The increasing urban population has significantly raised global energy consumption (EC) and fossil fuel demand, worsening environmental issues like greenhouse gas emissions and climate change (CC). Buildings contribute substantially to EC, requiring energy-efficient solutions such as passive systems (PS). This study investigates phase change materials (PCMs) for reducing heating and cooling demands while improving thermal comfort (TC) in Ismailia, Egypt-a hot, arid region. Five PCMs (PCM 21 (21 degrees C melting point), PCM 23, PCM 25, PCM 27, and PCM 29) were evaluated, with PCM 21 emerging as the most effective, achieving 17 % heating reduction and 13 % cooling reduction. Optimal PCM placement was determined using a 1.00 x 1.00 m2 grid system, revealing that 60 % PCM coverage across walls and floors maximizes efficiency. This configuration lowered energy use intensity (EUI) by 9.23 %, cooling demand by 11.49 %, and heating demand by 14.89 %, with the upper 50 % of walls and southern 50 % of floors proving most impactful. Economic analysis showed payback periods of 75 months for 55.11 % PCM and 104 months for 60 % PCM, confirming PCMs as a cost-effective PS for sustainable building design. These results highlight PCMs' potential to reduce EC while enhancing building resilience in extreme climates.