Resource Efficiency in the Construction Sector: Material Intensities of Residential Buildings—A German Case Study

Abstract

This article describes an approach for comparing material intensity values for residential buildings with different construction types. Based on the working drawings of the different construction types (wood and mineral), material intensities are calculated at the building level. Material intensities describe the materials used in a building in mass (tonnes (t)) in relation to the square meters (m²) of gross floor area (GFA) or the cubic meters (m³) of gross volume (GV). The method for determining material intensities at the building level is demonstrated. The results show that material intensities range from 0.61 t/m² GFA to 1.95 t/m² GFA for single-family residential buildings and from 1.36 t/m² GFA to 1.54 t/m² GFA for multi-storey residential buildings. The average material intensity for mineral buildings is twice as high as that for wood buildings, which means that there is a beneficial resource efficiency in building with wood instead of mineral materials. Therefore, benchmarks for a resource efficient building can be conducted based on these values. These values demonstrate a possibility to influence resource efficiency in buildings.

1. Introduction

In view of global warming and the European Union’s ambitious climate targets, the construction industry is facing radical changes. Increasingly scarce raw materials and rising greenhouse gas emissions mean that a rapid rethink is required in order to be able to achieve the climate protection targets that have been set.

Since the Paris Agreement [1], the building sector has played a central role in achieving climate protection goals. The sustainable development goals (SDGs) or various initiatives at the European level such as “green deal” [2] and “renovation wave” [3] for existing buildings, as well as the EU taxonomy [4], consciously focus on the building sector.

This is because the building sector plays an important role in achieving the climate protection targets in Germany due to its large consumption of resources and energy. The declared goal of reducing greenhouse gas emissions to such an extent by 2045 that a “greenhouse gas neutral building stock” is achieved in the building sector has been brought centrally into awareness by the ruling of the Federal Constitutional Court on the Climate Protection Act [5]. On 24 March 2021, the Federal Constitutional Court ruled that the German government’s climate protection law was inadequate and called for tightening. The requirements and implementation currently pose major challenges for players in the construction sector, as the steps taken to date are far from sufficient.

With a total of 40% of CO₂ emissions, the construction sector plays a central role in reducing greenhouse gases across all sectors in Germany [6]. To achieve this, it is necessary to focus not only on very high energy standards and energy generation in buildings, but also on the entire life cycle of buildings. In order to be able to achieve the targets in the climate protection legislation by 2045, the construction sector is one of the most important components, as it is responsible for more than one third of direct and indirect greenhouse gas emissions. The requirements for climate protection, resource conservation, energy efficiency and circular economy must be increasingly implemented here against the backdrop of the social transformation processes. After greenhouse gas reductions in the use-phase of buildings for new construction have already been significantly reduced in recent years, further opportunities for improvement can be found above all in the choice of construction. However, the importance of existing buildings, their energy improvements and modernizations, as well as redensification, should not be underestimated.

Adjustments and changed planning specifications must therefore be fed into the planning processes with immediate effect.

Resources can be classified into four categories. These are: the area, the energy, the material and the blue–green infrastructure [7]. The area on which a new building is constructed is thus sealed and the soil cannot contribute to other functions—the contribution of buildings to land sealing is currently 54 ha per day [8]. The resource energy is very relevant for the building in different respects. On the one hand, the building should consume as little energy as possible in its use phase and at the same time generate as much energy as possible from renewable sources at the building. However, energy is also used in the production process for materials. The materials from which a building is made should be chosen to use as little energy as possible in their production and, if possible, to make the building a carbon storage. The choice of construction methods and materials is one of the major adjusting factors on which design has a decisive influence. If other factors such as the choice of location, the space program or the density have often already been decided, there is room to manoeuvre when it comes to deciding on the construction method.

Climate protection and resource consumption are important parameters for the construction industry, which need to be considered more closely together. Economical and efficient use of raw materials as well as more recycling are needed to cover the increased demand for raw materials for energy transition in a sustainable way. If, for example, metal and mineral raw materials are recycled to a greater extent and are used more efficiently, this will also help to protect the climate, as fewer greenhouse gases will be produced [9].

