Carbon assessment of a wooden single-family building – A novel deep green design and elaborating on assessment parameters

The aim of this study was to investigate how the carbon accounting of a wooden single-family house is affected by (1) decreasing the carbon footprint by changes in building design, (2) differentiating biogenic carbon from fossil carbon and (3) including external benefits beyond the state-of-the-art system boundaries. The motivation of exploring different system boundaries, improved building design and investigating benefits aside of system boundaries rely on the fact of having the “ full ” picture of GHG emissions of building products. Changes in building design were analyzed by life cycle assessment (LCA) focusing on greenhouse gas (GHG) emissions, while the costs were assessed by using lice cycle cost (LCC). The findings showed that by including positive and negative emissions from the production phase for an improved building design within scenario 4 ‘Cradle to Gate + Biogenic Carbon + D module ’ has the lowest embodied GHG emissions when compared to other approaches with (cid:0) 3.5 kg CO 2 e/m 2 /y 50 . Considering the impacts of the whole building, the lowest GHG emissions are within the scenario 8 ‘Cradle to Grave + Biogenic Carbon + D module‘ for the improved building design with (cid:0) 0.7 kg CO 2 e/m 2 /y 50 . The results suggest that a change to sustainable alternatives for building components that makes the whole building to be constructed by wood, could lead to significant reduction of GHG emissions compared to conventional material choices. Economically, testing sustainable solutions, the highlighted results are the construction costs that are almost double higher for CLT elements for the foundation compared to concrete.


