Roof–envelope ratio impact on green roof energy performance |
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Authors: | Ryan Martens Brad Bass Susana Saiz Alcazar |
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Institution: | (1) Adaptation and Impacts Research Division, Environment Canada, 33 Willcocks Street, Toronto, ON, M5S 3E8, Canada |
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Abstract: | This paper addresses the impact of roof-to-envelope ratio on overall energy savings of a green roof design over conventional
roof designs. Simulations were performed using a modified version of the Environmental System Performance program simulator,
developed at the University of Strathclyde. The modified design employed a model developed by Columbia University and the
Goddard Institute of Space Science which models the evapotranspiritive effect of a green roof calculated using the Bowen ratio;
that is, the ratio of sensible heat flux to the surrounding air to the latent heat flux resulting from evapourative energy
losses. The resulting heat flux term is proportional to the external surface convection, but inversely proportional to the
surface Bowen ratio, which is held constant and chosen to match experimental results obtained for a given roof design. The
present study performed simulations for the month of July in a Toronto climate on square warehouse style one, two, and three-story
buildings, with windows occupying 10% of the area of each wall. For the first set of simulations, the internal building load
of each story was set to zero, and the roof–envelope ratio was increased by increasing the building width and length. For
the final simulations, several roof–envelope ratios were chosen, and the internal load of each story was increased from 0
to 50,000 W. As the roof–envelope ratio increases, the cooling load of the upper floor for multi-story designs approaches
the entire building cooling load. This indicates the importance of upper zone cooling in total building energy reductions.
Furthermore, the total energy savings of a green-roofed building over a conventional roofed building were far more significant
for single-story structures. A 250 × 250 m green-roof design with 50,000 W internal loading was found to have percentage energy
savings of 73%, 29%, and 18%, for a one, two, and three-story design, respectively. |
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Keywords: | Green roof Building energy simulation ESP-r Building envelope Latent flux |
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