Thermal modelling of the green roof substrate
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This research aims to improve the accuracy of a green roof thermal simulation by linking the moisture transfer in the substrate with the conduction heat transfer. The improved simulation is divided into two phases, which are the absorption and evaporation. For the absorption, this research applies the Sharp Front theory to predict the position of the boundary between the saturated and dry zones. The theory of evaporation from a porous medium is used to predict the moisture content in different substrate layers during evaporation. After the moisture content in each layer is determined, this moisture content data can be converted into thermal conductivity, and the time dependent effective thermal resistance can be calculated, which is used in the conduction heat flux calculation for the thermal simulation. Having developed the simulation of heat conduction through the substrate as a result of absorption and evaporation, the results are applied to a case study building to compare a green roof substrate in three moisture conditions (dry, saturated, and the varying simulated moisture content). The results confirm that it is possible to include the effect of water absorption and evaporation from a green roof substrate in the thermal simulation of building performance using the two theories. The improved simulation makes it possible to model an intensive green roof, where the depth of the substrate is likely to produce a greater distribution of moisture content in service. The simulation is an improvement over previous models and the case study shows that, assuming the substrate to be dry (as in existing models), underestimates the heat loss, whilst assuming it is saturated slightly overestimates the heat loss. The small, but significant, effect of moisture variation in service is important for designers of green roofs. If it is not taken into account it would be preferable to assume that the roof is continuously saturated as this gives a conservative estimate of the heat loss from a building.