Comparative LCA study of wood and mineral non-residential buildings in Germany and related substitution potential

The construction sector is a central source of greenhouse gas (GHG) emissions. Reducing environmental impacts along the life cycle of buildings is therefore an important target. Given recent innovations in low-energy buildings and energy supply systems with low climate impacts, additional reduction potential can mainly be found in mitigating GHG emissions in other life cycle stages. The focus of mitigation has thus shifted to emissions related to material input, and comparative life cycle analyses of buildings constructed with different material types are becoming increasingly relevant in guiding regulations to achieve emission reduction targets. This paper performs comparative life cycle assessments for 48 non-residential buildings, comparing GHG emissions according to the current European standardised calculation methods. A substitution potential is introduced to evaluate the advantage of using timber as a building material. Furthermore, a comparative method is presented for assessing the substitution potential on the building level. The results show that the substitution potential for the construction of the studied buildings ranges from 5 to 48%. Specific substitution potentials are differentiated between four subcategories of non-residential buildings. The lowest substitution potential was identified for agricultural buildings and the highest for office and administration buildings. Moreover, the current research shows that the specific materials, construction, the geometry and design all affect the substitution potential of a building. On the basis of these values, it is possible to make projections regarding GHG reduction potential in the construction sector on a national scale.


Introduction
One outcome of the 2015 Paris Agreement (UN 2015) is that climate protection has increasingly focused on closing carbon cycles and reducing greenhouse gas (GHG) emissions. The construction sector was identified as one important player, as the sector accounts for approximately 50 per cent of total primary energy consumption in the European Union (EC 2014). In the pursuit of carbon neutrality, the construction sector needs to balance emissions and removals of carbon. Recent policy developments in the European Union (EU), as reported by Frischknecht et al. (2019), tend towards setting top-down carbon quotas for buildings. Using this approach, the carbon budget for the construction sector is derived, for example, from the nationally determined contributions of the Paris Agreement (UN 2015) or from comparable climate legislation.
Reducing environmental impacts along the life cycle of buildings is thus an important target for climate change mitigation. Given recent innovations in low-energy buildings and energy supply systems with low climate impacts, additional reduction potential can mainly be found in mitigating GHG emissions in other life cycle stages. The focus, therefore, has shifted to emissions related to material input (Weißenberger et al. 2014). Numerous building life cycle assessment (LCA) studies published by different international stakeholders have compared, for example, conventional buildings and timber buildings to ascertain specific advantages and specificities of the latter (Takano et al. 2014;Heeren et al. 2015;Dodoo et al. 2012;Buyle et al. 2013;Cabeza et al. 2014;König 2016).
Because these studies differ significantly in scope, approach, system boundaries and database, they cannot be easily compared. They all, however, demonstrate that, on the level of construction material in the production stage, wooden materials hold advantages in terms of carbon storage capacity (named "biogenic GWP" in EN 15804:2020), and therefore result in lower GHG emissions in the production stage. They also reveal that calculations over the entire building life cycle show a slightly different picture: Advantages of wooden materials are generated mainly through benefits outside system boundaries, after the end of life (EOL) of the building is reached.
In Europe, EN 15978:2011(EN 2012) is a widely accepted framework to specify the assessment of environmental performance of buildings in LCA. Through dividing life cycles into modules, there is general agreement within the scientific community on calculating the benefits regarding energy recovery separately, as well as to further show all potential benefits, like recycling outside the life cycle of the building. This principle is also applied on the product level EN 15804:2012 + A1:2013 (EN 2014b). Less than a handful of studies (for example, , Eberhardt et al. 2019, Emami et al. 2019, however, comply with the current standards in European LCA calculations.  introduced a substitution potential to evaluate the advantage of using timber as a building material. Their calculations and the related data so far mainly assess residential buildings . Thus, there is a lack of reliable data to quantify the possible substitution potential of non-residential buildings. Additionally, there is a lack of data on average carbon storage capacity for such non-residential buildings. Carbon storage itself is balanced out over the life cycle, as carbon storage is calculated as benefit (−1) in the production stage and as emission (+ 1) at the end-of-life stage. Nevertheless, carbon storage benefits exist as long as the buildings exist and can be extended into the next life cycle if products are reused or their materials recycled without losing their carbon content (Hafner and Schäfer 2018). While substitution and temporary carbon storage capacity can be demonstrated on a building level, climate protection goals can only be applied to the entire national-wide construction sector through scaling up to a national statistical level.
The second period of the Kyoto-Protocol (2013-2020) alongside the Paris Agreement made the inclusion of forest management and harvested wood products in the calculations of Global Warming Potential (GWP) compulsory (UBA 2020). These accords stipulate that temporary and dynamic changes in the carbon pool of harvested wood are factors to be taken into consideration. According to Article 3.4 of the Kyoto Protocol, deductions are applied only on a national level and for domestic wood use only (Rüter 2017), thus accounting for the use of wood material in the construction sector. Quantifying expected climate impacts deriving from the increased material use of timber is therefore relevant on a national scale and can enhance the carbon sink of the forest sector. Robust and reproducible substitution potential data for calculating the climate impact of changes in material use in the construction sector are therefore essential to extrapolating data to a national level.
The potential global climate benefits of wood construction have been modelled, for example, by Churkina et al. (2020) and Amiri et al. (2020). Several studies provide models of the national implications of carbon storage. Hafner and Rüter (2018), for example, reported the potential GHG impact of wood consumption in the construction sector in Germany based on an expected future increase in the market share of timber in residential buildings. They also quantified the reduction potential through substituting energy intensive materials with a material choice with lower emissions, showing that there is a substantial potential to lower the GHG emissions through using timber in buildings . Kalt (2018) presented a similar approach and similar effects for Austria, showing the influence of increased carbon storage in timber buildings. Kalt (2018) and Vares et al. (2017) discussed the potentials for Finland. On the European level, Rüter et al. (2016) quantified the various ways in which the EU forest sector contributes to climate change mitigation. Through scenario analysis, consequences for GHG balances in the EU for possible policy choices are presented. A precondition for increased timber use is, of course, sustainable forestry alongside parallel active reforestation. This paper thus redresses the lack of robust and comparable LCA data on non-residential building stock according to a consistent framework and standards. The objective is to determine the substitution potential for newly constructed non-residential buildings and close the existing lack of data. The data made available can be combined with statistical building data from the construction sector to calculate substitution potentials on a national level. The method for calculating substitution potential has proven to be reliable for estimating the impact of the shift from mineral buildings to timber buildings on a national level.

