Humidity or air-speed? A climate chamber investigation into adaptive thermal comfort potential

High indoor humidity is known to impair sweat-induced evaporative cooling in the tropics. However, its role in the established adaptive thermal comfort standards was ignored until the recent development of the field-data derived relative humidity (RH) inclusive adaptive thermal comfort model (ATC RH ). The ATC RH model demonstrated the existence of an RH signal in adaptive thermal comfort for the first time by showing low RH is associated with higher comfort temperatures and vice versa. But it is unclear whether these comfort temperatures are due to simultaneous association with high or low air-speed in the field data. The aim of the present study is hence to investigate the potential relationship between ATC RH and air-speed. We utilise a unique well-controlled climate chamber - VSIMULATORS, that can virtually mimic real environments, thus providing a better match to field conditions than historic climate chamber research which was criticised for the artificiality of the study environment. Using a carefully created factorial experimental design protocol to account for order effects and counterbalancing, a series of experiments involving hypothesised pairs of warm-climate ‘adapted’ and ‘un-adapted’ subjects were undertaken. The subjects were exposed to two levels of air-speed (independent variable), 0.2 m/s and 0.8 m/s under low and high humidity associated comfort temperature bounds given in ATC RH . A total of 8832 subjective responses were obtained on standardised scales. Our warm-climate adaptation hypothesis is confirmed through observed discrepancies in the subjective responses in the factorial experiment. Results show that the ATC RH model is found to be comfortable regardless of air-speed at lower humidity levels but is comfortable only with a higher air-speed of ∼0.8 m/s at higher humidity levels. This clearly points to the need for the provision of elevated air-speed, through natural or mechanical ventilation, when using the ATC RH model in highly humid locales. Practical application The applicability of recently developed “Relative Humidity-inclusive Adaptive Thermal Comfort model ATC RH is tested using ‘long-term thermal memory’ of the occupants as a criterion, within a unique climate chamber – VSIMULATORS. Under extreme ATC RH model conditions of 34°C, 70% relative humidity, an elevated air-speed of 0.8 m/s maintains the thermal comfort of occupants at ‘neutral to slightly uncomfortable’, that is 0 to −1 on the ASHRAE’S 7-Point Scale. ATC RH model is found to be practically applicable for energy savings in the tropics. This study also found that prolonged exposure to ATC RH model conditions reduces the thermal comfort of the occupants by 0.54 units on the ASHRAE’s seven-point scale for each progressive hour, which has potential scope for further investigations into adaptive thermal comfort mechanisms.