Over the past 40 years, the concept of the embodied energy has gained a lot of attention for measuring the total energy required for the production of economic or environmental goods and services [
9]. For buildings, it refers to the energy used for all direct and indirect processes associated with the production of materials, products, systems or other elements that go into the construction of a building, maintenance, and replacement or demolition at the end-of-life [
10]. The indicator that is commonly used for the quantification of embodied impacts is the non-renewable primary energy use, accounted in megajoules (MJ) [
11]. Using a similar indicator like the operational energy use intensity, one can calculate the embodied energy intensity (EEI) that accounts for all the energy used to manufacture building materials and produce building elements or systems per unit floor area (MJ/m
2). These key indicators can be used in the early stages of the decision-making process to make the best renovation decisions and select among alternatives by accounting for the impacts of new materials and systems that will be added or the ones that will be removed during the lifetime of buildings. Ultimately, they can facilitate major building renovation activities and be used to best navigate future activities towards a decarbonized European building stock by 2050.
A Life Cycle Inventory (LCI) is a process that quantifies the data from raw material extraction, transportation, manufacturing, and distribution. There are several methods used in the LCI phase of an LCA to assess lifecycle energy that are extensively reviewed in several publications [
13,
14]. The most popular ones include the process-based analysis that takes into account the various inputs and outputs for all the processes during the lifecycle of a building material or product; the input-output (I-O) analysis that estimates the materials, energy use, and emissions for a given economic sector based on national statistics; or the hybrid analysis that is a combination of both. Since practically all methods have some type of limitations in terms of completeness, reliability, and specificity there is still no consensus on a globally accepted method [
10]. Depending on the selected method, the calculated EE from a hybrid analysis may be from 3.8 up to 4.9 times higher than the values derived using a process-based analysis [
15].
In this work, the focus is on a process-based LCA based on [
16,
17]. The boundaries of the analysis capture the initial and recurrent embodied energy. The initial embodied energy accounts for the total energy used by the production stage and construction processes. The most significant amounts of energy are used for the raw material extraction (i.e., quarrying or mining) and manufacturing, assembling, or other fabrication processes for delivering building construction materials, equipment, or system components to the factory gate. This is usually referred as “cradle-to-gate”, which may represent up to 92% of the total lifecycle embodied energy [
18]. The other basic boundary condition may include the additional energy for transportation from the factory gate to the building site and the energy used for the construction and installation. The transportation of materials and products may vary significantly depending on whether they are locally available (preferred) or imported and the mode of transport, while the construction and installation generally do not exceed 2% of the lifecycle impacts [
18]. Accordingly, the initial EE that is also known as “cradle-to-site” embodied energy may account up to 99% of the total lifecycle embodied energy [
18] depending on the materials and their function and use as different building elements.
The recurrent EE represents the energy consumed to maintain, repair, restore, refurbish, or replace materials and components (or systems) during the life of the building. Depending on the system boundary and analysis approach, the relative importance of the recurrent energy for residential buildings may range from 45% up to 60% over a 50-year building lifetime [
10]. Sometimes, the recurrent EE may even exceed the initial EE depending on the lifetime of the building, service life of its elements and systems, and renovation practices [
10,
19]. The frequency of the various works and the amount of the recurrent EE depends on the lifetime of the building materials or systems, and on the need to replace some building elements or add new systems as part of energy conservation and efficiency measures during building renovations.
1.1.2. Embodied and Operational Energy Intensity
As the operational energy used in high performing buildings continues to drop, accounting for the embodied energy of building construction materials and systems is gaining more importance [
20]. However, published values for the embodied energy of residential buildings vary significantly as a result of different calculation approaches, system boundaries, and other uncertainties [
13,
19].
The embodied energy can range from 6% to 20% of the lifetime operational energy use in conventional buildings, from 11% to 33% in passive buildings, 26% to 57% in low energy buildings, and 74% to 100% in net zero energy buildings [
15]. In the Netherlands, depending on the renovation rates of existing buildings, it is expected that the operational energy use of the building stock will decrease by 19% to 46% by 2050, while at over the same period, the share of embodied energy will increase by 26% with a renovation rate of 1.4% per year or by 35% with a renovation rate of 1.9% per year [
14].