Introduction
To fulfill the target from the Paris Agreement and limit global warming below 2 • C, the European Union (EU) needs to entirely decarbonize its economy by 2050 [1].To achieve the ambitious goal, it is recommended to reduce greenhouse gas (GHG) emissions by at least 50% by 2030 [2].According to recent data, more than 36% of GHG emissions in the EU belong to the building sector, therefore the transition towards net zero economy is needed [1].Sweden has set up an objective to mitigate GHG to net zero no later than 2045 [3].To achieve the target, some supplementary measures are promoted: raised net removal in forests, verified emissions from other countries and biogenic carbon storage [3].
The Swedish National Board of Housing, Planning and Building (Boverket) introduced climate declarations for new buildings from 2022 [4].The purpose of starting the regulations is to encourage manufacturers to produce low-carbon building materials.Further, Boverket developed a database for the main building components being used on the Swedish market.The database consists of generic climate data that are set conservatively and includes main characteristics of a product [5].When creating a list of building materials for a new building, this database can be used for calculation of carbon emissions.As Sweden has almost decarbonized the electricity supply, the emphasis in the future should be on minimizing embodied carbon emissions within building materials [6].Boverket further proposed [4] that the biogenic carbon storage in wooden building materials should be included separately in the climate declaration, as additional environmental information.
Using wood as a construction material for buildings has increased significantly in the Nordic building sector [7].The advantage of using wood in buildings rely on its ability to absorb and sequester carbon dioxide (CO 2 ) from the atmosphere and thus reduce the CO 2 level, preferably with higher longevity of a product [8].The CO 2 , sequestered in bio-based materials and wood, is labelled biogenic carbon.When the wood is combusted at the end-of-life, the sequestered carbon is released into the air, thereby readjusting the CO 2 level [8].Thus, woods' ability to absorb and release CO 2 from the air is often used as a strategy for mitigating the embodied GHG emissions of buildings.
However, to get an accurate value of the environmental impacts of wood is still under development, due to its complexity [8].Therefore, as to quantify biogenic carbon uptake and release in wooden materials, two different approaches are mostly used.The first approach, known as 0/0, the carbon uptake is equal to carbon released at the end-of-life and wood is considered as carbon neutral [8,9] and also state-of-the-art in life cycle assessment (LCA).The second approach, known as − 1/+1 includes specific handling of biogenic carbon in the life cycle stages, where during the product stage carbon uptake is referred to − 1 and during the end-of-life the carbon release accounted as an equivalent +1.Further, there is also a dynamic LCA method where timing of biogenic carbon uptake and release is taken into consideration [8,9].
Currently, assessment of the biogenic carbon is getting more attraction within the LCA society.Therefore, a new version of European Standards was developed by including the biogenic carbon content [10].According to that version, new environmental product declarations (EPDs) for building materials should declare building life cycle stages A1-A3 (production emissions), C1-C4 (end-of-life emissions) and D module (external benefits beyond the system boundary) and consider global warming potential GWP biogenic and GWP fossil separately.The inclusion of biogenic carbon content in LCAs and EPDs is significant in terms of overall estimation of carbon balance, considering its removal and storage [11].
Resch et al. [12] advocate the importance of operation emissions of buildings and benefits of biogenic carbon uptake in buildings for reducing the global warming potential in the next 100 years, while the end-of-life impacts will be neglected in a long run.Another study done by Hart et al. [13] shows a 43% reduction in GHG impact for a wood construction when compared to steel and concrete, likewise according to the study [14] wood has 9-56% reduced impact compared to mineral solutions.However, countries in Northern Europe (especially Sweden, Norway and Finland) have built single family houses using wood as the main construction material, with 85-95% of the total mass [15].A previous literature review of existing studies shows the lack of variability in LCA studies of wooden buildings [8].Therefore, it is difficult to explore the relationships between methodological choices and GHG emissions.The research gap should be covered by focusing on biogenic carbon accounting in LCAs, different types of detailed inventory data for wooden buildings and increased transparency in LCA studies [8].The majority of previous studies considering the biogenic carbon approach − 1/+1 within the system boundary "Incl.biogenic carbon + Cradle to Gate with Options" only includes the product stage (A1-A3) as mandatory stages with other construction, process or use stage as additional options.Further, released carbon emissions in the end-of-life stage typically present low embodied GHG impacts in total and therefore they were omitted [8].Similarly, the study by Hoxha et al. [9] identifies that 32 studies which have used "Cradle to Gate with Options" approach, covering the product stage and some also extended end-of -life stage with or without assessing module D. It is concluded that there is a lack of consistency due to various methods for biogenic carbon accounting, hence the reliable comparable analysis in results look unfeasible [9].Therefore, future studies would be more reliable to consider the whole system boundary in order to avoid misleading results by covering only the product stage [9] and also to add carbon release into account within end-of-life stage in order to fulfill more realistic outcomes [8].Additionally, the issue related to biogenic carbon accounting relies also on the fact that there are plenty of EPDs and other databases that do not properly consider this indicator.According to European Standards [16], environmental assessments of building products include only life cycle modules from A1-A3 in EPDs without declaration of the biogenic carbon content [16].In the recent report from World Green Building Council (WGBC) [17], it was stressed out that carbon emissions occurring beyond the system boundary, including reuse or recycle of building materials should be considered in forthcoming updates to European standards.Thus, it should be mandatory for EPDs to report D module together with other life cycle stages.
As buildings can last for minimum 50-100 years or even more, there is a lack of studies considering 100 years lifetime.There is also no clear evidence on how the biogenic approach was included in the case studies.The buildings mostly analyzed also involving biogenic carbon are multifamily buildings, while the single-family buildings are rarely reported.There is also a lack of case studies exploring wooden based buildings and materials in terms of carbon storage and the benefits after its end-of-life expressed in D module.Further, regarding material assessment there are limited findings regarding wooden foundation, wooden roof, and low carbon solar photovoltaic (PV) panels.In the literature, it can be noticed evident demand for environmental assessments on wood-based materials due to its great advantage for carbon storage, while on the other side its economic evaluation was also accounted as of great importance.Further, there is a lack of previous research based on low carbon roof system, therefore future studies would need to include more sustainable roof solutions.Indeed, roof as the large construction element could lead to significant environmental impact if made by conventional materials (steel or concrete tiles), therefore natural material such as wood could be taken into account and conducted more in details.Hence, the limitations mentioned in the previous studies were motivation for doing this study.
The economic performance of building components has a great role in the building industry.It is found that initial construction costs are the main contributors to total LCC (56%) [18], therefore, it is important to investigate and compare construction costs of building materials and installations.
The aim of the study is to elaborate carbon accounting through the life cycle assessment (LCA) of a reference building and its improved design by including biogenic carbon content into account and external benefits beyond the system boundary.The life cycle cost (LCC) method was additionally used for providing information about financial costs with emphasis on construction costs of building products.According to these goals three research questions were developed.
• How will building improvements change the overall climate impact and costs of the building products?• How can the biogenic carbon involved in the LCA change the overall climate impact on building and product levels?• How can the different system boundaries of LCA method influence the decision-making processes in the built environment?
By answering these questions, this paper will provide new insights to the understanding of sustainable solutions for future wooden singlefamily houses located in Nordic countries.

