Multi-Objective Optimization of Internal and External Insulation of Residential Building Walls in Cold Climates (Tabriz City)

The continuous growth of energy demand in Iran’s residential sector has made excessive building energy consumption one of the most serious national challenges, drawing attention to the urgent need for improving the thermal efficiency of building envelopes. This study conducts a comprehensive comparative analysis of internal and external wall insulation systems in residential buildings located in the cold and dry climate of Tabriz. The main purpose is to identify the most effective insulation configuration that minimizes heating and cooling energy consumption while achieving a balance between thermal comfort, environmental sustainability, and cost efficiency. Five insulation materials rock wool, mineral wool, wood fiber, expanded polystyrene (EPS), and sisal fiber were analyzed in combination with five commonly used façade materials, namely clay brick, fire brick, cement plaster, travertine, and granite. Energy performance simulations were performed using DesignBuilder software coupled with the EnergyPlus simulation engine to evaluate several critical indicators, including total energy demand, heating and cooling energy consumption, embodied carbon emissions, material and construction costs, and the number of annual thermal discomfort hours experienced by occupants. The analysis aimed to determine how both the insulation material and its position within the wall structure influence overall building performance under real climatic conditions.
To identify the optimal configuration, a multi-objective optimization framework was employed, combining the Pareto Front algorithm and the TOPSIS (Technique for Order Preference by Similarity to Ideal Solution) method. The Pareto approach was used to generate non-dominated solution sets that represent the best trade-offs between conflicting objectives, while TOPSIS was applied to rank each alternative based on its proximity to the ideal solution. In total, seventy-five wall configurations were modeled, covering various insulation types, positions (internal or external), and façade combinations. The results showed that the configuration employing external expanded polystyrene insulation with a clay brick façade provided the highest overall performance among all tested cases. This system reduced heating energy consumption by approximately 6.8%, decreased total annual energy demand by 17%, and improved thermal comfort by 18% compared to the non-insulated baseline model. In contrast, internal insulation systems exhibited weaker performance due to thermal bridging and reduced use of the wall’s thermal mass. Additionally, while materials such as mineral wool and sisal fiber had lower embodied carbon emissions, their overall insulation effectiveness under Tabriz’s climatic conditions was moderate. The findings highlight the importance of selecting insulation materials and placements that are compatible with local climate conditions and construction practices. Integrating energy simulation with multi-criteria decision-making provides a robust and repeatable framework for optimizing building energy performance. This approach can guide architects, engineers, and policymakers in designing sustainable, energy-efficient, and environmentally responsible residential buildings.
In conclusion, adopting external insulation systems combined with high thermal inertia façade materials represents an effective and sustainable strategy for reducing operational energy use, lowering carbon emissions, and enhancing indoor thermal comfort in cold-climate regions. The proposed methodology supports the development of resilient, energy-conscious building envelopes aligned with long-term national goals for energy efficiency and environmental sustainability.