EU Member States are also encouraged to consider integrating the embodied energy of the materials and other sustainability benefits as part of the EPBD national cost-optimal calculations that must consider the buildings’ complete lifecycle [
21]. Currently, embodied energy is clearly considered in cost-optimal studies only in Lithuania, while six other member states account only some of its aspects [
22]. Apparently, this topic should gain more attention to assess the cost-optimal minimum energy performance requirements for major renovations of existing building elements and technical installations.
Embodied energy is also one of the key indicators in the reporting framework for the sustainable building design, construction, and operation currently under development by the European Commission [
12]. Recognizing the emerging importance of the embodied energy, the EU building stock observatory includes it as a monitored indicator for new construction, and deep and major renovations, under the technical building systems thematic area [
23].
The embodied energy intensities of residential buildings for different building lifecycles can range from 0.9 to 23.1 GJ/m
2 for the initial EE, and from 1.22 to 20.4 GJ/m
2 for the recurrent EE [
14]. These large variations may be attributed to inconsistent system boundaries, the use of different methods and databases, etc. Other earlier studies have reported that the embodied energy intensities for residential buildings range from 3.6 to 8.8 GJ/m
2 (averaging 5.5 GJ/m
2), while more recent works have disclosed values from 1.7 to 7.3 GJ/m
2 (averaging 4.0 GJ/m
2) for conventional constructions and 4.3 to 7.7 GJ/m
2 (averaging 6.2 GJ/m
2) for high performance buildings [
24], and drop to values from 105 to 243 MJ/m
2 for low energy European residential buildings [
25]. For European residential buildings, the initial EEI that have been reported in the literature for different constructions average 2.6 GJ/m
2 for timber, 4.0 GJ/m
2 for brick, 7.0 GJ/m
2 for concrete, and 13.8 GJ/m
2 for steel [
19]. Compared to high-rise buildings, SFH have a higher EE intensity that ranges from 1.0 to 1.5 GJ/m
2 [
26].
In Greece, there is no official database related to the environmental impacts of common building construction materials, equipment, systems, etc. As a result, relevant studies have commonly used data on the embodied energy coefficients from international databases, like the very popular, open, and free-to-use ICE database [
27]; and information readily available in bibliography.
Studies have mostly focused on residential buildings, using example buildings with publicly available data sources or other international LCI databases to estimate the embodied energy [
15,
28]. Considering four real residential buildings (i.e., three single- and one multi-family house) as examples with representative constructions and different vintages, the initial embodied energy intensities ranged from 2.18 GJ/m
2 to 10.2 GJ/m
2 as a result of using different public databases for the EE coefficients of major construction materials [
29]. For an apartment building, the calculated embodied energy intensity for renovating the thermal envelope of old buildings to the new building thermal regulation standards in different climate zones ranged from 2.42 to 2.82 GJ/m
2 using conventional materials and from 1.52 to 1.56 GJ/m
2 using more ecological materials [
28].
Similar efforts have also focused on quantifying the materials and benchmarking the embodied energy in major electromechanical (E/M) installations and equipment, by auditing two real residential buildings—one house and one apartment building [
30]. The main installations included a single-pipe hydronic central space heating with oil-fired boiler connected to room space radiators, a hydraulic and DHW installation with solar collectors, hot water storage tank, local split-unit heat pumps, and electrical installations (e.g., main control panels, cables, wall plugs). Using the ICE data [
27], the work estimated an intensity of 421.0 MJ/m
2 for the house and 170.8 MJ/m
2 for the apartment building [
30].
Progressively over the years, some have also been collecting local and national data to enhance the knowledge base on the energy aspects of LCI databases for key construction materials and major mechanical equipment. These efforts are of particular interest for commonly used materials or equipment that are usually not imported but rather mostly produced in the country and used in the construction and operation of Hellenic buildings. For example, materials like thermal insulation [
31], steel rebar [
32], aluminium [
33], bricks [
34], and E/M system components like boilers [
35], solar collectors [
36], etc.
Initial work that exploited an adapted Hellenic dataset for commonly used construction materials in Greece considered only four typical SFH to estimate their EE intensities that ranged from 3.2 GJ/m
2 to 7.1 GJ/m
2 [
37]. This is significant when compared with the annual primary EUIs that range for the most recent building construction under the stringent national energy code from 0.3 to 0.5 GJ/m
2 in the different climate zones of Greece, or relatively less important if one considers the older pre-1980 dwellings that have high EUIs that range from 1.9 to 3.9 GJ/m
2 [
38]. The present work extends previous efforts by considering 16 case studies providing a comprehensive representation of the Hellenic residential typologies for single-family houses, for characteristic building construction periods and locations in the four climate zones in Greece.
