Thermal properties of green, white and other building roof materials and solar insolation: A case study in New York City

Abstract

This report presents results from a case study of surface temperatures of an urban green roof, a white roof, and other roof surfaces, at Hunter College, of the City University of New York. The analysis includes manual as well as automated observations between 2012 and 2016. The cooling effect, defined as the surface temperature differences between the green and white surfaces compared to a black surface, are often greater than 20 °C, and sometimes greater than 30 °C for both green and white roofs. Solar insolation is found to be the best predictor of the cooling effect, explaining over 72% and 77% of the variance for green and white roofs (respectively), with a 2-3 °C enhanced cooling effect for every 100 W/m2 increase in solar insolation. The response times of both green and white roofs to solar insolation is estimated to be 20 min (±10 min). Green roofs are found to have a slight cooling advantage (∼1.7 °C) over white roofs at higher solar insolation (∼1000 W/m2) and higher surface temperatures (∼45 °C). Key components of the surface energy balance are considered to help explain the observed thermal behavior of the green and white roofs. Manual observations, instrument maintenance, as well as data transcription and quality control, were performed largely by undergraduate students, demonstrating that green infrastructure can provide valuable learning experiences that may be difficult to otherwise provide in urban environments.