System boundaries and carbon assessment approaches
In terms of inclusion vs. exclusion of biogenic carbon, different studies have applied various system boundaries such as: Cradle to Gate with Options, Cradle to Grave excluding module D etc.Most of these studies include embodied GHG investigation, while details regarding the operational carbon was omitted.Data were normalized to kg CO 2 e/m 2 /y based on the different building areas while the 50 years as the RSP was mostly applied.In the study by Andersen et al. [8] only 2% of the scenarios in previous studies include biogenic carbon separately, while 98% of the studies do not include in details biogenic carbon.Furthermore, around 96% of the scenarios applied attributional LCA method, while only 4% used consequential LCA.It can be also noticed in the study [8] that results when using attributional LCA show high emissions, at around 4.4 kg CO 2 e/m 2 while the results achieved by consequential LCA show lower emissions, at 2.6 kg CO 2 e/m 2 mostly due to its potential to include external benefits.The average embodied GHG emissions found for single-family residential buildings is 4.7 kg CO 2e /m 2 for 50 RSP [8].
These results present significantly lower emissions than findings for the same building category investigated by Röck et al. that are within the range from 6.7 kg CO 2e /m 2 to 11.2 kg CO 2e /m 2 for the same RSP [19].However, it can be also seen that 74% of the 226 scenarios do not show the clear path of using biogenic carbon approach [8].According to results based on different system boundaries, the lowest embodied GHG emissions are found within the category "Incl.biogenic carbon + Cradle to Gate with Options" with − 0.2 kg CO 2e /m 2 /y for 50 RSP compared to other approaches included in the study [8].Negative embodied GHG is achieved due to applied − 1/+1 approach excluding the end-of-life stage in the analysis [8].Furthermore, the study by Hoxha et al. [9] also demonstrate that − 1/+1 approach can end up in negative GHG impacts with notice of lack of accurate data for decision making purposes.Other findings show that the system boundary "Incl.biogenic carbon + Cradle to Grave excl.module D" have lower GHG emissions compared to the system boundary "Excl.Biogenic carbon + Cradle to Grave excl.module D".The first category that includes biogenic carbon show 4.8 kg CO 2e /m 2 on average, while the second category without biogenic carbon approach presents 9.5 kg CO 2e /m 2 on average using 50 RSP.It is evident that results by using biogenic carbon accounting, mostly applying − 1/+1 approach show significant reduction of GHG emissions [8].

Carbon assessment results for wooden buildings
Wooden structures can store carbon emissions for a long time in their structures, and this process named as "carbon sequestration" [20] including removal of carbon dioxide from the air and creating "negative emissions" [21].Concrete that mostly consists of rock, cement, water and some additives releases carbon emissions during the production process, while during the use phase concrete can take up/absorb carbon when oxygen from the air is reacting with calcium hydroxide, forming calcium carbonate [21,22].
In the study by Andersen et al. [11], the results for concrete and cross laminated timber (CLT) buildings were investigated with significant difference in embodied GHG for the LCAs reference study period (RSP) of 100 years and cradle to grave approach.For the concrete building, materials account for 62% and 58% of the CO 2 emissions, taking into account baseline and biogenic scenarios.While for the CLT building, materials have shown significant reduction, 2% and − 54% of embodied GHG impacts for baseline and biogenic scenarios.The CLT building presented negative scores in the biogenic scenario as the GWP bio factor was included in the calculation, which automatically increased the climate benefits by including biogenic carbon sequestered in the building [11].
The results obtained from a Spanish single-family house based on CLT construction have shown 34 kg CO 2e /m 2 /y by considering 50 of RSP [23].The system boundary includes the construction phase, the use phase, maintenance and repairs, annual energy and material consumption until the demolition phase.The embodied GHG emissions from building materials and construction activities are 7.8 kg CO 2e /m 2 /y [23].The operational phase has the largest contribution in total environmental impact, with 26.5 kg CO 2e /m 2 /y, respectively [23].
The findings from a study in Sweden by Peñaloza et al. [24] show significantly lower emissions considering biobased materials in buildings and buildings with a shorter lifetime have lower benefits when replacing non biobased alternatives.Hence, the study elaborates dynamic LCA using GWP indicator and shows the climate benefits by substituting mineral-based materials with biobased alternatives.
In the LCA overview of wooden CLT buildings [25], the results presented a wide span in terms of different classifications of different studies.In the selected studies on CLT buildings, only one case was found for a single family building, located in Malaysia [26], with 695-833 kg CO 2e /m 2 for 50 RSP, including Cradle to Grave (excl.B1, B3-B7, C3-C4 and D1 modules) [25].Majority of results were published for multifamily buildings with 50 RSP and the overall climate impacts range between 0.05 and 6.3 tCO 2e /m 2 floor area including different system boundaries, lifespans, regional climate and other relevant parameters [25].In the selected case studies by Younis and Dodoo [25], considering biogenic carbon had a strong influence on the results.Based on their summary, CLT buildings with Cradle to Gate system boundary using 0% (no storage carbon) and 100% (included carbon storage) have a carbon footprint of 271 and 125 kg CO 2e /m 2 .Furthermore, for Cradle to Grave system boundary, using 0% and 100% carbon storage, results in 322 and 227 kg CO 2 e/m 2 [25].Another important finding is that reused CLT was the most environmentally preferable alternative as it not only reduces GHG emissions, but also supports Cradle to Cradle approach, circularity in buildings [25].