In 2012, the German government adopted the Resource Efficiency Program (ProgRess). The German government has undertaken to report every four years on the development of resource efficiency in Germany and to update the resource efficiency program. This was done for the first time on 2 March 2016 with ProgRess II. The German Resource Efficiency Program III was adopted on 17 June 2020. The Resource Efficiency Program defined goals, guiding ideas and courses of action for the protection of natural resources.

The strategy for increasing resource efficiency, which is anchored in this program, is part of the German Sustainability Strategy (DNS) [10]. In this context, sustainable development is anchored as a central objective of government and administrative action [11]. The goal is an average annual increase in total raw material productivity of 1.5 percent by 2030, taking into account the decoupling of natural resource use from growth and prosperity.

To determine raw material productivity, a quotient is formed. The gross domestic product (GDP) is related to the abiotic materials used in Germany. The abiotic materials include domestic raw material extractions and imported materials. Raw material productivity allows an initial trend statement on the efficiency of raw material use in our economy over a long time series.

The construction industry plays a key role in this context. The use of resources in the construction sector is very high, and thus offers great potential for optimizing resource efficiency. In Germany alone, 517 million tons of mineral raw materials are used annually. On the other hand, the construction industry generates 209 million tons of construction and demolition waste annually. In Germany, this corresponds to 52 percent of the total volume of waste [12].

Accordingly, the existing stock of buildings is more and more seen as a resource. The research series “Mapping of the Anthropogenic Stock” (KartAL) aims to understand materials as the capital stock of the future and to analyse and record it systematically [13]. The first project, “Mapping the Anthropogenic Stockpile in Germany to Optimize Secondary Raw Materials Management,” analysed the size and composition of the material stockpile and its dynamics of change. Durable goods were taken into account: structures of technical infrastructure, structures of building construction (residential and non-residential), building services, capital goods and durable consumer goods. For Germany as a whole, based on the year 2010, a material stock of approximately 28 billion tonnes has been determined. This figure is dominated by mineral materials with around 26 billion tonnes. These are mainly attributable to the mass building materials concrete and masonry and can therefore be assigned to the construction industry. The remaining 2 billion tons are divided among metals (2 billion tons), metals (1.2 billion tons), plastics (250 million tons), wood (350 million tons) and other (200 million tons) [13].

1.1. Literature

Resource efficiency means the efficient use of technical/economic and natural resources. According to German standard VDI (German Engineers Association) 4800 Sheet 1, it is defined as “the ratio of a certain benefit or result to the resource input required for it”. In this study, the ratio of a specific residential building (benefit) to the necessary input of materials (resource input) is analysed. The indicator of material intensity serves as a reference value. Material intensity is understood as the materials used in a building (mass in tonnes (t)) in relation to the square meters (m²) of gross floor area (GFA) or the cubic meters (m³) of gross volume (GV).

On an international level, there are several studies that use the bottom-up approach to determine materials in existing buildings. For the city of Vienna, a method for characterizing the material composition of buildings was presented in [14,15,16]. For this purpose, buildings were analysed before demolition, through site inspections as well as selective sampling on site. Hashimoto [17] quantified building stock in Japan by using statistical data to relate building stock in physical quantities per unit area to material intensity rates for several building types. Similar bottom-up approaches have also been conducted in Singapore [18], Norway [19], Sweden [20], Australia [21], Luxembourg [22], London [23], Rio de Janeiro [24] and Switzerland [25] with respect to residential buildings. All sources conclude that the quality of the identification of materials in existing buildings strongly depends on the origin and the available information content of the documents.

In German literature, material intensity is used as the basis for accounting the anthropogenic stock. The specific projections of residential buildings in [13] are based on statistical data (residential statistics [26] and construction activity statistics [27]), which are combined with building specific material intensities. The building specific material intensities are determined on the basis of two methods. Both methods have in common that specific material values are determined for buildings in order to perform extrapolations based on them. The first method [28] is based on the calculations of [29,30]. The building types describe synthetic buildings, which reflect the average characteristics of partial stocks of the building stock in Germany. A distinction is made between single-family houses, two-family houses and multi-family houses.

