PSI - Issue 55

Andréa R. Souza et al. / Procedia Structural Integrity 55 (2024) 143–150

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Andrea R. Souza et al ./ Structural Integrity Procedia 00 (2019) 000–000

1. Introduction Climate change will increase air and surface temperatures and change the urban climate, affecting the thermal performance of buildings and, consequently, energy consumption due to the urban overheating effect (Nazarian et al. 2022). Nowadays, the impact of buildings on greenhouse gas (GHG) emissions is well known, and some reports from the Intergovernmental Panel on Climate Change (IPCC) show that 33% of worldwide energy-related GHG emissions are due to building use (Robert & Kummert 2012). Therefore, in this scenario, it is essential to design and maintain the building with a strategy for lower energy consumption. The Building Performance Institute Europe (BPIE) estimates that about 97% of the European building stock is not classified as ‘A’ in the Energy Performance Certificate (EPC) and that at least 75% of the EU building stock should be renovated by 2050 to achieve decarbonisation of buildings (BPIE 2017). The building envelope plays a crucial role in improving the energy efficiency of buildings by regulating heat transfer between the external and internal environments, especially to maintain indoor comfort (Stocker & Koch 2017). External thermal insulation composite systems (ETICS), which consist of insulation boards and thin reinforced plaster, are often used in the building envelope, especially in facades, to solve heat transfer problems. Nonetheless, the effects of weathering lead to physical and aesthetic irregularities (Parracha et al. 2021). To mitigate the effects of thermal stress on ETICS, the European Association for External Thermal Insulation Composite Systems (EAE 2011) recommends a finishing coat with a solar absorption rate below 70% and surface temperatures not exceeding 80ºC. Surface temperature (ST) is strongly influenced by solar reflectance and infrared emittance from surfaces. In general, surfaces with low solar reflectance, such as dark colours like black, have higher ST than surfaces with high solar reflectance, such as light colours (Nazarian et al. 2022). Therefore, maintaining the reflectance of the surface is crucial to reducing thermal load and extending the durability of the building envelope once weathering changes the reflectance over the life cycle of the coating (Hradil et al. 2014). The use of light colours, which are considered high reflectance materials, reduces thermal stress. Nonetheless, reflectance often decreases rapidly in the first months of exposure due to the deposition of soot, dust or biomass associated with soiling and photodegradation (Santamouris et al. 2011). Dark colours might be better suited to resist such weathering and soiling effects, as noted by Revel et al. (2014). Nevertheless, maintaining dark surfaces while keeping them cool under solar radiation remains a significant technical challenge. Changes in near-infrared (NIR) reflectance for dark colours can significantly reduce surface temperatures compared to standard reflective coatings (Mourou et al. 2022). Some results show a reduction of peak temperatures by at least 5ºC and a reduction of surface temperatures above 50ºC by up to 10% (Ramos et al. 2021) when dark NIR ETICS facades are used. Furthermore, using dark NIR paints can improve the durability of ETICS, as shown in Dantas et al. (2022) and keep the aesthetic colour of the façade, as in Ramos et al. (2020). This study aims to determine the feasibility of using dark paints with high near-infrared (NIR) reflectance as an energy-efficient retrofit solution for building facades. It investigates the impact of high-reflectance dark colour paints on reducing the surface temperature of ETICS facades in different climatic regions of Portugal. The analysis includes estimating the surface temperature distribution with a steady-state calculation method under different reflectance conditions (aged, new, and retrofitted) for the same system. It is also investigated, the influence of climatic parameters such as solar radiation and air temperature on the surface temperature values. 2. Materials Description and Temperature Calculation 2.1. Façade system characterisation The performance of the retrofit was evaluated using an original ETICS sample (1 m 2 ) that was naturally aged for three years in a horizontal position on the roof of the Department of Civil Engineering of the University of Porto, with the thermal performance of the original system described in Ramos et al. (2021). The aged sample after 3 years of natural exposition (Fig. 1a) was removed from the roof. A 10 x 10 cm piece was cut from the original 1-metre square panel, washed with water, air dried (24 ºC) for 48 hours and renovated with black paint with high reflectance in the near-infrared range (Fig. 1b). The composition of the samples is listed in Table 1.