A Field Survey on Thermal Comfort of Occupants and Cold Stress in CLT School Buildings

Abstract

The goal of achieving a smart and sustainable built environment starts with the design, construction, and maintenance of an intelligent and sustainable occupied thermal environment. Such an environment must be designed and constructed to achieve thermal comfort and overall well-being of occupants. This paper presents a field investigation of occupants’ comfort and cold stress in cross-laminated timber (CLT) school buildings during the cold seasons (fall and winter). The study was conducted from October to November 2017 for the fall season and from December 2017 to February 2018 for the winter season. The case study comprises of spaces constructed with structural timber products. The case study is a LEED certified school building. It has been identified as one of the first green school buildings in the Northeast region of the USA. The building explores HVAC systems, and it utilizes ground source heat pumps for heating and cooling. The research employed physical measurements of environmental variables such as temperature, relative humidity (RH), dew-point temperature, air velocity and CO2 level in the selected spaces such as the administrative office, science, and art classrooms and the multi-purpose hall. The sensors were mounted on the internal walls at 1.1 m above the floor to measure the variables at every 60 min throughout the cold seasons. The study also calculated the Wet-Bulb Globe Temperature (WBGT) in the spaces to understand the average cold stress index within the thermal environment. The mean outdoor temperature was 11.3 °C in fall, and 0.5 °C in winter. In the fall season, the results showed that the mean indoor temperature was 21.2 °C. In the same season (fall), the mean RH was 50.7%, and the average dew-point temperature was 9.3 °C. In the winter, the average indoor temperature was 20.5 °C while the average RH was 23.9% and the mean dew-point temperature was −1.9 °C. The overall mean temperatures measured in the spaces during the cold seasons were within the comfort temperature thresholds (20.3 °C/23.9 °C) recommended by ASHRAE. In the fall, the mean RH was within the comfortable range (30–60%). The mean RH value was below the comfortable range in the winter. The study recorded a higher mean temperature, RH and dew-point temperature in the office space than the classrooms and the main hall during the cold seasons. Lower cold stress indexes were also calculated in the multi-purpose hall than the classrooms and office space. The study revealed occupants are more likely to experience cold temperatures in the hall than the office space and classrooms. The difference in the floor level (the main hall is on the lower floor while the classrooms are on the upper floor), hours of occupation (more extended hours of occupation in the office space), and floor area may be the contributing factors to the lower temperatures measured in the hall than the other spaces. By applying the WBGT mathematical model, the research recommends the WBGT of 16.0 °C and 13.7 °C as the cold stress indexes in the building for the fall and winter seasons respectively. Finally, the study recommends a WBGT of 14.9 °C as the average cold stress index in the spaces evaluated in this paper.