The Variable Performance of Green Roofs Across Buildings and Climates

Abstract

Residential and commercial buildings play a significant role in the energy, water, and climate nexus. Because much solar energy enters through the roof, many have studied the importance of roof upgrades to decrease demand in heating and cooling. Green roofs (GRs) provide additional insulation and evaporative cooling, thereby decreasing indoor air heating and cooling demand, and they reduce stormwater runoff by capturing and evaporating rainfall. It is still not well understood how a GR’s performance varies across buildings and climates. This thesis uses EnergyPlus to model a variety of buildings and climate zones over the United States and finds relationships between GR benefits (energy savings and stormwater reduction) and parameters of context (building and climate). Runoff reduction and irrigation demand are found to depend mostly on precipitation. Energy savings normalized by roof area appear to depend on the building’s relative amount of energy spent on heating and cooling (caused by internal schedules and loads, roof insulation, and/or climate) and the roof’s relative importance to the building’s envelope or volume. By a reduction of the urban heat island effect, a wide implementation of GRs is found to decrease cooling load significantly further. While GRs provide a variety of benefits, this research can help prioritize buildings and climates as candidates for GR retrofit.