Method
The LCA results are presented in GHG reduction potential [kg CO 2 eq.] at building level for non-residential buildings. The arrangement in groups of non-residential buildings follows the structure of calculations from the Federal Statistical Office (Destatis 2017) and is divided into four main groups: office and administration buildings, agricultural buildings, non-agricultural buildings and other non-residential buildings. The LCA calculations were performed following the general standards ISO 14040:2009(ISO 2009) and ISO 14044:2018(ISO 2018 and the standard at the building level EN 15978:2011 (EN 2012). The methodological approach is based on  and  and adheres to the same rules and framework as the above-mentioned studies.
Based on the same modular approach to life cycle stages, the implementation of LCA is regulated and standardized in Europe through EN 15978:2011(EN 2012 at the building level. Within the scope of standardization, uniform calculation rules are defined and the modules to be considered are specified. The life cycle herby is divided into module A (product stage), module B (operational stage), module C (end-of-life) and an additional module D (benefits and loads beyond system boundaries). For the current research the database: Oekobau.dat Version 2017-I (BBSR 2018) was used. Datasets that do not provide information on module C or D were supplemented by generic EOL datasets from Oekobau.dat in accordance with the EOL rules of the German certification systems BNB and DGNB (BBSR 2015;DGNB 2015). The table "Service life of building ele-ments…", published by the Federal Ministry of the Interior, Building and Community, defines the maintenance and replacement cycles for building components [BBSR 2017]. The LCAs were calculated over a life cycle of 50 years according to the German assessment system for sustainable construction (BBSR 2015). LCA calculations were performed with the LCA-tool LEGEP (Ascona 2018), a tool specially designed for the building sector. For further detailed information, see  and .