The second method, the calculations according to [31] and [32], uses synthetic age/class related type representatives. These type representatives represent a building type and construction era as well as dominant construction methods, construction design, construction elements and building materials. Material intensity was found to vary by reference size and building type. The material intensity for multi-family houses shows an average value of 2.2 t/m² HNF (main usable floor area), whereas single-family houses show a 27% higher material intensity with an average of 2.8 t/m² HNF [31]. Based on the determined building material intensities, calculations were carried out for urban areas of Freiburg, Zittau and Leipzig. The main focus was on characterizing and analysing urban structure types of residential developments. These “[…] analyses on the level of urban structure types of residential development have shown that material/energy parameters of building types are suitable to represent and describe housing stocks in their existing condition” [31]. The methodological approach to the building typology follows [31], in which a system for the use category “residential buildings” was developed. In this study, the material intensities can be compared to those of [31]. As the only source, [31] shows material intensities not only for existing buildings, but also for new buildings. A comparison of the material categories of the different construction types is not realizable in detail. The definition of the material categories is different. The analysis of material intensity in new buildings as an aspect of resource efficiency has not yet received much attention. The main focus in new buildings up to now has been on energy efficiency and therefore concentrates on usage period of buildings. With increasing development of low or nearly zero energy buildings, additional reduction potential is mainly found in the reduction of other life cycle stages. Therefore, the focus has shifted to emissions related to material input and the material used to construct buildings in general. When it comes to resource efficiency, energy efficiency must always be taken into account. In Germany, the new Building Energy Act (GEG) came into force on 1 November 2020. The minimum requirements for new buildings do not show any significant tightening compared to the previous regulation. However, with the introduction of the GEG, ultra-low energy buildings were set as the standard for new buildings. Low-energy buildings have a high level of energy efficiency. This offers the opportunity to integrate material intensity into the resource efficiency discussion.

In the context of climate discussions, which mainly revolve around the balance of CO₂ emissions, the debate on resource efficiency is also becoming increasingly important. In order to be able to lead a goal oriented discussion in this context, it is crucial to obtain knowledge of material intensities. This is necessary to influence construction as a next step. In order to develop future strategies for resource efficient construction, statements about the current state of material intensity of new buildings are needed as a basis for decision-making.

One practical example that focuses on the resource efficiency of new buildings is the project “Factor X” in the Rhenish mining area. The goal is to increase the resource efficiency of a building by a factor of X. Therefore, resources, energy and greenhouse gas emissions of a building are taken into account. This includes the consideration of resources, energy and greenhouse gases over the entire life cycle of the building. Only buildings that meet the specified factor X may be erected on the project site [33].

1.2. Aim of the Study

The aim of this study is to determine material intensities for residential buildings of different building types. The residential buildings are new buildings in Germany.

2. Method

In this study, material flow analysis (MFA) as input data for building life cycle assessments is used to determine the quantity and intensity of materials in residential buildings. The mass balance method is used for this purpose. The balancing of materials was carried out based on working drawings of constructed buildings. A total of 46 new buildings were analysed in this study. The buildings were divided according to building type (single-family house and row houses (EFH), multi-storey building (MRB)) and according to construction type (mineral construction (M), timber construction (W)). The specific material intensities are calculated as area-related values in t/m² GFA and volume-related values in t/m³ GV. The buildings are described in detail in [34,35]. There, the size, energy standard, and material choice are explained. By combining the specific material intensities with the construction types, the resource efficiency of the building types is derived (cf. Figure 1).

2.1. Determining Materials via Building Life Cycle Assessments

There is no standardized method for documenting building specific information in Germany. In order to obtain the required information on the buildings, mass balances of the building materials are carried out based on working drawing. Working drawing is the second most important stage in the planning of a building project. The working drawing follows the approval planning. The construction plan contains exact dimensions and sectional views of the planned building as well as illustrations of the individual construction parts.

The working drawings are drawn to a scale of 1:50, with details sometimes shown in even larger scale. Thus, the working drawing forms the information basis on which the planned building is created. These also provide the additional required information on the size, use and year of construction of the buildings. Working drawings are suitable for the calculation of volumes of building materials over the areas and thicknesses of the building parts. The main materials, such as concrete, masonry or wood, can be quantified using the volumes and densities of the materials. The mass balance serves as an input parameter for building life cycle assessments.

