Investigation of perception of thermal condition of naturally ventilated classroom environment during warm periods in Dhaka

Abstract

This research is based on a field investigation, for a detailed understanding of perception of thermal environments, by students, inside naturally ventilated classroom environment of Dhaka, Bangladesh, during the pre-monsoon (hot-dry) and monsoon (warm-humid) seasons of a year. One hundred individual responses provided comprehensive data, through a standard questionnaire survey, which was then used for analysis of their responses and preferences, with respect to the measured indoor thermal environment with running ceiling fans. The subjects were within 18-25 years age range, with average clothing insulation (0.35-0.5 clo), and with metabolic rate of 0.8 to 1.6 Met. The range of measured mean indoor air temperature, relative humidity and air velocity were found as 27.95-32.6ºC, 52.5-80% and 0.05-0.9 m/s respectively. Students’ thermal sensation voting showed a strong correlation with the mean air temperature, indicating that they were highly sensitive about the thermal conditions in their classrooms. The ‘neutral temperature’ was found to be 30.20 ºC and acceptable temperature range was 29.89 ºC to 30.54 ºC. This result shows that, the students are well adapted to their thermal environment, having a wide comfort zone. Voting also indicated the preference to be in cooler conditions, even if they voted their thermal sensation as ‘neutral’. The students expressed comfort over a very narrow range of relative humidity levels (65% to 68%) while the majority of students (70%) voted for more ventilation. To be comfortable, students adopted several behavioural measures; all were related to increased air flow. This research reveals that the ‘preferable’ temperature for the student population is 30.18 ºC, which is very close to ‘neutral’ temperature. The ‘preferred’ ‘relative humidity’ (57%) was found to be much lower than the identified ‘neutral’ relative humidity (66.5%). It was found that there exists a strong relationship between the ‘acceptability’ of thermal conditions and ‘perceived’ performance. Majority of them (57%) voted as the thermal conditions of classrooms were ‘not acceptable’ and 65% voted that these conditions were interfering on their class performance. The findings reconfirmed that sensation, comfort and preferences of this tropical country is markedly different from set international standards. This research can be used as a basis for further research to investigate other aspects and consequences, as mentioned in this section, of thermal condition of naturally ventilated classroom environment of Dhaka.