Evaluating the Impact of Horizontal Louvers on Natural Ventilation and Thermal Comfort in Polish Multi-Family Homes: A CFD Analysis Mohammad Mahdi Mohammadi 1 , Maciej Janowski 2 1 Poznan University of Technology, Poznan, Poland, mohammad.mohammadi@doctorate.put.poznan.pl 2 Poznan University of Technology, Poznan, Poland, maciej.janowski@put.poznan.pl SUMMARY: Climate change is intensifying summer overheating issues in buildings across cold regions like Poland, where designs have focused mainly on winter heat retention. This study evaluates the effectiveness of horizontal louvers as a passive heat-rejection strategy in Polish multi-family residential buildings, using computational fluid dynamics (CFD) to analyse thermal and ventilation performance. Over 300 residential building models were examined, categorized by fenestration and shading configurations, and 2 cases were chosen as prototypes. CFD simulations focused on horizontal louvers on sun-exposed facades, particularly in south-facing windows. Contrary to expectations, results indicate that horizontal louvers can increase indoor overheating. Louvers on sun-facing facades absorbed solar radiation, which caused heated air to enter the building—particularly with winds from the southeast, south, or southwest—increasing indoor temperatures. These findings suggest that while horizontal louvers may provide cooling benefits under specific conditions, they require careful design, material selection, color, and placement to avoid unintended heating effects. Alternative shading solutions or adaptive design strategies may offer more reliable summer comfort in cold-climate buildings facing climate-induced temperature extremes. Keywords: Natural Ventilation, horizontal louvers, multi-family residential buildings 1. BACKGROUND AND MOTIVATIONS With climate change bringing hotter summers, countries with cold winters, like Poland, are increasingly experiencing indoor overheating challenges. Buildings in these regions, historically designed for winter heat retention, now face summer discomfort for occupants, as their designs often lack adequate summer cooling strategies (Bugenings & Kamari, 2022; Konstantinou & Prieto, 2018). In this context, shading solutions like horizontal louvers are widely promoted to mitigate solar heat gain in warm seasons (Bugenings & Kamari, 2022; Nazari et al., 2023). However, this study provides an in-depth evaluation of horizontal louvers in 2 Polish residential buildings, specifically examining their impact on thermal comfort and airflow in naturally ventilated homes. Using computational fluid dynamics (CFD), this research investigates the conditions under which louvers may exacerbate rather than mitigate overheating and offers insights into optimizing their use. The residential building sector in Poland consumes approximately 38% of the nation's total energy (Anna Dworakowska, 2016; Tao et al., 2021), primarily for heating (Attia et al., 2022). Yet, as