Studies have demonstrated the energy savings potential of mixed-mode ventilated office buildings. Yet, it is important to widen the knowledge about how those buildings have been designed and built in practice, and which design parameters have greater influence on its energy performance. The aim of this paper was to evaluate how building envelope design parameters influence the energy performance of cellular mixed-mode office buildings, in order to identify key design variables. The analysis presents a comparison among literature research studies and typical construction practices from a sample of buildings located in the city of Sao Paulo, Brazil. According to a base case model, established based on the real buildings sample, three sensitivity analysis techniques were performed to obtain relative parameter sensitivity to thermal loads: OFAT, Morris and Monte Carlo. Results showed the importance of the window opening effective area and the reduced impact of the window-to-wall ratio on the energy performance of mixed-mode office buildings. By applying a multivariate regression model, it showed significant in predicting 78.1% of the variance in annual thermal loads. The accurate determination of annual thermal loads into mixed-mode office buildings can be used to optimize the envelope characteristics based on a combination of input data and the building geometry. Findings from this study could also be applied to other locations, provided that similar climatic environment and urban context are taken into account.

Introduction

Mixed-mode ventilation (MMV) is an option that allows to combine natural ventilation and mechanical cooling systems as a possible solution to provide cooling, natural ventilation, Indoor Air Quality (IAQ) and thermal comfort to users, while reducing energy use (Brager et al., 2000). This is a relatively new subject; there is no specific guidance on how to simulate or even design such buildings (Salcido et al., 2016), which makes their energy performance analysis a challenging task. Thus, it has attracted attention from researchers, with an increased number of publications on the past two decades (Salcido et al., 2016). IEA ECBCS Annex 35 (Heiselberg and University A, 2002) and CBE database (Center for the Built Environment (CBE), 2013) were pioneer research studies about the subject, summarizing design principles and performance prediction techniques of MMV buildings. According to Annex 35 report (Heiselberg and University A, 2002), in order to reduce the energy consumption in MMV buildings, designers are required to understand its requirements since the early design stages, which are different from mechanically ventilated buildings. In that sense, the building envelope is a key aspect for the MMV system to work properly. The characteristics of the building envelope, such as glazing surface area, window protection and window type affect the heat gains and losses of the interior spaces, and also help promoting a better use of the natural ventilation system (Hamdaoui et al., 2018). As a result, it provides less use of the air-conditioning system and, therefore, greater energy efficiency (Brager et al., 2000; Mendler et al., 2006).