In comparison with the operational energy use, the final EUIs of residential buildings for normal climate conditions average 624 MJ/m
2 in EU-28 and 446 MJ/m
2 in Greece [
23]. New buildings in the nZEB era have a very low primary energy use that range from 72 MJ/m
2 to 720 MJ/m
2 according to the different national definitions [
39]. In Greece, the upper limit of the calculated primary energy use for space heating, cooling, ventilation, and DHW in nZEBs is 288 MJ/m
2 for new buildings and 342 MJ/m
2 for major renovations of existing buildings.
1.1.3. Building Materials
European residential buildings on an annual basis use about 12.1 million TJ for their operation [
1], compared with an estimated embodied energy in new building materials of 2.0–2.8 million TJ [
18]. Over half of the total embodied energy in building products is accounted for by steel (27.0%) and aluminium (23.9%), followed by concrete (16.5%), timber (11.6%), bricks (10.0%), glass (4.1%), copper (3.7%), aggregates (1.0%), insulation (0.9%), stone (0.6%), and clay (0.5%) [
18].
The most common building materials are cementitious products, cement, wood, steel, asphalt, and brick that have different environmental footprints [
40]. About 20–25% of cement is used in reinforced concrete (stone aggregates and cement as binder) and a comparable amount in mortar (sand particles and cement) that is prepared on site and used to fill the gaps between building blocks (e.g., bricks, concrete masonry units, stones) or plastering.
Among metals, the most energy intensive production processes per unit mass are for the production of aluminium [
33] and steel [
32]. Recycled metals have a significantly lower embodied energy, but even in this case, they have a higher EE compared to other materials [
18]. On a positive note, about half of the European steel production is made from recycled scrap.
The production of thermal insulation materials is also energy-intensive, resulting in a high EE per unit [
31,
41]. On the other hand, thermal insulation materials play a key role in reducing heat losses through the building envelope (e.g., walls, roofs, load bearing structure) and reaching high energy performance in new and renovated buildings. As a result, considering the significant energy savings resulting over the buildings’ lifetime, they outweigh the materials’ cradle-to-grave impacts by 3.8 to 270 times [
18]. In Greece, there are several manufacturing facilities for the production of various types of insulation materials, including polystyrene, stone wool, etc. Previous national studies have used values of 80.8 MJ/kg for expanded polystyrene, 87.1 MJ/kg for extruded polystyrene, 24.6 MJ/kg for mineral wool, and 92.1 MJ/kg for polyurethane foam [
42].
The cement industry ranks in third place as an industrial energy user and in the second place as an industrial carbon dioxide emitter in the world [
43]. Cement is the most energy intensive ingredient for the production of concrete and the component that has the highest contribution to its high embodied energy. The embodied energy coefficients for cement range up to 4.5 MJ/kg clinker, which is twice as much as the 2.171 MJ/kg for cement mortar (cement and sand) and about four times that of concrete (cement, gravel, and water) with 1.105 MJ/kg [
44].
Clay bricks and tiles are very common building construction materials manufactured from clay or adobe soil. Bricks are commonly used for external envelope walls and internal separation walls, while tiles are used for roof or floor coverings. The brick production industry requires large amounts of raw materials and energy use for the clay extraction, crushing, screening, water mixing, shaping with machine moulding or extrusion, drying, and finally a baking process through a long furnace that is the most energy intensive stage [
34]. For glazed bricks and tiles, an additional firing process is used to melt and then to adhere the glazed finish that usually contains glass on the finished surface. The embodied energy coefficient averages 3.6 MJ/kg for ordinary bricks, 4.6 MJ/kg for ceramic roof tile, and 15.6 MJ/kg for ceramic tiles [
44]. An audit of a brick production plant in Greece and a follow-up LCA provided an embodied energy coefficient of 2.1 MJ/kg for cradle-to-grave, with pet-coke being the main energy source (86%), followed by diesel-oil (11%) and electricity (3%) [
34].