Functional unit of buildings
For the product system "building", the functional unit over the life cycle of 50 years is set at 1 m² gross external area (GEA). The definition of a functional equivalent establishes the comparability of the life cycle assessment results. Construction types such as Cross Laminated Timber (CLT) and timber frame are grouped within the term "timber building". The term "mineral building" groups the construction types of brick, sand-lime brick, porous concrete and reinforced concrete. The classification of individual elements into a group of construction elements and finishing elements is shown in Table 1. The differences between the timber and mineral buildings are found in the construction elements. Therefore, only the construction elements are highlighted in Table 1 and considered, since the same finishing elements could be chosen for all buildings independently of the primary construction material (mineral or timber). The fulfilment of the minimum technical and functional requirements is ensured by the functional equivalent. All analysed buildings comply with the minimum legal requirements and the state of the art and thus meet the structural safety and load-bearing capacity of the structure. Requirements for fire protection according to the building regulations and sound insulation are likewise fulfilled. Furthermore, buildings that are subject to the Energy Conservation Act (EnEV, 2009) meet at least the requirements from 2009 and thus achieve the structural physical prerequisites in all seasons. In addition, the buildings that are compared with each other have the same technical equipment for heating and hot water generation.
The essential requirements of the building elements and the minimum standard over the entire life cycle of a building is established via the functional equivalent. The different types of construction lead to gradual differences in terms of exceeding certain minimum requirements (for example, fire protection requirements in mineral buildings). In some examples, fulfilling one minimum requirement leads to over fulfilling another requirement.
The indicator GWP of timber and mineral buildings is compared in order to determine substitution potential on the building level.

Selection of the analysed buildings
The conducted comparative LCA study is based on planned and realized buildings as well as model-generated building counterparts. The analysed buildings for this research are.
Overall, 48 buildings were assessed and included in the comparison. Whereas, agricultural buildings (ST) are a unique feature from other building typologies and are generally not directly comparable. The construction of an agricultural building is highly dependent on the end-use, including livestock type, management, and use, and therefore varies conditionally in building codes. The agricultural buildings (STs) studied were designed, constructed, or modelled between 2012 and 2018 and vary in size with one story and gross floor area ranging from 906 to 2555 square meter. Table 2 shows information on the area, number of floors and year of construction for each building studied.
All studied buildings are representative for the market and accord with the common construction technique for timber buildings (CLT and timber frame) and mineral buildings (brick, sand-lime brick, porous concrete, reinforced concrete or steel).
Functional equivalence is achieved by meeting the same technical and functional requirements and ensures the comparability of building life cycle assessments. Table 3 describes the type of building data used in this study. The building data of the "original" buildings originate from architecture and planning offices and are representing in most cases built examples.  For each "original" timber building, at least one functionally equivalent counterpart was modelled for mineral construction materials, so that there are one to four mineral counterparts for each timber building. The timber buildings and their respective counterparts have identical energy properties, heat transmission coefficients for construction components, facade surfaces, gross external area, durability, functional performance (fire protection, sound insulation, etc.) and correspond to the current state of the art. Thus, the building models are identical, only the material concepts were changed. Furthermore, when creating the counterparts, a distinction was made between construction and finishing materials: the construction elements were replaced by mineral materials, the components of the finishing elements correspond to those of the "original" buildings. The finishing elements are not further considered in this study or included in the calculations of the substitution potential. The classification of construction elements and finishing elements is shown in Table 1.

Substitution potential on the building level
Substitution on the building level is equated to the GHG displacement factor of wood product substitution as defined by Sathre and O'Connor (2010). Substitution calculations on the building level were adapted from  and transferred to the non-residential building sector.
Substitution on the building level is defined as the difference in greenhouse gas emissions that would result if, instead of a mineral building, a functionally equivalent timber building was built. Substitution at the building level thus describes the replacement of an entire building with a functionally equivalent building. The substitution is considered modularly over the building life cycle (modules A, B and C) of the building and is expressed through substitution potential SF G : with: (1) kgCO2eq.  A positive result means that GHG emissions can be reduced by replacing a mineral building with a timber building. Conversely, a negative factor means that more emissions can be produced. The higher the factor (positive or negative), the more emissions can be avoided or produced.