Biobased construction materials
A significant reduction of carbon emissions from the building construction sector could in the future be achieved through higher utilization of biobased materials.Historically, wood as a construction material was broadly applied in buildings [27].According to previous studies, timber in general and CLT as a wood construction material, have shown large reduction of GHG impacts compared to concrete [28][29][30], even a negative climate change impact [31].Additionally, the alternative suggested in the paper is that obsolete concrete can be reused, e.g., as ballast in newly produced concrete [21].From an economic perspective, CLT material is considered more expensive than concrete.However, due to structural properties there are significant advantages of using CLT instead of concrete for flooring.The CLT elements can be constructed efficiently and in short time, which results in shorter production time, while the concrete slab needs a long manufacturing process [21] due to drying process which is also energy consuming.Further advantages are that a CLT element is a lighter material and weighs about 1/6 of concrete, thus, an easier construction can be achieved with reduced logistic costs [21].Even in the investigated scenario where concrete involve carbon storage compared to wood that excludes carbon storage, wooden materials are found as favorable solutions from an environmental perspective [21].It can be concluded from environmental point of view that wood has a clear advantage over concrete, thus its can be also seen that Sweden has a long tradition of sustainable forestry and great possibilities to use the wood as construction material [21].If more buildings are built with wooden frames in the future, the carbon storage will increase and that will have direct impact on the reduction of climate impact [21].

Embodied carbon in solar photovoltaic (PV) panels
Different countries provide different climate potential of the electricity mix and therefore the embodied carbon emissions from the production process of solar PV panels in each country could significantly vary.Following the study by Karaiskakis et al. [32] where different types of PV systems were investigated, the embodied carbon intensity is in the range of 159 kg CO 2e /m 2 to 199 kg CO 2e /m 2 .Furthermore, in the study [33], producers of solar PV panels display different embodied carbon results.The estimated released emissions per m 2 of solar PV panels are investigated showing impact of 125 kg CO 2 (Germany), 103 kg CO (USA), 17 kg CO 2 (Brazil), 182 kg CO 2 (China) and finally 82 kg CO (Japan) [33].However, Brazil generated 83.7% of its electricity from renewable sources in 2018.Therefore, the carbon emission amount is lower compared to other countries mentioned in the paper [33].
Müller et al. [34] highlight the importance of the electricity mix in the country where solar panels are installed.It can be concluded that the highest values of GHG emission savings can be reached when the PV systems are manufactured in low carbon countries and installed in countries with carbon intensive electricity mix and great solar irradiation [34][35][36].
Valuable results, investigated in the study [33], identify that energy required to produce solar panels is 9.52 times higher than the energy needed for recycling process.Thus, it is important to include recycled B. Petrović et al. components in the manufacturing process of solar panels [33].Moreover, it was stated in the paper that in the recycling process of panels, up to 58% of energy savings can be achieved compared to energy needed for new panels.In other words, recycled panels could contribute 42% to mitigating GHG emissions [33].Likewise, according to Ref. [37], the benefits of recycling glass and aluminum could lead to great reduction in total life cycle emissions of PV panels.
Another way of reducing the embodied carbon emissions is to replace traditional roofs with PV systems.In the study [38], buildings could have installed integrated photovoltaic (BIPV) systems where PV modules are used instead of conventional roofing system.It is noticed that proper installation of PV systems on roofs, can significantly reduce the quantity of roof materials and associated emissions from material manufacture.It is also stated that BIPV module do not damage the building envelope when need to be removed.Due to dual purposes, the integrated systems can provide cost and material savings [38,39].
Following the case study investigated in Spain for a single-family house, it is also concluded that while the electricity become fully decarbonized, installing PV panels would be the best environmental and economic solution [23].

