White , black, or green? The role of roof design in affecting the urban heat island

Urban heat island mitigation research to date has focused largely on the potential of high albedo roof and paving surfaces and street-level vegetation to reduce urban air temperatures. These assessments have typically been conducted using mesoscale atmospheric modeling with crude parameterizations of the urban surfaces involved. In this study we focus more specifically on the surface energy balances and ask the question “how does roof surface treatment impact the convection of heat into the urban atmospheric environment”? This study will move beyond a simple comparison of “cool” high albedo membranes with their darker counterparts. Specifically, we compare thermal performance of dark and light membrane roofs with vegetated "green" roofs, typical rooftop photovoltaic treatments, and a combination system involving photovoltaic panels installed over a green roof system. The estimates of convection heat flux rates per unit area of flat roof are based on a combination of measurements in the field and building energy simulations. While addition of photovoltaic panels above a roof provides an obvious energy generation benefit, it is important to note that such systems – whether integrated into the building envelope, or mounted above the roof – can also result in an increase of convective heat flux into the urban environment. Our analysis shows that integration of green roofs with photovoltaic panels can partially offset this undesirable side effect, while producing additional benefits.