Evaluation of Overall Thermal Transfer Value (OTTV) for commercial buildings constructed with green roof

Overall Thermal Transfer Value (OTTV) is a measure of average heat gain into a building through the building envelope. It is a widely adopted measure in many countries for enhancing energy-efficient building design. In the past decade, there is increasing application of green roof into commercial buildings for enhanced building insulation, leading to reduction in heat gain through the roof area as well as cooling requirement of a building. Since the current OTTV equations and coefficients were originally developed for buildings with traditional bare roof construction, building designers have difficulty to compute the OTTV for building constructed with green roof. The aim of this study is to revise the existing OTTV calculation method and derive a set of correction factors for OTTV evaluation of green roof integrated buildings. An experimental setup of a green roof system with sensors was installed on the rooftop of a commercial building. The measured data were used for validation of a building energy simulation program EnergyPlus incorporated with a green roof model Ecoroof. Four building cases with typical and traditional roof constructions were modeled using the validated computer simulation program. Through a series of parametric computer simulations, a correlation between OTTV and annual heat gain through the roof area was established with that a set of correction factors ranging from 0.03 to 0.99 was developed. Theses correction factors can be used by building designers to compute the OTTV of building constructed with green roof. The details of methodology and findings are reported in this paper.