Investigating the role of glass thickness and air layer in increasing the thermal performance of window openings in buildings with a climatic approach in Tehran.

Subject Areas : َArchitecture

Ashkan Hassani ¹ , shooka khoshbakh bahramani ² , Vahid Ghobadian ³

1 -
2 - Architecture, Faculty of Architecture and Design, Islamic Azad University, Tehran Central Branch, Tehran, Iran.
3 - Department of Architecture, Islamic Azad University, Central Tehran Branch

Received: 2025-02-26 Accepted : 2025-06-30 Published : 2025-09-21

Keywords: Thermal optimization of windows, heat transfer, hot and dry climate of Tehran, building energy simulation, sustainable architecture,

Abstract :

In recent decades, optimizing energy consumption in buildings has become a critical concern in architecture and building engineering, particularly in hot and dry regions such as Tehran. Windows play a pivotal role in heat transfer, and improper design and selection significantly increase energy consumption across different seasons. A lack of quantitative data and precise analytical tools for assessing the parameters influencing the thermal performance of windows remains a major challenge in this field. This study aims to provide a quantitative and analytical framework to minimize heat transfer through windows and enhance their thermal performance in buildings located in hot and dry climates, with a specific focus on Tehran.The research is based on simulations conducted using DesignBuilder software. Parameters such as glass thickness, the number and type of internal air layers, and various insulation types were analyzed using real data and the climatic conditions of Tehran. The results were compared with existing standards to identify the impact of each factor.The results indicate that increasing glass thickness, optimizing the design of air layers, and utilizing appropriate insulation can significantly reduce heat loss. This study presents a comprehensive framework for window design that assists architects in making optimal decisions during the design phase. The findings serve as a valuable reference for improving facade openings and reducing energy consumption in buildings with similar climatic conditions.

 

Extended Abstract

 

Introduction

Under the heightened worldwide energy need and pressing environmental issues, construction and building design practices are undergoing considerable scrutiny. Cities, especially with desert and warm climates like Tehran, are very susceptible to excessive energy requirements due to heating and cooling activities. Of the numerous architectural features, windows are among the most effective elements that regulate heat transfer between internal and external environments. Inefficient design and inappropriate material choice for windows can largely enhance thermal loss or gain, thus contributing substantially to energy use in residential buildings.In spite of the significance of the matter, there is still a considerable deficiency of research on quantitative and site-specific investigation of thermal optimization of multi-layer window systems for the particular climate of Tehran. Thermal properties of glass layers, interstitial air gap, and the window profiles themselves are all parameters commonly examined in isolation rather than through integrated simulation methodologies. In order to fill this literature gap, the present study strives to introduce an integrated simulation-based framework that is capable of guiding the thermal design of multi-glazed windows with a special emphasis on heat balance and energy efficiency improvement.

 

Methodology

This research is analytical and quantitative in type, and simulation and comparison of the thermal performance of various types of double-glazed windows in an average Tehran residential building are simulated with DesignBuilder simulation software. The simulation parameters are:

• Inner and outer glass thicknesses whose values are variable (from 2 mm to 20 mm),
• A consistent 2-mm thickness of the air gap between the glasses,
• Application of PVC profiles on all types of windows.

The case study is a flat with regular spatial layout, where one living room of 29 m² is selected as the simulation zone. Boundary conditions include indoor comfort temperature ranges (12–21°C in heating, 25–28°C in cooling), lighting target (150 lux), supply rate of fresh air (10 l/s/person), and mean occupant activity (eating/drinking, metabolic factor = 0.9).

Ten window types were simulated with their glass thickness varied systematically to see the effect on thermal heat balance. The performance measure is the heat balance in kilowatts (kW), positive for gain and negative for loss.

 

Results and discussion

Simulation findings revealed that thermal insulation enhancement can be measured by increasing inner and outer glass panes' thickness to reduce total heat transmission across window systems. Specifically, windows with outer glass thickness between 2–10 mm showed the largest range of performance with optimum improvement of heat equilibrium at inner glass thickness ranges of 12–14 mm.

Key observations include:

• Window Type 1 (2 mm outer glass): Recorded initial heat loss as -1.72 kW. Inner glass thickness was raised to 12 mm and improved heat balance by +0.23 kW.
• Window Type 5 (10 mm outer glass): Recorded initial heat loss as -1.68 kW, which was improved by +0.22 kW when inner thickness was altered to 12 mm.
• Window Types 8–10 (outer thickness 16–20 mm): Experienced decreasing returns with improvements in performance levelling off at more than 16 mm, pointing to an economic inefficiency to spend more on improving thickness.

In both cases, the fact that a 2 mm air gap was present assisted in improving the insulating advantage but further opening of this gap (beyond the scope of this research) would be preferable. Notably, simulation results compare with research that has experienced reduced benefits once a barrier of glass thickness is passed and has emphasized the importance of economic in addition to thermal performance.

The final-deliverables are 10 synthesizing data matrices of combining glass thickness and corresponding thermal balance, together with technical drawings showing each window design.

 

 

 

Conclusion

The paper provides a comprehensive simulation-based approach to thermal performance analysis and optimization of double-glazed window systems for hot and dry urban climates. The key findings are that:

1. Thermal resistance improves with increasing inner and outer glass thickness, but the gain saturates after 12–16 mm.
2. 12–16 mm outer pane thickness in the windows provides the optimum cost-thermal efficiency ratio.
3. Inner thicknesses of spectacles are also implicated, and 12 mm is a satisfactory value for most applications.

The matrices and technical reports outlined here can be an important aid to architectural decision-making at the design stage. Additionally, the research provides raw material for subsequent studies of cost-benefit analysis, lifecycle analysis, and long-term durability of materials.

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