1. Introduction
Spain is, among other countries, a signatory of the United Nations Commission on Sustainable Development (CSD). As stated by the organizing committee of the 2015 United Nations Climate Change Conference, COP 21 or CMP 11 that was held in Paris, from the outset of the talks, the expected key result was an agreement to set a goal of limiting global warming to less than two degrees Celsius (°C) compared to pre-industrial levels [
1]. The construction sector has a significant impact on the environment [
2]. According to some studies, around 20% of the total impacts are related to manufacturing, construction, demolition processes, and final disposal of building materials, elements and systems [
3], but the operational energy is, in many cases, considered the most significant aspect.
Energy in buildings can be categorized into two types: firstly by energy for the maintenance/servicing of a building during its useful life, namely operational energy (O.E.) and, secondly, by energy capital that goes into production of a building using various building materials, named embodied energy (E.E.). Study of both types of energy consumption is required for a complete understanding of building energy needs. Embodied energy of buildings can vary over wide limits depending upon the choice of building materials and building techniques. Reinforced concrete walls, fired clay brick masonry, concrete block masonry, bream and block slabs represent common conventional systems forming the main structure of buildings in Spain. Similar building systems can be found in many other developed and developing countries. Alternative building technologies, such as stabilized soil blocks, can be used for minimizing the embodied energy of buildings [
4,
5,
6,
7,
8,
9,
10]. Generally, the materials used for the structure of buildings represent more than 50% of the embodied energy in the building [
11]. In this sense, the use of alternative materials, such as mortar/concrete blocks, stabilized soil blocks, or fly-ashes, instead of materials with a high embodied energy, such as reinforced concrete, could save 20% of the cumulative energy over a 50-year life cycle [
12]. In addition, recycling building materials [
13,
14] is essential to reduce the embodied energy in the building. For instance, the use of recycled steel and aluminum saves more than 50% in embodied energy [
15].
There are many studies in the literature dealing with energy analysis in buildings [
16], some of them have been published thus far on the evaluation of environmental impacts using a life cycle assessment (LCA) tool on the operational energy in the life cycle of residential dwellings. These studies have appeared during the last 10 years [
17,
18]. Furthermore, in different countries some other LCA case studies for operational energy of residential dwellings were performed, as in the following examples:
Sartori and Hestnes [
19] performed a literature survey on buildings’ life cycle energy use of 60 cases from nine countries. This study concluded that operating energy represents by far the largest part of energy demand in a building during its life cycle.
Ortiz
et al. [
20] have applied LCA methodology to evaluate environmental impacts on a Spanish Mediterranean house located in Barcelona with a total area of 160 m
2 and a projected 50-year life span. The same authors studied [
21] the importance of operational energy in life cycle assessment (LCA). The comparison was applied within the residential building sector for two buildings, one in a developed country (Spain), and one in a developing country (Colombia).
Chung
et al. [
22] conducted an energy input–output (E-IO) analysis in Korea. The results showed that accounting for energy intensities and greenhouse gas (GHG) emission intensities is becoming an essential step in proper understanding of the energy usage structure.
Thormark [
23], in Sweden, analyzed, within its CEPHEUS project (cost efficient passive houses as European standard) in the European Thermie program, on how far the design phase of housing was relevant with regard to reducing operational energy and how the choice of building materials may affect both embodied energy and recycling potential during the 50-year life span of the building.
Stephan and Stephan [
24] quantized the life cycle energy and cost requirements associated with 22 different energy-reduction measures targeting embodied, operational, and user-transport requirements. It evaluates a case study apartment building in Sehaileh, Lebanon.
Koesling
et al. [
25] established a model valid for various types of building sector and applied it to estimate the amount of embodied energy in the building envelopes of 20 dairy farms in Norway.
Guan
et al. [
26] studied the energy used in all three phases of construction, operation, and demolition of eight residential buildings in and around Brisbane, Queensland, Australia. It was found that the main contribution to the operational energy in residential buildings comes from the use of general appliances in homes.
Brown
et al. [
27] analyzed how to mitigate climate change through operational energy reduction in existing Swedish residential buildings.
Zhu
et al. [
28] developed a new optimization method for building envelope design in order to get the lowest carbon emissions of building operational energy consumption using an orthogonal experimental design.
Pinky and Sivakumar [
29] presented a case study of life cycle energy analysis of a residential development consisting of 96 identical apartment-type homes located in Southern India. They considered that the life cycle energy of the building includes the construction energy, operational energy and demolition energy. Construction refers to initial construction as well as recurring maintenance and repair work.
