN BOGOTÁ, COLOMBIA. An Australian field study7 tested the question, “Do prolonged exposures to air conditioning make people acclimatize to cooler, or perhaps a narrower band of indoor, temperature conditions?” They classified occupants as “heavy A/C users” and “light A/C users.” The research found significant differences in the use and duration of the A/C between these two groups of users. The mean room temperature was consistently about 2°C lower in the “heavy A/C user” group compared to the “light A/C user” counterparts. Using a comfort scale, “heavy A/C users” felt “slightly warm” when the room temperature was about 24.5°C, whereas “light A/C users” reported the same level of thermal sensation at a higher room temperature of 26.5°C. However, researchers warn that this is not enough evidence to suggest households’ acclimatization to A/C. In Figure 3, from the same field study5 , researchers developed four predictive curves of adaptive thermal comfort behaviours. In Figure 3, between about 21°C and 28°C, less than 20 per cent of the people rely on mechanical heating and cooling, indicating that this range is most conducive to natural ventilation, minimizing the dwellers’ reliance on mechanical heating, cooling and ventilation appliances. 0% 20% 40% 60% 80% 100% 5 10 15 20 25 30 35 40 45 Percent (%) Outdoor air temperature Ta(out) (°C) AC-cooling on Heating on Fan on Window/Door open FIGURE 3: THE PERCENTAGE OF DIFFERENT ADAPTIVE STRATEGIES IN USE IN RESIDENTIAL BUILDINGS, AS A FUNCTION OF OUTDOOR AIR TEMPERATURE (REPRINTED WITH PERMISSION OF KIM ET AL.5 ). With due consideration to increasing urban densification and the heat island effect in larger cities, low-energy designs can be achieved as long as some fundamental priorities are followed (Figure 5): 1) design for the local climate and the dwellers’ particular needs and expectations on their homes; 2) maximize the design of passive strategies, combined with proper zoning and a high-performance enclosure; and FIGURE 4: ADAPTIVE COMFORT PRINCIPLES APPLIED IN THE DESIGN OF SCHOOLS IN A HOT-HUMID TROPICAL CLIMATE (REPRODUCED WITH PERMISSION FROM “PLAN:B ARQUITECTOS”11 . PHOTO CREDIT: ALEJANDRO ARANGO). SPRING/SUMMER 2019 19