Economic performance of wooden buildings
In the study done by Lechon et al. [23] for a single-family house based on CLT construction, the total costs are 41.21 €/m 2 in 50 years RSP, where 77% are the construction costs while the rest are operational costs 23%.Within construction costs of the single-family house the most expensive material is the CLT structure (23%).In a similar study, the single-family house constructed with CLT and 50 years of RSP, the building materials' production contributes significantly, by 89% to the total cost [26].Silva et al. [40] identified that CLT can lead up to 30% reduction in construction time, which consequently shows lower labor costs.Additionally, the timing period for construction per floor of CLT material could be up to 4 days, compared to 21 days for concrete floor [41].Further, in the study [42], it is noted that CLT can be used for both single and multistorey buildings where speed construction might reduce costs, while on the other side the planning phase would be more time consuming.Therefore, understanding CLT as manufactured wood material and as a construction material will lead to more efficient decision process on how to use the benefits of its properties [42].

Methodology
The LCA method is used for calculation of released CO 2 e emissions for building materials and installations, while the carbon compensation method is proposed by using biogenic carbon accounting.Furthermore, the LCC was conducted in line with LCA for both reference and improved building, using the software One Click LCA [43].A sensitivity analysis with comparative analysis and interpretation of results was made in MS Excel.In line with a previous study [44], both 50 and 100 RSPs were applied.The most common RSP of 50 years mentioned in previous work was used as benchmark, while 100 RSP was used since wooden houses in Sweden and other Nordic countries could last for an even longer period, therefore the difference in GHG emissions could be of great interest.The inventory data for materials and energy calculations were based on previous studies [44,45], and the results were updated by the most recent data on material manufacturing localization target (where emissions from electricity used in manufacturing are adjusted to represent the power source mix in the chosen location using energy grid and other data) based on Boverket with estimated Swedish electricity mix from year 2021 [43].
Selected building materials were mostly EPDs from Sweden and Norway, with a minority of EPDs from other EU countries.However, due to the lack of EPDs on energy systems, the data were based on the generic data provided by the software.The biogenic carbon content within the database of One Click LCA software was provided as a value

Table 1
Data description for LCA and LCC.obtained from EPDs, else if the value was not declared the software provided close estimation explained in the document [46].The end-of-life method chosen is based on default option, where material type-specific scenarios were used for the end-of-life impacts [47].Description of LCA and LCC methods within different system boundaries are shown in Table 1.
Different LCA approaches are applied for reference and improved building design scenarios for 50 and 100 years RSP.Considering biogenic carbon accounting, the case study applies a − 1/+1 approach by involving both carbon uptake during production phase of wood products and carbon release during its end-of-life phase.

Case study building
The building model used in this paper is Dalarnas Villa [44,45], a single-family house located in Dalarna region in Sweden built in 2019, shown in Fig. 1.The house is used as a reference building with total gross floor area of 180 m 2 .The reference building envelope consists of a wooden framework and wood panel facade including cellulose insulation for external walls and for the roof, wood fiber insulation for internal walls and foundation based on concrete.Windows are triple glazed and wooden-aluminum framed.The roof is made of steel, while doors are wood/glass for external use and wood for internal use.Energy systems applied in the building are solar PV panels, exhaust ventilation system and ground source heat pump.The inventory data can be seen in detail in a previous study [45].Further, the operational energy use was calculated within TMF Energy program based on building physics, climate, heating and ventilation systems and occupancy [45].These data were transferred in One Click LCA for further estimation of operation carbon emissions.
The reference building was compared with improved design case by substituting the high embodied materials with sustainable choices.The comparable analysis in the result section was conducted for reference vs. improved building design, in terms of GHG emissions, based on fossil and biogenic carbon emissions provided in Table 2.
The further comparable analysis was done in terms of LCC.The  purpose of using this house is to investigate differences by changing various materials that contributed in large share to total emissions.Motivation for introducing the improved building design rely on our previous findings where few materials, such as concrete installed only for the foundation, roof made by steel and PV panels produced with EU electricity mix contributed as a large share of embodied carbon in total results.Therefore, to mitigate carbon impacts, we introduced biogenic carbon and included different methodological aspects.Further, cost estimation was included for testing economic sustainability aspect.Therefore, to decrease the embodied carbon footprint, the improved building design has substituted concrete slab for CLT elements and roof made by steel with thermo-wood construction.Solar PV panels produced in Europe are replaced with panels produced in Sweden, that account for around 90% decrease in embodied carbon.