Praseeda
et al. [
30] discussed the embodied energy and operational energy assessment of a few residential buildings in different climatic locations in India. The study shows that the balance of O.E. and E.E. in LCA greatly depends upon the types of materials used in construction and extent of space conditioning adopted. In some cases E.E. can exceed O.E. in the whole life cycle. Buildings with reinforced concrete frames and monolithic reinforced concrete walls have very high E.E.
Stephan
et al. [
31] presented a framework which takes into account energy requirements at the building scale,
i.e., the embodied and operational energy of the building and its refurbishment, and at the city scale,
i.e., the embodied energy of nearby infrastructures and the transport energy (direct and indirect) of its users. This framework has been implemented through the development of a software tool which allows the rapid analysis of the life cycle energy demand of buildings at different scales.
Islam
et al. [
32] described the life cycle assessment and life cycle cost analysis of a typical Australian house under the design phase in their paper. The implications of life cycle environmental impacts and life cycle costs were evaluated and the optimum assemblage design is reported using an optimization algorithm. A set of best solutions is found depending on different factors: the model assumptions, range of environmental and economic indicators considered, and the chosen quantitative criteria.
Iddon and Firth [
33] developed a building information model (BIM) tool to simultaneously estimate embodied and operational carbon over a 60 year life span for a typical four bedroom detached house. Using the tool, four different construction scenarios are evaluated, representing a range of current construction methods used in present day house buildings in the UK.
Finally, Ibn-Mohammed
et al. [
34] took a retrospective approach to critically review the relationship between embodied and operational emissions over the lifecycle of buildings in their paper. This is done to highlight and demonstrate the increasing proportion of embodied emissions, which is a consequence of the efforts to decrease operational emissions.
The current study takes an environmental perspective when comparing various conventional technologies for building walls to others that use new low-impact materials. By identifying and quantifying the materials used in the manufacturing and construction processes and the consequent operational energy, by applying LCA methodology, we identify the environmental impact of each alternative building material studied. Therefore, the aim of this research is to compare the environmental aspects and potential impact associated with the construction, maintenance, use, and disposal of walls in three-storey buildings, determining the option with the lowest negative impact in relation to insulation and material characteristics. A life cycle assessment was made of three models of housing blocks erected with load-bearing walls that varied according to their material structure. The options compared involved conventional and unconventional building materials.
There are several previous studies above mentioned comparing different structural system in terms of LCA, but tend to focus just in one of the aspects. This study implements three different kinds of parameters in a single case study, the structural comparison, the material comparison, and the environmental comparison. The last variable included is to compare the results in two real climate conditions and real scenarios.
The aim of this research is to determinate which material type produces the least environmental impact during the different stages of its life cycle. The purpose in this article is to demonstrate the relationship between materiality, architecture, and design, and the environmental impact produced by different construction systems. In order to establish this relationship, life cycle assessment is the tool used to analyze the environmental impact produced by the construction materials studied in its life cycle.
The main limitations of the study are related to the variables used. Only two alternatives to the stabilized earth block have been selected, being the most widely used materials in this kind of load-bearing wall. A specific building type has been chosen as a case study, whereas this decision comes from the abundance of its use in Europe in the twentieth century. Two climate scenarios have been selected, the most different ones among the Spanish climate zones. Just one type of insulating material has been introduced, being the most used in this type of construction solutions. Previous studies, comparing three different insulation materials, provided no significant differences in results, increasing the complexity of the data presentation and understanding.
3. Life-Cycle Assessment Goal and Scope
The particular focus of the application of the life-cycle assessment (LCA) in this study is to obtain the values of the embodied energy and global warming potential impacts (GWP) categories associated with the construction of three types of bearing walls: fired clay brick masonry walls (BW), concrete block masonry walls (CW), and stabilized soil block masonry walls (SW). A three storey construction is evaluated. To establish a suitable comparison framework, after calculating the necessary thickness of the walls, thermal conductivities have been unified in order to obtain equivalent operational energy parameters for all three materials.
According to the proposed framework, this study should answer the following question: what are the impacts produced by the processes related to the construction for each one of the combinations proposed?
According to the objective of this study the functional unit established is the total surface of walls in each case.
The assessed system is composed of every process that takes part in the production, construction, maintenance, deconstruction, and final disposal of every component of the building structure. It has excluded every process related to the operational phase of the dwelling. The system includes the following processes:
Manufacturing of the building products phase. For each building material involved in the building every good and service from cradle to grave are considered. The manufacturing of employed machinery and territorial infrastructure processes has been considered.