Results
Key data of the analysed buildings relevant for the interpretation of the results are compiled in Table 2.

LCA results on the building level
LCA results are exemplarily shown for the GWP indicator per functional unit of one m² GEA for an office and administration building (OB). They are transferable to the other studied building types. The results are presented in modules A, B, C, and D, although module D is included only as information module and is not further analysed. The results are presented only for primary building elements, This also means substitution potential is also shown only for construction material. Substitution potential of finishing elements still needs further research bearing in mind that finishing elements can be included not only in timber buildings. Figure 1 shows the LCA results in GWP per m 2 GEA for the "original" timber construction of an office and administration building and three mineral counterparts. The total 1 3

Office and administration building (OB)
GWP is the sum of fossil and biogenic GWP. According to EN 16485:2014 (EN 2014a), the biogenic GWP has a negative value because carbon is temporarily stored by the material properties of wooden products. This material property applies to all renewable materials and means that all renewable material masses installed in the building function as temporary carbon repositories. This leads to the conclusion that the more renewable raw materials are used in a building, the higher the carbon storage is. Due to higher wood mass in the timber building, higher biogenic GHG emissions occur than in the mineral buildings, shown as negative values in module A. It can be seen that the mineral counterparts of the building also have carbon storage, although it is not significant. The occurrence of biogenic carbon in the mineral counterparts is due to the use of wood in the internal wall elements (see Fig. 1). In addition to biogenic GHG emissions, fossil GHG emissions occur during the production phase of building products, for example due to energy supply, transport to the manufacturing site, etc. The biogenic (negative) GWP partially offsets the fossil GHG emissions. As seen in Fig. 1 module C, the same amount of biogenic carbon as in module A leaves the product system, but with a positive value in module C. It becomes clear that the biogenic carbon is balanced over the entire life cycle of a building. This corresponds to EN 16485:2014 (EN 2014a), which defines renewable raw materials as carbon neutral over its entire life cycle. Results for the entire life cycle for the GWP indicator are obtained by adding the results of modules A, B and C. The total results demonstrate that the timber building has a lower GWP than the mineral variants. The impact of each individual construction element considered in this calculation (see Table 1) is shown exemplarily for the "original" and counterpart in Fig. 2 for building OB_1. The construction components, which were built from mineral building materials in the "original" building, have been retained in the counterpart. Therefore, the impact of the foundation and balcony construction elements is the same. The differences between the ceiling component in the "original" and the counterparts are due to sound insulation performance. Since mineral building materials have a much higher density compared to wooden materials, ceiling constructions made of mineral building materials can be built with less material, while the timber construction has to be much thicker to achieve the same sound insulation level. Compared to the counterparts, the roofing construction component of the "original" building has a smaller impact. This can be explained by the timber material, which has a lower emission impact than mineral materials. It can clearly be seen that the external wall component has the greatest impact on GWP than any other construction component in the building. The external wall components largely cover the load-bearing structure of a building, therefore the mass of replaced timber material is higher. Only differences between the construction components of the "original" building and the construction components of the mineral counterparts have an impact on the substitution potential. This also means that the foundation and the balcony have no effect on the substitution potential. Figure 3 illustrates the LCA results shown in 3.1.1., for an "original" hybrid construction of a non-agricultural building and one mineral counterpart, which is a reinforced concrete construction type. The load-bearing construction elements of HA_1_H consist mostly of timber elements and to a lesser extent of mineral elements. The biogenic carbon, which acts as a temporary carbon store in module A, is visually identifiable by the negative value. Due to higher wood mass in the hybrid building, higher biogenic (negative) GHG emissions occur in module A than in the mineral building, although the counterpart also has a carbon storage, as in module A. The biogenic (negative) GWP partially offsets the fossil GHG emissions. As seen in Fig. 3 module C, the same amount of biogenic carbon as in module A leaves the product system, but with a positive value in module C. The overall result shows that the hybrid building has a lower GWP than the exclusively mineral variant.

