Cool Roof Impacts on a School-building Thermal and Energy Performance in Athens, Greece

Abstract

This study presents the results of a thorough investigation of a cool-roof application on a public school building (pilot building) located in Athens, Greece. An impact assessment was conducted by means of both energy monitoring and numerical analysis of whole-building's thermal behavior. A twin nearby school building (reference building) was used for comparison purposes. The measuring process in both buildings included the recording of thermal performance indicators (TPIs), i.e. roof internal and external surface temperature, indoor air temperature and relative humidity. The ambient air temperature and incident solar irradiance were also recorded at the roof of the reference building. The duration of the monitoring period was approx. one month, two weeks before and two weeks after the application of the cool roof material. TPIs’ differences before and after the application were based on comparisons to the reference building recordings to ensure the same outdoor climatic conditions. The impact of the cool roof in the building's energy performance was based on hourly dynamic energy simulations (in annual basis). The model developed uses as input conditions: a modified weather file based on the onsite weather measurements, the thermo-physical properties of the building materials taken from the available insulation study, and the optical properties measured from surface samples. Inspection surveys’ with interviews, which provided building's systems operation and occupancy schedules, were also used as input schedule files. The model was validated with comparisons of calculated and measured indoor air temperatures. The results showed that, ten days after the application, the reduction of the daily mean indoor air temperature below the cool roof ranged in 1.3-2.30C and 1.6-1.90C as provided by measurements and simulations, respectively. In terms of annual energy consumption, a reduction of up to 30% in cooling demand was estimated in summer, while the heatingpenalty for winter period reached 12%.