Six different external wall systems for residential buildings in climate of Tehran (Iran) will be compared in this study from an environmental standpoint to demonstrate how the common wall systems work in our climate since data on their performance is not generally available and has not been compared before. LCA technique is useful for evaluating and quantifying all environmental inputs and outputs, from raw material extraction stage (cradle stage) to operational and finally end of life stages (grave stage) (Amiri Fard et al. 2021). It should be mentioned that the proposed analysis for the current research is known as “cradle to grave”.
System, Boundaries, and Functional Unit
LCA methodology distinguishes between two approaches to product analysis. The first one is known as the attributional approach that states that the study of product has no impact on the system. The second one is referred as the consequential approach in which the final product is affected by the study of system(Balasbaneh et al. 2018). As a result of the adoption of the alternatives studied, no changes in flows within the supply chains would be expected, thereby an attributional LCA approach was chosen in the current research.
The BS EN 15978 standard (BRE 2018) defines the stages of the life cycle of building. Life cycle stages A, B6, and C are included in this research. Operational energy is defined to be only the consumed energy at stage B6 for heating and cooling loads, whereas all embodied energy is considered at stages A1–A5, and C1–C4, as illustrated in Fig. 1. It should be mentioned that life cycle stages of B1–B5 were left out of the system boundary because related impacts are hard to measure and thereby are presumed to be the same across all building types. Even though including these life cycle stages would offer more reliable results, it does not directly serve the study's purpose and are therefore excluded from the system boundary and scope (Hong et al. 2020).
The functional unit in the current research is the measurement value for quantification of the derived results in a LCA, according to ISO 14040 (ISO14040 2006). This is extremely important since each system/product might have a variety of functions, and the products would be compared on a common foundation. The emissions, energy consumption, and materials used in this study are based on 1 m2 of exterior wall. Each wall sample studied is being used in new construction works in the region of Tehran (Iran). This selection of functional unit makes it easier to compare the six compositions that were tested, allowing for a more accurate assessment of the results. This criterion has been used for ensuring which functional unit produces credible findings in earlier studies (Llantoy et al. 2020). It was assumed that service life is 50 years for the mentioned building, as was assumed in the previously distinguished researches by Balasbaneh et al. (2022) and Balasbaneh et al. (2021).
The materials' LCI and related applied processes in this research were calculated using the Ecoinvent 3.7.1 database. Ecoinvent is regarded as comprehensive and transparent on a global scale, with reliable data on resource exploitation, energy, and material supply(Frischknecht and Rebitzer 2005). It has been frequently used in past LCA evaluations of buildings (Batouli et al. 2014). Because of the lack of Iranian-specific LCI data, its application to the main goal of this study would be regarded as trustworthy and consistent when evaluating data primarily related to the Asian region. The Iranian national regulations for buildings provided the materials' quantity which compound elements in the project required. Nonetheless, due to the fact that some material information was not included in mentioned charts, additional sources were required, including information provided directly by companies and professionals.
Impact Assessment Method and Categories
The ReCiPe 2016 was picked as an environmental impact methodology due to the fact that it is suitable for use in the Iranian market and contains worldwide normalization factors. Furthermore, its indications are expressed in kilograms of substance equivalent, making them technically and objectively expressible and enabling comparison with other studies. The default approach was used, which was the hierarchist ReCiPe version with average weighting set as well as global normalization. It should be mentioned midpoint (problem-oriented) and endpoint (damage-oriented) impact classes are included in ReCiPe 2016.This research demonstrates how to classify various midpoint impacts that lead to endpoint classifications (as illustrated in Fig. 2). It should be noted that the endpoint and midpoint characterization has a robust relationship to the areas of protection the elementary flows with lower modeling uncertainty, respectively. It should be noted that the endpoint approach produces easier-to-understand results, yet they are less transparent and subjective. As a result, the endpoint categories can be seen as presenting more easily comprehendible data on the environmental significance of the defined flows (Huijbregts et al. 2017). A single score indication is used to make it easier to comprehend the impact of procedural selections in the LCI modeling on the derived results. This approach includes both endpoint and midpoint categories, and also a set of weighting criteria that allow a single score impact to be calculated (Buyle et al. 2019).
To assist in the administration of information on environmental impacts, the software Simapro version 184.108.40.206 was employed, which enabled the LCA methodology to be used to assess, organize, and classify the resources used and their environmental effects. This software has previously been utilized in researches with favorable findings(Günkaya et al. 2021; Özer and Yay 2021). Simapro provides comprehensive and unrestricted access to the ecoinvent database for compiling a data inventory for projects and production systems.
Six different external wall systems which are commonly used in Iranian residential constructions were chosen for this study. The related details and properties are shown in Table 1 and Fig. 3. The external walls are made up of three layers. The exterior layer is the first layer that is normally exposed to the thermal environmental conditions. The next one is a structural layer and the last layer is an interior layer that is in direct exposure with the building habitational area. It was assumed that the exterior layer for all wall solutions is made of granite stone, which is a common choice between Iranian construction sector. Also, for the interior layer, a gypsum layer was considered.
