Present problems with building performance simulation of green roofs

Green roofs are increasingly popular with designers and performance simulation is necessary to ensure a building’s energy efficiency but there are some problems with present theories that support green roof modelling. For example, it is assumed that a substrate layer is continuously saturated, which is not true in practice. Most models ignore the drainage layer, which is an important service layer integrated with current green roof systems. Where included, models have used an inappropriate mechanism. In a substrate layer, the liquid absorbs into the soil by capillary attraction, and the contact surface is saturated first, while soil underneath remains dry and requires some time to reach saturation. Sharp Front Theory can be used to understand this situation and the rate of liquid absorption by the material is called its sorptivity. This can be used with the one-dimensional conduction heat transfer. In the drainage layer, the theory of conduction of porous material is applied by using Hadley's weighted average of Maxwell upper bound method to estimate the effective thermal conductivity. This varies with porosity and can be used to calculate the transient conduction heat transfer. A concept for improved performance simulation models is presented.