2.2. Case Study

The list of buildings analysed in this study originates from the life cycle assessment comparison within the research project “Greenhouse Gas Balancing of Wooden Buildings—Implementation of New Life Cycle Assessment Requirements and Determination of Empirical Substitution Factors (GHG Wood Construction)” [35]. The buildings show an average of established constructions in the last 20 years and all fulfil the building regulations at the time. Additionally the buildings are located all over Germany, and therefore include an average mix of residential buildings.

The analysed buildings are real constructed buildings. Buildings of the category SFH as well as MRB were analysed. Additionally, building pendants were generated. The building pendants correspond in their functional and technical unity to those of the original buildings. For example, if a building was built as a wooden building, in the original, the same building was generated as a counterpart as a mineral building in different variants. This ensures the functional equivalence of the product system [35].

A total of 46 buildings was assessed. The buildings in the SFH category are shown in

Table 1. Building SFH, modified after [35].

Abbreviations	Year of Construction	GFA	GV	Number of Floors	Building Design
					Exterior Wall	Interior Wall	Ceiling	Roof	Foundation
1.1_H	2012	215	577	SFH (GF + 1)	Solid timber with cellulose insulation	Cross-laminated timber	Wooden beam ceiling	Cross-laminated timber with cellulose insulation	Reinforced concrete
1.1_M1					RCC and mineral wool insulation, facing bricks	Reinforced concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.1_M2					RCC and ETICS	Reinforced concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.1_M3					Timber, single-shell	Reinforced concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.1_M4					Aerated concrete	Reinforced concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.1_M5					Sand-lime brick and ETICS	Reinforced concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.2_H1	2009	176	557	SFH (GF + 1)	HTB with mineral wool insulation	Wooden stud wall	Wooden beam ceiling	Wooden beam with mineral wool insulation	Reinforced concrete
1.2_H2					Solid timber with mineral wool insulation	Wooden stud wall	Board stack	Solid timber
1.2_M1					Timber and ETICS	Sand-lime brick	Reinforced concrete	Wooden beam with mineral wool insulation
1.2_M2					Timber, single-shell	Vertically perforated brick	Reinforced concrete	Wooden beam with mineral wool insulation
1.3_H	2012	379	1069	Duplex (GF + 1)	Solid timber with cellulose insulation	Cross-laminated timber	Cross laminated timber	Cross-laminated timber with cellulose insulation	Reinforced concrete
1.3_M1					Timber and insulating plaster	Vertically perforated brick	Reinforced concrete	Wooden beam with mineral wool insulation
1.3_M2					Sand-lime brick and ETICS	Sand-lime brick	Reinforced concrete	Wooden beam with mineral wool insulation
1.3_M3					Aerated concrete	Aerated concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.5_M	2011	245	685	SFH (GF + Attic)	Timber and insulating plaster	Vertically perforated brick	Hollowcore ceiling	Wooden beam with mineral wool insulation	Reinforced concrete
1.6_H	2011	190	483	SFH (GF + 2)	HTB with mineral wool insulation	Metal stud wall	Wooden beam ceiling	Wooden beam with mineral wool insulation	Reinforced concrete
1.6_M1					Sand-lime brick and ETICS	Sand-lime brick	Reinforced concrete	Wooden beam with mineral wool insulation
1.6_M2					Timber and insulating plaster	Vertically perforated brick	Hollowcore ceiling	Wooden beam with mineral wool insulation
1.7_H	2011	127	384	SFH (GF + 1)	HTB with mineral wool insulation	Metal stud wall	Wooden beam ceiling	Wooden beam with mineral wool insulation	Reinforced concrete
1.7_M					Timber and insulating plaster	Vertically perforated brick	Vertical perforated brick	Brick solid roof
1.8_H	2011	224	564	Duplex (GF + 2)	HTB with mineral wool insulation	Metal stud wall	Wooden beam ceiling	Purlin roof	Reinforced concrete
1.8_M					Timber, single-shell	Vertically perforated brick	Hollowcore ceiling	Purlin roof
1.9_H	2009	209	536	SFH (GF + 1)	HTB with mineral wool insulation	Wooden stud wall	Wooden beam ceiling	Wooden beam with mineral wool insulation	Reinforced concrete
1.9_M1					Sand-lime brick and ETICS	Sand-lime brick	Reinforced concrete	Wooden beam with mineral wool insulation
1.9_M2					Aerated concrete	Aerated concrete	Reinforced concrete	Wooden beam with mineral wool insulation
1.9_M3					Timber and insulating plaster	Vertically perforated brick	Reinforced concrete	Wooden beam with mineral wool insulation

RRC = Reinforced concrete, ETICS = thermal insulation composite system, HTB = Wood panel construction.