Substitution potential results
Following Eq. 1, substitution potentials are calculated in two steps.
In the first step, we calculated the difference in GHG emissions between mineral and timber buildings in module A + C. In the second step, the resulting difference is divided by the GHG emissions of the mineral building. The higher the GHG difference or substitution potential, the greater the GHG reductions which can be achieved if the timber variant is built instead of the mineral counterpart. What also becomes clear is that the GHG reduction potential or substitution potential decreases the more similar the materials of the building pairs ("original" / counterpart) are. Furthermore, the durability of the construction components has a major impact on emissions, as elements such as external walls extend beyond the product life cycle. Thus, no replacement of the construction components takes place and module B remains unaffected. Figure 4 shows the GHG differences of the LCA results of mineral buildings compared to timber buildings for module A + C (for office and administration buildings). From this result, substitution potentials at the building level were derived, as shown in Fig. 5, and for the other groups of nonresidential buildings in Figs. 6, 7 and 8. The higher the GHG difference or the substitution potential, the more GHG savings can be achieved if the timber variant is used instead of the mineral counterpart.
Building design is influenced by various factors, for example, by the development plan, the wishes of the client, kg CO2 eq. /kg CO2 eq.
Building code with construction Comparative building OB: Module A+C: Substitution potential for various building constructions corridor the spatial conditions, etc. These factors lead to a multitude of possibilities for construction variants. Due to the high variances, the results of the analysed buildings are distributed along a value corridor, which is limited by the minimum and maximum substitution potential.

Office and administration building
The GHG differences of the analysed office and administration buildings are shown in Fig. 4 and vary between 17 and 177 kg CO 2 eq./GEA, demonstrating that all analysed timber buildings emit less CO 2 than their mineral counterparts for modules A + C. The substitution potential range is between 0.06 and 0.48 (see Fig. 5), which indicates that, on the building level, there is a GHG reduction potential of 6-48% if timber is used in the building construction instead of mineral materials. The range results from the different construction type's size, materials used in the construction of the buildings and their individual design. The analysed multi-story buildings have different building designs: two to four floors with internal or external Fig. 6 Substitution potential ("original"-"counterpart") for all building constructions of agricultural buildings for module A + C and corridor of values. Each column shows the substitution potential of one timber building and one mineral counterpart. A division is made between various mineral constructions. Abbreviations for the buildings are shown in Table 2. The first part of the abbreviation shows the code of the building type, the second part shows the building number and the third part notes the counterpart variant  Fig. 7 Substitution potential ("original"-"counterpart") for all building constructions of non-agricultural buildings for module A + C and corridor of values. Each column shows the substitution potential of one timber building and one mineral counterpart. A division is made between various mineral constructions. Abbreviations for the buildings are shown in Table 2. The first part of the abbreviation shows the code of the building type, the second part shows the building number and the third part notes the counterpart variant The timber content in the construction elements of the "original" building OB_5 is low, leading to overall lower substitution potential. Thus, lowest substitution potential is calculated in the building counterparts OB_5_M1, which results from replacing the external wall insulation and roof components, while maintaining the building's supporting structure. In comparison to the substitution potential seen in Fig. 5, the building OB_10 in the reinforced concrete construction counterpart variant has the highest substitution potential. This value can be explained by considering the building components (see Table 3). All structural components in the "original" timber design (external wall, internal wall, ceiling, roof and balcony) are replaced by mineral alternatives and the existing large proportion of wood is thus minimized.

Agricultural buildings (ST)
The GHG differences of the analysed agricultural buildings are between 10 and 70 kg CO 2 eq./GEA, backing the evidence that all analysed timber buildings emit less CO 2 than their mineral counterparts for module A + C. The substitution potential of the investigated agricultural buildings is within a value corridor of 0.05 and 0.37 (cf. Figure 6); for module A + C, there is a GHG savings potential of between 5% and 37% for the buildings investigated. The variance is mainly due to the construction types and materials of the buildings and the regulations observed. The lowest substitution potential can be found in the design variant ST_2_M1 (cf. Figure 6); this can be explained by the overall low proportion of timber in the original construction. The highest value in the building ST_3_M1 is calculated by a total replacement of the timber components.