Assembly and construction phase. This covers every process aimed at integrating all products and services in the site in each studied dwelling. The transportation of building materials from the factory to the site, the placement of building products has been considered
Maintenance and repair phase. This includes all repair operations and maintenance of building components. The renewal of those materials which have a lower durability has been considered.
Dismantling and demolition phase. Every process carried out at the end of the life of the building to remove and demolish the dwelling has been taken into consideration: demolition, removal of building elements, and transportation of demolition materials to recycling or disposal have been included.
Disposal and recycling phase. This covers all processes suffered by demolition materials after dismantling i.e., the deconstruction of building materials.
The environmental data of wool and algae have been extracted from the recent studies conducted by Barber and Pellow [
40] and Resurreccion
et al. [
41], respectively. The environmental data of the rest of the building materials were obtained from the well-recognized ECOINVENT database [
42]. The calculation procedure to obtain the life cycle inventory was the described by García-Martínez [
43]:
- (1)
Identification and quantification of the initial building products and auxiliary materials—including replacement materials that take part in the life cycle.
- (2)
Identification and quantification of the basic processes associated with the construction and deconstruction. The determination of the energy consumed during the construction and demolition is obtained as a factor of the total building material volume, following the procedure as described by Kellenberger
et al. [
44]. The following procedure has been taken:
- (a)
Basic materials have been grouped into unit processes (
Table 3). Construction systems, structural elements, walls and roofs, windows, doors, and finishing materials (from floors, ceiling, and walls) has been considered. The dimensioning of these elements is according to the obtained structural and thermal values (
Table 1).
- (b)
Division into groups and listing product specifications. Each case study has been divided into building elements according to the Building Cost Data Base of Andalusia (BCCA). The materials, the building machinery, and the labor has been related to each building element.
- (c)
The building elements have been quantified using the construction management software Presto V.8 by RIB Software AG, Stuttgart (Germany).
- (d)
The basic materials used in each case study has been obtained from the results given by Presto.
- (e)
Once quantify each basic material and considering its physics properties, its mass and volume has been obtained.
- (3)
Determination of input and output of each unit process. The ECOINVENT database and published LCA studies have been used to obtain environmental information of unit processes (see
Table 3). Final disposal processes for the plastics, metals, bitumen-based and wood materials have been considered. Other materials have been considered inert from the point of view of their final disposal. The quantification of the final disposal processes has been obtained from the quantities the initial basic products (
Table 4).
- (4)
Inventory and assessment. The impact assessment is carried out using the CML 2001 method in relation to the GWP impact category. The “cumulative energy demand” in relation to the embodied primary energy (
Table 3).
- (5)
Operational energy data are considered according to the benchmarks included in the Spanish ministry report SPAHOUSE [
45] for houses located in Spain, considering two of the climates mentioned, location 1, corresponding to the warm Mediterranean climate and location 2, corresponding to the inner continental areas of the peninsula. Including insulation, all building envelope U values are considered equal for all three materials and subsequently, the operational energy will be considered to be the same for all three cases. Insulation is placed inside the building envelope so as to minimize the differences of thermal inertia among them.
5. Discussion
Regarding cumulative energy demand and global warming potential (GWP), for the different LCA phases, a proportion can be seen between the consumption of the three different materials employed. Manufacturing phase PH1 is the most relevant phase for embodied energy for both CO2-eq and MJ. The contribution of the manufacturing phase to these results is significant, representing mean percentages in relation to the total stages. In this phase there are significant differences that make the brick wall a more unfavorable material from energy consumption and emissions associated point of view, being more than 65% higher than the stabilized soil wall. The values of both soil and concrete block walls are quite similar. Nevertheless, in all cases the embodied energy is lower for the SW than for the other two materials except for the cumulative energy demand of location 2 due to greater energy employed in soil transportation.
Taking into account the different locations, there are no big differences between the values of embodied energy for both climates. The differences between materials are higher in global warming potential than in the cumulative energy demand.
If different phases are compared, both CO
2 emissions and cumulative energy demand, the greatest differences among both climates takes place in PH2. Analyzing
Table 5, consumption data per m
2 and year, they show remarkable differences between SW and BW values of total embodied energy. CO
2 emissions for BW rise up to 1.6 times higher in cold climates. For warm climates, BW exceeds 1.5 times the values of SW.
The operational energy emissions of the building are higher than the ones associated with the embodied energy for all three materials. Accordingly the operational energy emissions of the building represent more than 200% of the embodied energy for the SW case. However for BW operational energy represents only 130% of embodied energy.
Accordingly with these results it could be said that the energy consumed to build houses with brick walls in warm climates represents 165% of the energy invested to erect the same building with stabilized earth walls.