Iranian Sustainable Building Scientific Association International Journal of Urban Management and Energy Sustainability 2538-1628 6 4 2025 10 01 Impact of Thermal Insulation on Energy Behavior of Residential Building Envelopes (Tehran’s District 15) 248 259 729219 10.22034/ijumes.2025.2060021.1311 EN Reza Salimi Gargari Department of Architecture, S.R.C., Islamic Azad University, Tehran, Iran 0009-0009-9699-708X Seyyed Majid Mofidi Shemirani Department of Architecture, S.R.C., Islamic Azad University, Tehran, Iran 0000-0001-5388-7045 Haniyeh Sanaieian Department of Architecture, Iran University of Science and Technology, Tehran, Iran 0000-0003-2792-1777 Journal Article 2025 05 07 Buildings account for approximately 40% of global energy consumption, with poorly performing building envelopes significantly contributing to unsustainable energy demands. This issue is particularly critical in semi-arid climates such as Tehran, where extreme seasonal temperatures result in high residential energy use for heating and cooling. This study aims to address Tehran''s energy efficiency challenges by optimizing the insulation thickness of residential façades in District 15. Using validated simulations conducted in DesignBuilder and informed by GIS data and on-site measurements, three scenarios of rock wool insulation 3 cm, 5 cm, and 7 cm were modeled and analyzed. The results demonstrate that a 7 cm insulation layer achieves the lowest energy consumption (Cooling: 13.01 kWh/m²; Heating: 306.7 kWh/m²), while the 5 cm option represents the most cost-effective solution, yielding an annual energy savings of approximately 2,500 kWh/m². Furthermore, the study confirms that compliance with Iran’s National Building Regulations, specifically Topic 19, leads to an 18.7% reduction in natural gas consumption. These findings offer valuable insights for urban energy policy, particularly in arid and semi-arid environments. Future research should investigate the integration of phase-change materials and conduct multi-climatic simulations to generalize the applicability of the proposed optimization model https://www.ijumes.com/article_729219_ca6ecbd23810fb38d85dc7d99ea0f79f.pdf