Extended Abstract

This study examines the optimization of thermal insulation thickness and its placement within the wall assembly of the building envelope in the cold climate of Tabriz using a multi-criteria decision-making approach. The subject is important because the thermal performance of exterior walls in cold regions depends not only on the type and thickness of insulation, but also on its position within the wall layers and its interaction with thermal mass, heating demand, construction cost, and practical implementation conditions. In many building projects, insulation design is still treated mainly as a matter of meeting minimum thermal resistance requirements. However, such an approach does not fully reflect the complex relationship between insulation thickness, energy saving, economic efficiency, and envelope behavior. This research addresses this limitation by evaluating insulation alternatives as integrated wall systems and by identifying the configuration that provides the most balanced performance under the climatic conditions of Tabriz.

The main assumption of the study is that the optimal insulation solution cannot be determined only by selecting the greatest thickness or the lowest thermal transmittance. Although increasing insulation thickness generally reduces heat loss through the wall, its benefits gradually decrease after a certain point. Beyond this range, additional insulation may lead to limited extra energy saving while increasing material use, construction cost, and wall thickness. Therefore, optimization requires identifying the point at which the thermal benefit remains significant and proportionate to the economic and technical consequences. This perspective is especially relevant in cold climates, where reducing heating demand is essential, but where design decisions must also remain feasible, economical, and compatible with construction practice.

The placement of insulation in the wall assembly is another central issue in this research. Insulation location can influence the dynamic thermal behavior of the wall, particularly through its relationship with the thermal mass of masonry or other heavy layers. When insulation is placed toward the exterior side of the wall, the inner layers may remain within the conditioned zone and contribute more effectively to heat storage and indoor temperature stability. This arrangement can reduce rapid indoor temperature fluctuations and improve the overall thermal response of the envelope during cold periods. In contrast, placing insulation on the interior side may reduce the effective contribution of the wall’s thermal mass to the indoor environment. For this reason, the study considers insulation placement not as a minor construction detail, but as a key variable in the optimization of building envelope performance.

The climatic context of Tabriz gives the research additional significance. As a city with a cold climate and considerable heating requirements, Tabriz provides a suitable setting for evaluating the effect of wall insulation on energy consumption and indoor thermal performance. In such climates, exterior walls play a major role in controlling heat transfer between indoor and outdoor environments. Poorly insulated walls can increase heating loads, raise operational costs, and reduce thermal comfort. Conversely, well-designed insulation systems can contribute to lower energy use, improved comfort, and reduced environmental impact. The findings of this study are therefore relevant not only to theoretical discussions of envelope design but also to practical decisions in architectural and building engineering practice.

The use of a multi-criteria decision-making framework is one of the main methodological strengths of the study. Insulation alternatives cannot be judged adequately by a single criterion, because each option involves advantages and limitations. An alternative may perform well in terms of energy saving but may be less attractive due to higher initial cost or greater implementation complexity. Another option may be economical but may not provide sufficient thermal improvement. A multi-criteria approach makes it possible to evaluate these competing aspects together and to rank the alternatives according to their overall performance. This allows the final decision to reflect a more realistic balance among thermal, economic, technical, and practical considerations.

In this research, insulation materials and configurations are assessed not only through their nominal properties, but also through their role within the wall assembly. Expanded polystyrene (EPS), for example, is considered as one of the common insulation materials because of its thermal resistance, availability, and relatively favorable cost-performance relationship. However, the evaluation does not imply that one material is universally superior under all conditions. Rather, the suitability of each material depends on its thickness, location, safety requirements, and compatibility with the wall system. This integrated view is important because material selection in building envelopes should not be separated from layer arrangement and climatic performance.

The results emphasize that increasing insulation thickness improves thermal performance up to an optimal range, but the improvement is not unlimited. The relationship between insulation thickness and energy saving follows a pattern of diminishing returns. At lower thickness levels, adding insulation can produce noticeable reductions in heat loss and heating demand. After the optimal range, however, further increases in thickness provide smaller benefits while increasing costs and material consumption. This finding is important for designers because it shows that the most efficient solution is not necessarily the thickest one. Instead, the optimal solution is the one that achieves an acceptable balance between reduced energy consumption and reasonable construction cost.

The findings also indicate that exterior placement of insulation can offer meaningful advantages in cold climates when implemented in accordance with technical and safety requirements. By positioning insulation outside the main structural or masonry layer, the thermal mass of the wall can remain more effectively connected to the indoor space. This can support indoor thermal stability and improve the wall’s response to outdoor temperature changes. In cold climates, where maintaining indoor warmth is a major design objective, this effect can be particularly valuable. The study therefore suggests that the position of insulation should be carefully considered alongside its thickness, rather than being selected only on the basis of construction convenience.

From a practical perspective, the study provides useful guidance for architects, engineers, and energy consultants involved in the design of building envelopes in Tabriz and similar cold regions. It shows that insulation design should be based on a combined assessment of material type, thickness, and placement. It also demonstrates that relying only on minimum regulatory requirements may not lead to the most efficient or balanced solution. Instead, performance-based evaluation can help identify wall configurations that reduce energy consumption while remaining technically and economically reasonable. This is particularly important in contexts where construction budgets are limited and where long-term operational savings must be carefully weighed against initial investment.

The research also has implications for future studies and technical discussions related to building energy regulations. Its findings should not be interpreted as a direct or sufficient basis for immediate revision of building codes. Building regulations require broad evidence across different building types, climates, materials, construction practices, and safety conditions. However, the study provides analytical evidence that can support future investigations and contribute to more detailed discussions about improving envelope performance standards. In this sense, the research offers complementary scientific support for performance-based approaches to insulation design and for possible future refinements of technical guidance in cold-climate construction.

Another contribution of the study is its emphasis on integrated decision-making in sustainable building design. Energy efficiency is often discussed in terms of individual components, such as insulation material or thermal conductivity. However, the actual performance of a wall depends on the interaction of several factors, including layer sequence, insulation position, thickness, thermal mass, construction quality, and climate. By considering these factors together, the study moves beyond simplified comparison and presents the wall as a complete performance system. This approach is more consistent with the realities of architectural design and building construction, where decisions must respond to multiple constraints at the same time.

Overall, this research provides a scientific and practical assessment of thermal insulation optimization in the wall assembly of the building envelope under the cold climatic conditions of Tabriz. Its main contribution is the simultaneous evaluation of insulation thickness and placement through a multi-criteria decision-making approach. The results show that the optimal solution is not necessarily achieved by maximizing insulation thickness, but by selecting the configuration that provides the best balance among energy performance, economic efficiency, material suitability, and constructability. The study also demonstrates that insulation placement within the wall layers has an important effect on thermal behavior and that exterior insulation can improve the effective use of wall thermal mass in cold climates.

In conclusion, the research highlights the need to treat insulation design as an integrated optimization problem rather than a simple material selection process. The findings support more informed decisions in the design of exterior walls and provide a useful reference for future studies on energy-efficient building envelopes in cold regions. By combining thermal analysis with multi-criteria decision-making, the study offers a balanced framework for identifying suitable insulation configurations and contributes to the development of more rational, economical, and climate-responsive envelope design strategies.