Redefining green roof systems with climbers: Simulation of a conceptual model for thermal-radiative performance and plant vitality

Purpose: Green roofs are strategies for the ecological intensification of cities and a measure of meeting some of the sustainable development goals (SDGs). They have widely been adopted as an adaptation strategy against an urban heat island (UHI). However, they are conventionally soil-based making it difficult and expensive to adopt as a strategy for greening existing buildings (GEB). This paper, therefore, develops a novel green roof system using climbers for thermal-radiative performance. The paper explores the vitality of climbing species as a nature-based strategy for GEB, and for the ecological improvement of the predominantly used cool roofs in sub-Saharan Africa (SSA). Design/methodology/approach: Simulation for the same building Kejetia Central Market (KCM) Redevelopment; the existing aluminium roof (AL), soil-based extensive green roof (GR1) and the proposed green roof using climbing plants (GR2) were performed using ENVI-met. The AL and GR1 were developed as reference models to evaluate and compare thermal-radiative performance of the conceptual model (GR2). The long wave radiation emission (Qlw), mean radiant temperature (MRT) and outdoor air temperature (Ta) of all three roofing systems were simulated under clear sky conditions to assess the performance and plant vitality considering water access, leaf temperature (Tf) and latent heat flux (LE0) of GR1 and GR2. Findings: There was no short wave radiation (Qsw) absorption at the GR2 substrate since the climbers have no underlying soil mass, recording daily mean average Qlw emission of 435.17 Wm−2. The soil of GR1, however, absorbed Qsw of 390.11 Wm−2 and a Qlw emission of 16.20 wm−2 higher than the GR2. The AL recorded the lowest Qlw value of 75.43 Wm−2. Also, the stomatal resistance (rs) was higher in GR1 while GR2 recorded a higher average mean transpiration flux of 0.03 g/sm3. This indicates a higher chance of survival of the climbers. The Ta of GR2 recording 0.45°C lower than the GR1 could be a good UHI adaptation strategy. Research limitations/implications: No previous research on climbers for green roof systems was found for comparison, so the KCM project provided a unique confluence of dynamic events including the opportunity for block-scale impact assessment of the proposed GEB strategy. Notwithstanding, the single case study allowed a focussed exploration of the novel theory of redefining green roof systems with climbers. Moreover, the simulation was computationally expensive, and engaging multiple case studies were found to be overly exhaustive to arrive at the same meaningful conclusion. As a novelty, therefore, this research provides an alternative theory to the soil-based green roof phenomenon. Practical implications: The thermal-radiative performance of green roofs could be improved with the use of climbers. The reduction of the intensity of UHI would lead to improved thermal comfort and building energy savings. Also, very little dependence on the volume of soil would require little structural load consideration thereby leading not only to cheaper green roof construction but their higher demand, adoption and implementation in SSA and other low-income economies of the global south. Social implications: The reduction of the consumption of topsoil and water for irrigation could avoid the negative environmental impacts of land degradation and pollution which have a deleterious impact on human health. This fulfils SDG 12 which seeks to ensure responsible consumption of products. This requires the need to advance the research for improvement and training of local built environment practitioners with new skills for installation to ensure social inclusiveness in the combat against the intractable forces of negative climate impacts. Originality/value: Climbers are mostly known for green walls, but their innovative use for green roof systems has not been attempted and adopted; it could present a cost-effective strategy for the GEB. The proposed green roof system with climbers apart from becoming a successful strategy for UHI adaptation was a so able to record an estimated 568% savings on topsoil consumption with an impact on the reduction of pollution from excavation. The research provides an initial insight into design options, potentials and limitations on the use of climbers for green roofs to guide future research and experimental verification.