Our group conducts research into the building energy and environmental consequences of green roof design decisions.

In the lab we have tested the thermal properties of various green roof growing media under varying levels of moisture content. We have created a wide variety of sample media using varying amounts and types of aggregate, sand, and organic matter. These data, which have been published in Energy & Buildings, clearly show that thermal properties of green roof growing media vary substantially both seasonally and diurnally as a function of moisture content. For example, a wet soil layer may have 40% higher thermal conductivity than the corresponding dry soil. Hence, any attempt to include green roofs in building energy analyses must account for the time varying thermal properties and thermal storage within the growing media.

In the field we have been gathering data for use in understanding the energy budget of green roofs and in validating model performance. Specifically, we have an automated weather station on the Portland State Broadway Housing Building green roof - a 10 story mixed-use building. This green roof has a spatial extent of 18,000 sq. ft. Sensors on the roof include air temperature, humidity, wind speed, atmospheric pressure, solar radiation, and leaf wetness. Within the soil we have sensors at multiple locations and depths, recording soil temperature and moisture. All data are sampled at 1-minute intervals and logged every 30 minutes. Data have been collected since July 2007.

We have also developed a computational model of the energy balance of a green roof. This canopy-soil model accounts for long wave & shortwave radiative exchange, convection, evapotranspiration, and conduction/storage. It has been validated against observations from several green roofs across the US (Florida, Pennsylvania, and Oregon). It has been adopted by the US Department of Energy as part of their standard release of the EnergyPlus building energy simulation program.