a b s t r a c tIn hot dry climates, it is estimated that almost half the urban peak load of energy consumption is used to satisfy air-conditioning cooling demands in summer time. Since the urbanization rate in developing countries -like the case in Egypt -is rising rapidly, the pressure placed on energy resources to satisfy inhabitants' indoor comfort requirements is consequently increasing too. This paper introduces passive cool roof as a means of reducing energy cooling loads for satisfying human comfort requirements in a hot climate. A designed algorithmic hybrid matrix was used to simulate 37 roof design probabilities alternating roof shape, roof material and construction. The result of using a vault roof with high albedo coating shows a fall of 53% in discomfort hours and saves 826 kW h during the summer season compared to the base case of the conventional non insulated flat roof in a typical Cairo residential buildings. It is recommended that the selected cool roof solution be combined with natural ventilation to increase the indoor thermal comfort, and with passive heating strategies to compensate the increase in heating hours. The application is intended for low cost residential buildings in a hot dry climate.

HIGHLIGHTS Seven approaches present solutions for urban densification  Successful roof stacking requires integrating urban, engineering and architectural aspects  European cities have a great potential to be densified through roof stacking  30% of the population increase could be covered by roof stacking in Brussels ABSTRACTFacing the need to accommodate a growing number of inhabitants in major European cities, this research aimed to establish a methodology that facilitates decision making on urban densification through roof stacking.The methodology adopts a systematic approach on three consecutive levels: urban, engineering, and social.Multiple criteria are identified to assess and map the roof stacking potential in terms of location and number of added floors. The Brussels Capital Region was chosen as a case study to experiment with the developed workflow chart and validate the proposed approach, using ArcGIS software, by creating a map of the urban densification potential through roof stacking of Brussels at the city scale. The results show a realistic potential of accommodating 30% of the expected population increase in Brussels by the year 2040 using only roof stacking, provided that the current urban regulations are respected. In addition, a theoretical potential to accommodate more than the expected population increase by the same year is proposed provided that urban planning regulations are relaxed in relation to the height of buildings. Further applications to other cities in 2 Europe would help create additional opportunities to develop an automated tool for estimating such potentials on a wider scope.

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The interest to find cost-optimal zero-energy solutions for building, using multi-objective optimization, has risen dramatically over the last decade. Accordingly, several studies have been carried out, proposing new methods and tools. None, however, has introduced a simplified approach that is viable by a broader range of users. This study addresses this lack, offering a methodology that supports the decision making process on cost-optimal zero energy building, using a novel approach, namely Multi-Objective Parametric Analysis (MOPA), rather than optimization algorithms. This study adds to the domain of roof stacking construction by setting the weight of construction as a third objective. The current methodology is applied to a newly developed theoretical Reference Building (RB) for a Belgian passive roof stacking house. Different options of the building's superstructure components (walls, roof, and windows) have been examined. MOPA follows three consecutive steps: modeling setup, parametric simulation, and ends up with evaluation and selection. The results show cost-optimal zero-energy and lightweight packages of design variables for the building envelope.

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