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Purpose: Rooftop greenhouses (RTGs) are increasing as a new form of urban agriculture. Several environmental, economic, and social benefits have been attributed to the implementation of RTGs. However, the environmental burdens and economic costs of adapting greenhouse structures to the current building legislation have already been pointed out as a limitation of these systems in the literature. In this sense, this paper aims to analyze the environmental and economic performance of RTGs in Barcelona. Methods: A real RTG project is here assessed and compared to an industrial greenhouse system (i.e., multi-tunnel), from a life cycle perspective. Life Cycle Assessment (LCA) and Life Cycle Costing (LCC) methods are followed in the assessment. The analysis is divided into three parts that progressively expand the system boundaries: greenhouse structure (cradle-to-grave), at the production point (cradle-to-farm gate), and at the consumption point (cradle-to-consumer). The applied LCIA methods are the ReCiPe (hierarchical, midpoint) and the cumulative energy demand. A Cost-Benefit analysis (CBA) approach is considered in the economic analysis. For the horticultural activity, a crop yield of 25 kg•m-2 is assumed for the RTG reference scenario. However, sensitivity analyses regarding the crop yield are performed during the whole assessment. Results and discussion: The greenhouse structure of an RTG has an environmental impact between 17 and 75% higher and an economic cost 2.8 times higher than a multi-tunnel greenhouse. For the reference scenario (yield: 25 kg•m-2), 1 kg of tomato produced in an RTG at the production point has a lower environmental impact (10-19%) but a higher economic cost (24%) than in a multi-tunnel system. At the consumption point, environmental savings are up to 42% for local RTGs tomatoes, which are also 21% cheaper than tomatoes from multi-tunnel greenhouses in Almeria. However, the sensitivity assessment shows that the crop efficiency is determinant. Low yields can produce impacting and expensive vegetables, although integrated RTGs with energy from the building can lead to low impacting and cheap local food products. Conclusions: RTGs face law limitations that make the greenhouse structure less environmentally-friendly and less economically competitive than current industrial greenhouses. However, as horticultural systems and local production systems, RTGs can become an environmentally-friendly option for further develop urban agriculture. Besides, attention is paid to the crop yield and, thus, further developments on integrated RTGs and their potential increase in crop yields (i.e., exchange of heat and CO 2 with the building) are of great interest.

Urban agriculture (UA) is spreading within the Global North, largely for food production, ranging from household individual gardens to community gardens that boost neighborhood regeneration. Additionally, UA is also being integrated into buildings, such as Urban Rooftop Farming (URF). Some URF experiences succeed in North America both as private and community initiatives. To date, little attention has been paid to how stakeholders perceive UA and URF in the Mediterranean or to the role of food production in these initiatives. This study examines the promotion and inclusion of new forms of UA through the practice of URF and contributes to the nascent literature on the stakeholder and public perceptions of UA. It seeks to understand how those perceptions shape the development of new urban agriculture practices and projects. Barcelona (Spain) was used as a Mediterranean case study where UA and URF projects are growing in popularity. Through semi-structured interviews with 25 core stakeholders, we show that UA is largely perceived as a social activity rather than a food production initiative, because the planning of urban gardens in Barcelona was traditionally done to achieve leisure and other social goals. However, several stakeholders highlighted the potential to increase urban fertility through URF by occupying currently unused spaces. As a result, the positive valuation of URF depends on the conceptualization of UA as a social or food production activity. In turn, such conceptualization shapes barriers and opportunities for the development of URF. While most UA-related stakeholders (e.g., food coops , NGOs) preferred soil-based UA, newer stakeholders (e.g., architects) highlighted the economic, social and environmental opportunities of local and efficient food production through innovative URF.

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Urban rooftop farming favours local food production. Although rooftop farming is perceived as a sustainable system, there is a lack of quantitative studies. There, we set up experiments in the community rooftop garden of a public housing building in Bologna, Italy, between 2012 and 2014. We grew lettuce, a leafy vegetable, using three techniques: nutrient film, floating hydroponic and soil cultivation. We also grew tomato, chilli pepper, eggplant, melon, watermelon on soils. Data was analysed by life cycle assessment for environmental and economic performance. Results reveal that the best techniques of lettuce cultivation to address global warming were floating in the summer, with 65-85 % less environmental impact per kilogran than nutrient film; and soil production in the winter, with 85-95 % less environmental impact. Furthermore, floating production was 25 % cheaper in summer, and soil was 65 % cheaper in winter, compared to the nutrient film technique. For soil production, eggplants and tomatoes showed the best environmental performances of about 74 g CO 2 per kg. Eggplant production in soil was cheapest at 0.13 € per kg.