. The main materials divided by parts of the building are given. The accounted SFH and partly semi-detached houses have 2–3 floors. The basements are not considered. The living areas are between 97 m² and 248 m². All buildings were planned or built between 2009 and 2012. They are market-representative wooden buildings and mineral buildings.

All buildings studied here (SFH, MRB) comply with the current state of the art in terms of energy properties and fire and sound insulation requirements. The timber buildings correspond to the current timber construction (timber panel construction, solid timber construction, prefabricated timber construction), the mineral buildings are standard buildings as they are currently built [35].

The buildings in the MRB category are shown in

Table 2. Building MRB, modified after [35].

Abbreviations	Year of Construction	GFA	GV	Number of Floors	Building Design
					Exterior Wall	Interior Wall	Ceiling	Roof	Foundation
2.2_H	2013	1.394	3.965	MFH (GF + 5)	HTB and ETICS	Wood and metal stud wall, reinforced concrete	Wood-concrete composite	Cross-laminated timber	Reinforced concrete
2.2_M					RCC and ETICS	Reinforced concrete, metal stud wall	Reinforced concrete	Reinforced concrete
2.3_M	2007	7.016	21.943	Residential complex (GF + 3)	Aerated concrete resp. RCC and ETICS	Reinforced concrete, sand-lime brick	Reinforced concrete	Reinforced concrete	Reinforced concrete
2.4_M	2010	1.478	4.277	MFH (GF + 3)	Sand-lime brick and ETICS	Reinforced concrete, Metal stud wall	Reinforced concrete	Reinforced concrete	Reinforced concrete
2.5_H	2006	6.152	19.072	MFH (GF + 2)	HTB with cellulose and mineral wool insulation	Wood and metal stud wall, reinforced concrete	Cross-laminated timber	Cross-laminated timber	Reinforced concrete
2.5_M					Timber and ETICS	Wood and metal stud wall, sand-lime brick	Reinforced concrete	Reinforced concrete
2.6_H	2013	2.717	8.646	MFH (GF + 3)	HTB with mineral wool insulation	HTB, metal stud wall	Structural solid wood	Wooden beam ceiling	Reinforced concrete
2.6_M					Aerated concrete resp. RCC and mineral wool insulation	Reinforced concrete, metal stud wall	Reinforced concrete	Wooden beam ceiling
2.7_H	2011	2.033	6.172	MFH (GF + 7)	Solid timber with mineral wool insulation	Solid wood, Wooden stud wall	Cross-laminated timber	Cross-laminated timber	Reinforced concrete
2.7_M1					Aerated concrete	Reinforced concrete., metal stud wall	Reinforced concrete	Reinforced concrete
2.7_M2					Timber and insulating plaster	Reinforced concrete, metal stud wall	Reinforced concrete	Reinforced concrete
2.7_M3					Timber and mineral wool insulation, facing bricks in facade cladding	Reinforced concrete, metal stud wall	Reinforced concrete	Reinforced concrete
2.9_H	2011	1.257	3.876	MFH (GF + 3)	HTB with cellulose and mineral wool insulation	Solid wood, wooden stud wall	Cross-laminated timber	Cross-laminated timber	Reinforced concrete
2.9_M					Sand-lime brick and ETICS	Reinforced concrete, Sand-lime brick	Reinforced concrete	Reinforced concrete
2.10_H	2010	723	2.404	MFH (GF + 3)	Solid timber with mineral wool insulation	Wooden stud wall	Solid timber	Wooden beam ceiling	Reinforced concrete
2.10_M1					Aerated concrete	Reinforced concrete	Reinforced concrete	Reinforced concrete
2.10_M2					Timber and insulating plaster	reinforced concrete	reinforced concrete	Reinforced concrete
2.10_M3					Timber and mineral wool insulation, facing bricks in facade cladding	Reinforced concrete	Reinforced concrete	Reinforced concrete
2.12_M	2014	1765	5363	MFH (GF + 4)	Sand-lime brick and mineral wool insulation, facing bricks in facade cladding	Reinforced concrete, metal stud wall	Reinforced concrete	Reinforced concrete, wooden beam	Reinforced concrete
2.13_M	2010	1348	3933	MFH (GF + 5)	RCC and ETICS	Reinforced concrete, metal stud wall	Reinforced concrete	Reinforced concrete	Reinforced concrete

