Accumulated snow layer influence on the heat transfer process through green roof assemblies

Publication date: May 2015
Source:Building and Environment, Volume 87
Author(s): Mingjie Zhao , Jelena Srebric , Robert D. Berghage , Kevin A. Dressler
A green roof can reduce the peak thermal cooling loads and reduce the building energy consumption during the summer. It is also necessary to understand the thermal performance of these green roof assemblies during the winter when affected by an accumulation of snow on the rooftops. This study presents an experimental investigation and discusses the snow influence on the heat transfer processes through green roof assemblies. The on-site experiments were conducted in the outdoor test facility in Pennsylvania, U.S.A. during the winter of 2010/2011. The experiments were conducted on green roof buildings and on reference buildings for comparison. The collected data included the local meteorology, building operation data, and manually measured snow properties. The measured heat fluxes show that the heat flow though the green roof assemblies compared to the typical roof assemblies were reduced by approximately 23% when there was not an accumulated snow layer. However, this difference in the heat flux was only 5% when the roof structure had an accumulated snow layer. To quantify the snow effects on the heat transfer through green roof assemblies, the Johansen method was then used for snow conductivity calculations on rooftops. These equations should be a part of the total energy balance for the snow covered green roof assemblies because the snow layer significantly altered the heat transfer through these roof assemblies.