Operational energy assessment
A conceptual design for a typical single-story residential building in Tehran, Iran was selected as the case study since it is one of the most frequent residential construction style in Tehran nowadays. The shape, typology, and area of the building are typical of a common residential building in Tehran. Thermal simulation in Autodesk Green Building Studio application in Autodesk Revit was used for determining the average cooling and heating loads for the house and various external wall alternatives. Green Building Studio is dynamic platform for building performance analysis which substantially simplifies the process of carrying out building performance analysis utilizing DOE2 as an established and verified simulation engine for delivering data on energy usage, water consumption, and carbon emissions (Autodesk.). Using basic thermal balance concepts, the software calculates usage of energy as a function of the building envelope. Only space heating and cooling are included in the operation energy calculation. The Green Building Studio software generates numerous building simulations based on the building's model design, requirements, and orientation, as well as the qualities of construction materials. The results of this phase aid in the development of a life cycle energy analysis for buildings in their operational period. The plan of the studied building model is shown in Fig. 4.
The characteristics of the building and the construction alternatives examined have a direct impact on operational energy usage. Natural gas and electricity are the most common heating and cooling sources in Iran. The impact of cooling and heating the building with a heat pump system (10 kW) was studied during the operational phase. Different assumptions, such as thermal characteristics, type of building, phase of project, operating schedule of building, location, information of outdoor air, building spaces and zone, and system of HVAC must be filled in accurately for achieving reliable results. Table 2 displays the energy building simulation settings used in Green Building Studio for evaluation of the building model.
It should be noted that thermal properties of different building's components are shown in Table 3.
The first phase involves selecting one of six detached house models with six different external wall types. Except for the type of wall type, which is necessary to differentiate models of buildings; the models are based 145 m2 area house project with the same heating system, location, and construction assemblies. The sole distinction between the studied models is related to the indirect or direct result of a wall type connected with the relevant model. For LCA and energy efficiency analysis, all relevant wall assembly factors are taken into account. Other considerations, including as cost of construction, seismic performance, and structural performance, are not included in this study. The models selected for the assessment have a 50-year lifespan and are built with U-values that correspond to the Iranian national regulations for buildings. As shown in Fig. 5, the proposed approach can be visually displayed.
Life Cycle Inventory (LCI)
After the samples were described, their functional equivalence were confirmed, and the LCA's goals and scope were set, the research's inventory was carried out.
Production of material, transportation of material to the site, and construction procedures on-site were all part of the construction phase. The amounts of required material for configuring each component of the exterior wall were computed per the square meter for evaluation of the product stage (A1–A3). Waste coefficients were assigned to each element investigated.
This study assumes that roadways are the main route of transportation for all phases of construction projects in Iran. The weight of materials (tons) and the plant distances to site (km) are taken into account while assessing A4. The distances were calculated by beginning at each material's factory and ending at a central position in Tehran, which was done using route optimizer and geo-referencing program. The closest factory to the construction site is selected for each type of material. Movement impacts is obtained by multiplying average distances and weight of material. For carrying the external wall components, a lorry with the EURO3 standard and a load capacity of 10 tons was selected.
Stage A5 entails the employment of machinery to move materials to each story of the building, as well as the creation of the binders used in the wall construction. Many researches deemed the energy spent on construction sites to be of minor importance (Nemry et al. 2010; Monteiro et al. 2020). However, some researches proposed to consider the effect of assembly phases in site in terms of consumptions of water and electricity equal to five percent of the EE of all the building materials (Asdrubali et al. 2013; Cornaro et al. 2020). This study considers the assembly phase effect on the construction site similar to mentioned studies that is equal to five percent of the total EE of each external wall system.
As mentioned before, the Autodesk Green Building Studio tool in Autodesk Revit software is used for predicting operating cooling and heating loads for calculation of stage B6. The thermal characteristics of materials were presumed to remain constant over time due to the fact that thermal properties decay overtime is out of the scope of this study.
It should be noted that in Iran, the end-of-life stage is divided by disposal and recycling. It relates to the fact that less than 30% of construction waste is recycled in Iran, with the rest ending up in landfills and vacant lots (Khoshand et al. 2020). As a result, for the end of life phase, it was expected that the steel components of walls would be recycled and other parts would be dumped in landfills. Construction waste is headed for a landfill with 37 km distances from project site or is processed for recycling with a 91 km displacement at the final phase.
The proposed method was used in six different types of exterior walls in the case study of this research. The proposed approach can be used to any other components or an entire building. Nevertheless, because of its not only great frequency in building projects(Crippa et al. 2018) but also high environmental impacts (Hong et al. 2020; Tushar et al. 2021), the evaluation of mentioned specific construction element was selected for calculations.