RRC = Reinforced concrete, ETICS = thermal insulation composite system, HTB = Wood panel construction.

. The main materials divided by parts of the building are given. The designated MRB have between 3–8 floors without basement. The living space is between 488 m² and 4256 m². The buildings were constructed between 2006 and 2014 [35].

A mass balance was calculated for each building, divided into the following different material fractions.

• Mineral building material;
• Renewable building materials;
• Metal;
• Plastics, fossil;
• Waterproofing, protective coatings, adhesives, roofing, sealants;
• Floor coverings, screeds;
• Insulation materials;
• Plaster, finishing panels, facade, ceiling cladding;
• Coatings;
• Technical finishing;
• Translucent components.

Each Material fraction was calculated in volume and then converted with the specific density of each material to achieve an overall MFA. The specific density values were taken from the open access German database “ÖKOBAU.DAT” (BMWSB) [36], which includes datasets for all common materials used in buildings. As the “ÖKOBAU.DAT” is a commonly used standard to calculate lifecycle assessment, the publication of densities for the specific materials is mandatory.

3. Results

In order to be able to compare the analysed buildings, the material intensity was calculated as a specific parameter for each building. The GFA and the GV were selected as reference values for the study, since these values describe the total space occupied by the buildings and need further improvement in terms of resource efficiency.

3.1. Results Regarding Area Related Material Intensity

The results regarding area-based material intensity are presented in the section below. The buildings are shown separately by construction type. The columns are presented in mass per m² GFA, ordered by the main material used. For each material category, an average is formed. Additionally, a total average of the wood and mineral constructions is shown. The results regarding area-related material intensity of SFH show values in the range of 0.61 t/m² GFA to 1.95 t/m² GFA, see Figure 2. The columns show the mass per area, sorted by the main material used. For each material category, an average is calculated. In addition, the total average of wood and mineral is shown. Buildings made of aerated concrete show the highest values in this case with an average of 1.48 t/m² GFA. At 0.70 t/m² GFA, the average material intensities of buildings made of solid wood are the lowest.

The average material intensity for mineral buildings at 1.39 t/m² GFA is nearly twice as high as that of buildings with a wooden construction. Here, the average material intensity is 0.71 t/m² GFA. Overall, the material intensity of mineral buildings shows a significantly wider range from 0.71 t/m² GFA to 1.95 t/m² GFA. Buildings with a wooden construction show a significantly lower range of material intensities from 0.61 t/m² GFA to 0.75 t/m² GFA. Figure 2 shows the results of the SFH for the area related material intensity [t/m² GFA].

Basically, the buildings with a solid wood construction (Cross laminated timber) show the lowest area-related material intensities on average, followed by wood panel constructions, sand-lime brick constructions, reinforced concrete, vertically perforated bricks, as well as aerated concrete constructions with the highest values on average.

Figure 3 shows area related material intensities for MRB. The calculations of the area related material intensity of MRB buildings result in values in the range of 0.66 t/m² GFA to 1.54 t/m² GFA. The buildings made of sand-lime bricks examined here show the highest values in this case. With 0.66 t/m² GFA, solid wood buildings also show the lowest values for the MRB.

Likewise, the results of the MRB confirm that the average material intensity of mineral buildings (1.44 t/m² GFA) is nearly twice as high compared to buildings with a wooden construction (0.75 t/m² GFA). Overall, the material intensity of mineral buildings shows a range from 1.36 t/m² GFA to 1.54 t/m² GFA. Buildings with a wooden construction material intensities of 0.66 t/m² GFA up to 0.79 t/m² GFA feature a significantly lower value range, even for MRB.

