Performance comparison of two low energy cooling systems for top floor residential buildings in moderate climates

Background Overheating in top floor residential buildings is more frequent in comparison to lower floors because they are more exposed to external elements through the roof and have increased solar heat gains because of less overshadowing by adjacent buildings. Purpose : The exposed roof of top floor apartments provides an opportunity to install systems to reduce overheating. Research Design: The method of this paper is a comparison of two such systems which were both installed in an operational building and compared through analysis of the measurements and energy simulations. The chosen installed systems are; cool paint which was applied to the roof to reflect solar radiation and a ventilative cooling system with Phase Change Materials (PCM) thermal storage. Although different systems (one passive and the other mechanical) they have the same objective to reduce the building cooling load. Measurements were carried out for one summer period after the installation. Modelling was performed using IESVE to study their performance throughout the year using a calibrated model based on the measurements. Results: Measurements confirmed that the systems work efficiently; cool roof reduced surface temperature of the roof and the ventilative cooling system with PCM maintained typical set-point internal temperatures. Simulations indicated reduction of cooling energy demand in comparison to typical air-conditioning systems. They also confirmed the heating energy penalty for the cool roof system although there is a reduction in total energy demand for heating and cooling through the year. In terms of costs, cool paint has lower cost and is easy to install but ventilative cooling with PCM also improves Indoor Air Quality. Practical application This study demonstrates through measurements and modelling the performance of two low energy cooling technologies suitable for retrofit of top floor apartments. The Cool Roof reduces overheating with a small heating penalty and the Ventilative Cooling with Phase Change Materials (PCM) system reduces energy demand. A Cool Roof is a lower cost system in comparison to Ventilative Cooling with PCM and easier to install, however the later additionally provides improvement to Indoor Air Quality. The results can help building designers to make decisions about implementing the systems in retrofits to reduce cooling energy demand and improve overheating.