Results
In this section, the results for the reference building and the improved building design are presented.The results are normalized and presented in kg CO 2 e/m 2 /y in Figs. 2 and 3 by taking into account 50 and 100 RSPs for different system boundaries applied for reference vs. improved cases.Further results are presenting the LCC analysis covering cradle-to-grave.Finally, there are results on the sensitivity of building materials and installations in terms of emissions and costs for both buildings.
Tables 3-4 present the LCA results of the reference building and the improved building.The 2 cases (original and improved designs) have been investigated in 8 scenarios for two RSP (50/100 years).This means there are 32 final results on GWP, depicted in Figs. 2 and 3.There is also a break-down of these GWP results into building components (Figs. and 5) as well as break-down of construction costs into building components for the two cases (Fig. 6).

GHG emissions including different system boundaries
Figs. 2 and 3 present GHG impacts for reference vs. improved building design applying different system boundaries for 50 and RSPs.It can be pointed out that the improved building design shows significantly lower embodied GHG emissions compared to the reference building for 50 and 100 RSPs for all scenarios, respectively.The results on Fig. 2 show that including emissions from products' level in scenario 1 and buildings' level in scenario 5 without considering biogenic carbon and external benefits from D module led to the highest total GHG emissions.However, including the negative carbon within wood materials using biogenic accounting and external benefits derived from second used products present the best outcome on products' level in scenario 4 for improved building with − 3.5 kg CO 2 e/m 2 /y 50 , respectively.Considering the impacts on buildings' level, the lowest GHG emissions are within scenario 8 with − 0.7 kg CO 2 e/m 2 /y 50 , mainly due to higher negative emissions derived from wooden materials along the whole life span of the building.Another reason behind the overall lower emissions for the improved building design outcome is based on high content of biogenic carbon within wooden materials that surpasses benefits considered in D module for scenarios 2 and 6 compared to scenarios 3 and 7. Fig. 3, presenting emissions for a period of 100 years, does not show negative emissions on a buildings' level, mainly due to higher replacement rate of solar panels during the long lifespan of a building.Taking into account different RSPs, it can be pointed out that considering improved building scenarios involving biogenic carbon and D module, 50 RSP present significantly lower total emissions compared to 100 RSP.The reason behind these results relies on the fact that wooden choices provide greater environmental performance and rise the amount of negative GHG emissions.However, decreased impact when applying cradle-to-gate scenarios (1-4) are due to that only the material assessment is taken into evaluation, meaning that biogenic carbon or benefits after the service life of products will show significantly reduced emissions compared with whole life cycle approach where the replacement rate of some materials will increase the total amount of released GHG emissions.Therefore, this study presents a complete analysis by adding other life cycle stages and covering the cradle-to-grave approach.