The comparison of chiefly used materials for load bearing structures shows the lowest area related material intensities for solid wood, followed by wood panel construction, vertically perforated bricks, reinforced concrete, aerated concrete and the highest values for sand-lime bricks.

The order of wood construction (mass timber and then wood panel construction) is identical for the SFH and MRB. For mineral buildings, the order changes. Differences can be seen in mineral buildings. The buildings with a sand-lime brick construction show the lowest material intensities on average in mineral SFH, whereas buildings made of sand-lime brick show the highest values for MRB. There are also changes in the order for vertically perforated brick constructions. These buildings show the second highest values in the SFH, while in MRB they show the lowest values on average for mineral building.

There is a clear difference in the weight of the buildings examined here. With an average of 310 t, mineral constructions are 2.1 times heavier than the wooden constructions at 150 t. A comparable result can be seen for MRB. With an average of 3400 t, buildings with mineral construction are 1.8 times heavier than the buildings with wood construction (1900 t).

Basically, the differences in material intensities between SFH and MRB as well as the material types can be attributed to the different material quantities and their individual weights.

3.2. Results Regarding Volume-Based Material Intensity

When considering volume related material intensity (see Figure 4 and Figure 5), other value ranges emerge. Thus, the volume-related material intensity for SFH shows a value range from 0.24 t/m³ GV to 0.54 t/m³ GV and for MRB from 0.21 t/m³ GV to 0.51 t/m³ GV. The columns are shown in mass per m³ GV, ordered by the main material used. For each material category, an average is formed. Additionally, a total average of the wood and mineral constructions is shown.

Volume related material intensity is lower for SFH with wooden construction on average with 0.25 t/m³ GV compared to than mineral buildings with 0.50 t/m³ GV. For MRB, mineral buildings can be described with an average volume related material intensity of 0.47 t/m³ GV and wood buildings with 0.24 t/m³ GV.

In summary, buildings with a solid wood construction show the lowest volume related material intensities on average for both the SFH and the MRB. This is followed by buildings with a wooden panel construction for SFH and MRB.

The volume related material intensities of SFH show the lowest values for constructions made of aerated concrete, followed by sand-lime bricks, vertically perforated bricks and reinforced concrete. A different sequence of values results for MRB. Here, the buildings with a vertically perforated brick construction show the lowest values, followed by reinforced concrete constructions, aerated concrete and with the highest average values, the sand-lime brick constructions.

Accordingly, the unequal volume-related material intensities can be attributed to the differences in material quantities (cf. Section 3.1).

4. Discussion

Based on working drawings of residential buildings (SFH, MRB) from the years 2006 to 2014, the amount of material of different construction types could be calculated. In combination with the building volume or the building area, the specific material intensity of the different building types in this case study could be identified.

A significant difference of material intensities depending on the materials of the structure was found. It can therefore be said that buildings with a wooden construction have a material intensity that is about half that of buildings with a mineral construction. The decisive influence on the higher material intensity of the buildings analysed here is the weight of the construction materials. In the case study conducted here, residential buildings in Germany were analysed. For a sensitivity analysis of the values, an additional number of buildings should be analysed.

Basements were not taken into account. Currently, there are no materials that could fulfil the requirements for a basement construction as an alternative to mineral materials. Accordingly, the basement is comparable in all buildings with regard to the material. For this reason, the basement has no influence on the material intensity when comparing the buildings and their counterparts. The system boundary for the calculations is the outer skin of the building. Attachments such as garages were not considered. In the calculations, the materials of the building structure were taken into account. The building structure includes exterior walls, interior walls, roof, floor slab, false ceilings, as well as the built-in windows, doors, floor coverings and stairs. The technical installations were not taken into account.

For buildings in Germany, material intensities can be derived from literature [31,32]. In Gruhler [31,32], a material intensity of 1.20 t/m² GFA is determined for a synthetic SFH from 1991 to 2010. Material intensities for more recent construction years are not available. In comparison, the value determined in this study for the material intensity of mineral SFH is approximately 14% larger and for the material intensity of wood buildings approximately 41% lower. Comparable deviations are shown for the MRB. According to [31,32], a synthetic MRB from 1991 to 2010 shows a material intensity of 1.36 t/m² GFA. The value determined here for the material intensity for mineral MRB are 6% larger and for wooden buildings 45% lower. The comparison shows that the values of material intensity for the mineral buildings.