Non-agricultural buildings (HA)
The GHG differences of the analysed non-agricultural buildings are between 10 and 170 kg CO 2 eq./GEA. The substitution potential of the non-agricultural buildings range within a value corridor of 0.14 and 0.44 (Fig. 7) for module A + C. This means there is a GHG savings potential between 14% and 44% for the examined buildings. The variance is mainly due to the construction types and materials of the buildings and their individual design. Influences on design potential include the existing floor plan, the location of the building, specifications of the development plan, wishes of the building owner or architect, etc.
The building HA_2 is built in hybrid skeleton construction, therefore material consumption is lower compared to the other analysed buildings. The material consumption in HA_2 also results in a lower percentage of replaceable building components, which translates into a low GHG savings potential. The highest substitution potential is calculated for building HA_5, where the share of timber is the highest compared to the other analysed buildings. In this building, the original external wall timber with cellulose insulation is replaced by sand-lime brick and ETICS. The wood content in the timber construction already results in a GHG savings potential, which is increased by the cellulose insulation.

Other non-residential buildings (YC, UAS)
The GHG differences of the other non-residential buildings investigated range within a value corridor of 37 and 133 for module A + C. The substitution potential, shown in Fig. 8,   Fig. 8 Substitution potential ("original"-"counterpart)" for all building constructions of other non-residential buildings for module A + C and corridor of values. Each column shows the substitution potential of one timber building and one mineral counterpart. The substitution potential is calculated according to Eq. (1) indicates a GHG savings potential between 13% and 46% for the investigated buildings. The lowest GHG reduction potential of 13% is identified in the building UAS_3_M1. The "original" construction elements of the building, such as the external wall, internal wall and ceiling construction, consist of timber and mineral elements, which leads to a lower replacement potential. The highest GHG reduction potential of 46% was identified for building UAS_2_M1, where the external wall and ceiling construction were replaced with mineral building elements.

