Center for the Built Environment
Thermal comfort in naturally-ventilated and air-conditioned classrooms in the tropics.
- Author(s): Kwok, Alison G
- et al.
Designers use thermal comfort standards, such as Thermal Environmental Conditions for Human Occupancy by the American Society of Heating Refrigeration, and Air-conditioning (ASHRAE Standard 55-1992) and Moderate Thermal Environments - Determination of PMV and PPD Indices and Specification of the Conditions for Thermal Comfort by the International Standards Organizations (ISO 7730-1994), to design systems to provide a physical environment appropriate for thermal comfort. This thesis examines the comfort criteria of ASHRAE Standard 55-1992 for their applicability in tropical classrooms. The Standard specifies exact physical criteria for producing acceptable thermal environments: minimum and maximum limits for temperature, air speeds, and humidity that are often difficult to apply, particularly in hot and humid tropical climates. The Standard’s requirements are based in part on climate-controlled, laboratory experiments in temperate climates. The primary questions here ask: Are laboratory-based air-conditioning standards applicable in tropical climates? Does a different set of criteria exist for people accustomed to hot and humid climates than for those living in temperate climates? Preference for, or acceptance of, thermal factors beyond the prescriptions of the standard might suggest wider latitude for environmental control and air-conditioning set points.
Borrowing primarily from previous thermal comfort studies in office buildings and adapting them for the school setting, I used a variety of methods to collect the data: survey questionnaires, physical measurements, interviews, behavioral observations, and statistical analysis techniques. Hawaii serves as a case study where 3,544 students and teachers completed questionnaires in 29 naturally-ventilated and air-conditioned classrooms in 6 schools. Concurrent measurements of the physical environment were made during each class visit.
The majority of classrooms failed to meet the physical specifications of the ASHRAE Standard 55-1992 comfort zone. Analysis of subjective responses using the thermal sensation, preference, and other scales and environmental indices, found votes of more than 80% acceptability by both naturally-ventilated and air-conditioned occupants whether in or out of the comfort zone. Responses from these two school populations, suggest not only a basis for separate comfort standards, but energy conservation opportunities through raising thermostat set points and certainly by choosing to design optimized naturally-ventilated environments.