The building characteristics in [31,32] are based on synthetic buildings. These are each aggregated from a number of representative buildings with similar characteristic values. The synthetic building is formed from these building representatives and reflects the average of the buildings common in the respective period. Thus, the values for the SFH of [31,32] represent buildings in masonry construction (brick, aerated concrete, sand-lime brick), but also in wood construction (wood frame construction). A detailed differentiation of the constructions is not presented.

This shows that the values reported by [31,32] represent very good general average values. For the goal of a resource-efficient construction, a more detailed differentiation of the values by building construction is needed.

The case study shows that with the combination of material quantities with building specific areas (GFA) or volumes (GV) of the different building and construction types, the resource efficiency of a building can be estimated. Here, wooden buildings show essential lower resource intensities than mineral buildings. A benchmark factor for buildings could contribute to a better understanding of the resource efficiency of residential buildings. In addition, a benchmark for the material intensity of a building could help to increase resource efficiency. In addition, one could add that if part of the large mineral mass came from secondary resources, this would also increase resource efficiency.

In this way, resource efficiency for new buildings could be integrated in building requirements in the future. A binding benchmark for resource efficiency could thus be required for new buildings. The integration of threshold values for the materials proposed in the planning process for new buildings, as called for in ProgRess, offers great potential for careful and at the same time efficient use of natural resources in Germany.

In addition to positive resource efficiency, wooden buildings also show an advantage in terms of the CO₂ balance and wood products have the characteristic of temporary carbon storage. CO2 footprint (GHG emission) can be determined by the method of building life cycle assessment. Wooden buildings in the life cycle assessment store a large amount of carbon storage temporarily. This storage is created during the growth of the tree that removes carbon from the atmosphere.

However, small amounts of wood can also be used in the construction of mineral buildings. Accordingly, biogenic greenhouse potentials can occur here as well. Typically, buildings in the SFH sector have a roof truss made of wood, regardless of the exterior wall construction materials.

In addition to biogenic GHG emissions, fossil GHG emissions occur during the manufacturing of the building products (e.g., electricity use, transportation expenses to the factory, etc.). The biogenic GHG potential partially offsets the fossil GHG emissions. Nevertheless, the choice of materials has a decisive influence on GHG emissions. The larger the quantities of wood used, the greater the carbon storage.

This results in substitution potentials of the GHG potentials by the material choice on the building level. It can be seen that between 35 and 56% less GHG emissions are generated when constructing an SFH from wood instead of a mineral building. For an MRB, between 9 and 48% GHG emissions can be saved when building predominantly with wood construction. This means that the comparison of buildings shows a significant difference in GHG emissions between buildings with mineral and wood construction, which is solely due to the different materials used for the supporting structure [35].

These benefits can be considered in the design of new buildings to reach climate protection goals and consider resource efficiency. The combination of resource efficiency and substitution in new buildings can make a decisive contribution to climate targets.

However, in terms of resource efficiency use of the material and the sensible use of wood structures, the trade-off between carbon storage and material efficiency use of wood must be made again for each building project.

5. Conclusions

In this study, material intensities for the assessment of resource efficiency were determined using the example of German residential buildings.

The results of the material intensities are verified by studies from literature, but differ slightly in comparison to [31] due to different calculation methods. This study provides results of material intensities on a building level, since the calculations were performed based on the data from [34]. Furthermore, the analysed buildings are representative buildings that were constructed or produced using state of the art construction methods in Germany. A total of 26 SFH with different construction types and 20 MRB were part of the analysis. Thus, the material intensities can be identified as representative values and can now be used as a basis for further calculations.

Further resource efficiency potential could be achieved if, in addition to the material intensity, the deconstructability and use of the materials as secondary resources were taken into account and documented with limit values. Relevant initial approaches were developed in the research project [38].

Future research should also separate calculations for hybrid buildings and non-residential buildings.