Discussion
Following the current LCA standards (EN 2012(EN , 2014b, the investigated building constructions of all building types achieve positive substitution potential. The analysis considered construction elements such as foundation, external wall, internal wall, ceiling, roof and balcony. The classification of a building into the relevant building type -timber or mineral -was based on the load-bearing structure. Further elements are the finishing components, which include doors, windows, stairs, flooring, roofing, facade and the technical equipment of the building. The finishing components were, however, not included in the present calculations due to the influence based on subjective preferences, and because the data availability and quality are yet not sufficiently reliable to map the market situation. Further research is required on that issue.
The substitution potential of the studied non-residential buildings varies for: • office and administrative buildings between 0.06 and 0.48; • agricultural buildings between 0.05 and 0.37; • non-agricultural buildings between 0.14 and 0.44; • other non-residential buildings between 0.13 and 0.46.
Thus, the GHG reduction potential of the studied buildings ranges from 5 to 48%. The lowest substitution potential has been identified for agricultural buildings and the highest for office and administration buildings. The used building materials, the building construction, the building geometry and the building design all affect the substitution potential.
The calculated substitution potential or GHG reduction potential is divided into building use categories in nonresidential building construction; a comparison between building use categories is currently not possible as it would require further research and analysis.
The substitution potential correlates with the GHG reduction potential, that is, the higher the substitution potential by replacing a mineral construction with a timber construction, the greater the GHG reduction potential. The material differences between the "original" timber building and the mineral counterparts have great effect on substitution potential. Thus, comparing an "original" timber building, which has a high proportion of mineral elements, and a mineral counterpart, which has both mineral and wooden elements, results in a low substitution potential. Thus, the greater the proportion of wood elements in a wooden building compared to its mineral counterpart with a low wood content, the higher the substitution potential will be.
Wooden-building elements not only have positive effects on the substitution potential but can also temporarily store carbon over the building life cycle due to the natural material properties as also shown in, for example, Amiri et al. (2020). The carbon storage is presented as a negative biogenic GWP in module A (Fig. 1), leaves the system in module C and therefore is not included in the substitution potential. A significant influence on the carbon storage is based on wooden elements used in structural components (Amiri et al. 2020). The longer a wood product is in use as a building material, the longer the carbon storage effect is maintained on the building level (Kuittinen et al. 2021).
Since the substitution potential is calculated according to the type of construction of the counterpart, it is not possible to specify the substitution potential in one indicator value. Thus, the resulting substitution potentials are presented in a range between the minimum and maximum values, as shown in  and . To calculate a representative substitution potential, statistical data (Destatis 2017) providing information on building type, construction method and building materials must be taken into account and analysed. Following data analysis, a weighted factor must be determined for each mineral construction type. Thereafter, the influence of the substitution potential on the overall construction activity can be calculated, as described in .
Comparing the present results to recent literature on the same topic (e.g., D´Amico et al. 2020;Humekoski et al. 2021), it becomes obvious that in the literature often a mix of a general substitution factor for the whole material use of wood combined with scenarios (e.g. upscaling to future scenarios) is done. Following the criticism of overestimating the substitution factor (Hammon 2019), realistic substitution potential should follow clear and transparent rules, rely on actual product specific data, and the replacement of building products must be shown clearly (Hammon 2019). All these specifications are fulfilled in the present calculations with a transparent presentation of numerous realised buildings and the use of actual environmental product declarations, for example for timber products in Germany. Howard et al. (2021) emphasises that more sophisticated data is needed for the building sector. The present results can satisfy this need as it shows substitution benefits for various building categories. Additionally, the results are 1 3 based on a large range of calculated buildings and not only by single building comparison (Geng et al.,2019;Himes and Busby 2020). What the present data does not integrate is the changes of future energy-mix or the increasing share of recycling in the future (Brunet-Navarro et al. 2021), which will certainly lower the substitution potential until the 21 century. However, for achieving realistic GHG emission reductions in the short term, the presented substitution potential is reliable.
The results reveal two major influences. Timber buildings indicate lower GHG-emissions over the entire life cycle than mineral materials for the functional unit of m² GEA. Additionally biogenic carbon is sequestered in buildings -shown in LCA results as negative biogenic GWP.
There is a need, however, to differentiate between (a) substitution potential and the captured and stored carbon on the building level as is demonstrated in this paper and (b) the calculation of carbon pools on national scale by using harvested wood products in buildings (IPCC 2006). On a national scale, the carbon stock level cannot be solely estimated from the quantity of carbon uptake. To estimate the carbon stock magnitude of selected pools, inventory as well as flux data methods need to be applied. The latter are based on information on the magnitude of carbon inflow to a pool as well as its carbon outflows (Rüter 2017). To assess the GHG impact of deviating development pathways, the accounting must be checked against a reference case, as shown in Rüter (2017) and Hafner and Rüter (2018).
Looking to future research, the next step would be to scale the results to a national level and to link them to the national calculation of GHG emissions. Hafner and Rüter (2018) demonstrated how integrating data from buildings' LCA with calculations on national scale could be employed to predict the GHG reduction potential for the national construction sector. The data derived in the present research allow these calculations to be broadened. Presently, only 17% of non-residential buildings in Germany are built using timber construction (Purkus et al. 2019).

Conclusion
This study presents transparent and scientifically validated LCA results and substitution potentials on the building level for various groups of non-residential buildings.
Further GHG reduction potential could be mobilized if, alongside construction elements, finishing elements such as doors, windows, flooring, outer cladding were also accounted for. The potential of finishing elements for GHG reduction should not be underestimated, since timber finishing products can be installed in every building, independent of the building's construction.
In general, the results reveal two major influences. Timber buildings indicate lower GHG-emissions over the entire life cycle than mineral materials for the functional unit of m² GEA. Secondly, biogenic carbon is sequestered in buildings -shown in LCA results as negative biogenic GWP. The carbon sequestration effect of timber buildings has recently gained more attention, for example, in the context of the European Green Deal.
The present calculations of substitution potential show that a shift to increasing the rate of timber buildings on national level could create a significant potential for future GHG reductions. Thus, the more detailed data we have on the calculated substitution potential of really built buildings that are connected to the input on the national level, the more valid data can be modelled to precisely calculate the input of material choice for a national GHG balance sheet.