Determining Thermal Specifications for Vegetated GREEN Roofs in Moderate Winter Climats
Publicatie van Kenniscentrum Duurzame HavenStad
C.M. Ravesloot | Artikel
Because local weather conditions in moderate climates are changing constantly, heat transfer specifications of substrate and vegetation in vegetated green roofs also change accordingly. Nevertheless, it is assumed that vegetated green roofs can have a positive effect on the thermal performance of construction in winter conditions. Is there proof from scientific research and field testing for this assumption? To answer this question, research is conducted with the main research question: Which parameters defining thermal performance for vegetated green-roof construction for a moderate winter climate like that in the Netherlands can be determined from existing literature, and how do these parameters influence thermal performance? Literature research was executed on monitoring and testing of thermal specifications of vegetated green roofs. Models with physical parameters on vegetated green roofs were studied and compared. The first goal was to make a list of all physical parameters and corresponding variables valid in the Dutch moderate-winter climate. None of the models that were found in the literature seemed to cover all physical processes. These models use parameters and variables to calculate the overall u-value of substrate and vegetation. Nevertheless, one nearly complete model was used for a sensitivity test on variables. Maximum and minimum values of variables were calculated in the model to determine the influence on the outcome in terms of u-value. From this analysis, a distinction could be made between variables influencing the u-value strongly and other variables influencing the outcome weakly. The modelling showed that three variables were influencing the model calculations moderately strongly and therefore the thermal performance of the vegetated green-roof substrate and vegetation. These variables are not consistent with parameters modeling or calculating u-value in constructions. This finding means that contribution to thermal insulation by extensive vegetated green-roof substrate and vegetation in terms of u-value would be negligible. Only a small theoretical contribution to thermal insulation can be argued from weak variables. To be sure about this small theoretical contribution to the u-value of the roof construction, this u-value was used as input for energy-use calculations for residential buildings. These calculations show that such a small increase of the u-value leads to no visible reduction in energy use for heating in winter conditions. The contribution is negligible compared to the influence of the u-value from extra insulation under the roof. For vegetated green roofs in such moderate winter climates as in the Netherlands, additional u-value will have to be proven specifically, because the modelling shows that, in general, no contribution to thermal insulation can be expected.