Embodied GHG impacts and cost estimation of building products
Figs. 4 and 5 present embodied GHG emissions, expressed in kg CO 2e /m 2 , from the production stage of building materials and installations for reference vs. improved building design considering.It can be seen that evaluation of biogenic carbon and additional benefits expressed in negative values provide better view on different building products.By substituting the materials with a high carbon footprint to alternatives with lower CO 2 emissions, embodied carbon will be significantly reduced.It can be noticed that wooden materials (such as CLT, wood for framework and facade, thermo-wood, wooden roof, parquet, cellulose insulation and wood fiber insulation) show the lowest positive (released) emissions during their production process.Additionally, wooden products present negative emissions that are stored during its service life.Further, these products show benefits that are considered after the end-of-life when the second "life" is considered.It can be noticed that biogenic carbon stored in wooden materials including external impacts from D module can significantly reduce embodied carbon.Therefore, substituting concrete foundation with CLT elements, metal roof with wooden structure and solar PV panels produced in EU with the panels produced in Sweden will lead to extremely low embodied carbon emissions.Comparing insulation materials, it is evident that cellulose insulation and wood fiber insulation are the best solutions to reduce emissions.
By comparing the results obtained in Fig. 6, it can be also noted that sometimes wooden materials are not always cost-efficient solutions as the prices on the market can differ.It can be noticed that choosing CLT structure for foundation instead of concrete will increase costs significantly.Roof steel is more expensive than wooden construction, while PV panels production costs, by comparing that from Sweden and EU, are negligible.However, CLT elements is a lighter material than a concrete slab, which has faster production time and consequentially reduced logistic costs.This could be a profitable solution in the long run [21].Furthermore, even the concrete has more economic benefits that CLT, the important factor to consider is variation of prices on the market dependent on various economic indicators.
Nevertheless, each investigation of environmental impacts and economic solutions of building materials and installations will provide worthwhile information for decision makers in building industry located in cold climate.

Benchmarking overall results with previous studies
In line with our study, Andersen et al. [8] explored wooden buildings and identified that the embodied GHG emissions within category 'Incl.biogenic carbon + Cradle to Gate with Options' have the lowest values compared to other approaches in their study where the average value is − 0.2 kg CO 2 e/m 2 /a 50 .Further their results have shown that the scenario 'Incl.biogenic carbon + Cradle to Grave excl.module D' results are in average 4.8 kg CO 2 e/m 2 /a 50 , compared to our improved building design that significantly reduce the impacts to 1.0 kg CO 2 e/m 2 /a 50 for scenario 6.Other findings in Andersen et al. [8] within the scenario 'Excl.biogenic carbon + Cradle to Grave excl.module D' are in large share of embodied GHG emissions, on average 9.5 kg CO 2 e/m 2 /a 50 , compared to our improved building design presenting 5.2 kg CO 2 e/m 2 /a 50 for scenario 5.In their study, they summarized different studies including various wooden based building types, building cases, building areas, regions, LCI modeling, LCA approaches etc.Thus, the average embodied GHG emissions for their study present 4.7 kg CO 2 e/m 2 /a 50 for the category 'single-family house', while the findings in our study shown the lowest emissions for improved building design with average value of 0.8 kg CO 2 e/m 2 /a 50 .The results in their study [8] do not include B6 and B7 modules, therefore the complete benchmark analysis was not possible to conduct.Expended system boundaries including module D and biogenic carbon can have a significant contribution to the "real" value of a building product.The main reason of including different scenarios provide different perspective on inclusion/exclusion of biogenic carbon stored in wooden building materials and external benefits explored during recycling/reusing processes of building products.The secondhand products can highly extend the service life of products and at the same time decrease and provide negative emissions in total accounting.If a wooden building is built with reused building materials the embodied carbon will be significantly lower compared to a building based on new produced materials.Therefore, there is also high importance on the RSP of secondhand products.In order to have full picture of all carbon flows for a wooden building, the 100 RSP would be a good reference as in general wooden houses can least even longer.

Wooden foundation and benefits of using wood as a material
One of the assumptions of having remarkably lower impacts comparing to Andersen et al. [8] is that in our study both reference and improved cases are wooden constructions, while specifically, the improved case is completely made by wood, even the foundation, what is rarely found in previous work.In most cases, the foundation is made by reinforced concrete that leads to very high embodied carbon during its lifetime.However, another recently built wooden single-family house in Sweden is villa zero [49] that represents carbon neutral concept.The specific novelty of this house is wooden foundation made by CLT elements.As there is a great debate on having wooden structure in the foundation, the company [50] that built this house will further investigate the moisture content in wooden walls, ceiling, and the foundation.In Swedish comparative study conducted by Hassan et al. [21], CLT flooring and concrete slab flooring were analyzed in terms of structure, economic and environmental impact.The study shows significant reduction in GHG emissions of CLT flooring and its ability to store carbon is higher that the capacity of concrete storage.However, the reduction of embodied carbon within foundation could be achieved by replacing ordinary concrete with green alternative.However, considering wooden based materials the biogenic accounting play vital role, especially if the material has long service life and if the product is recycled or reused.Due to climate emergency and resource scarcity, the imperative in the building sector is to find sustainable solutions that will compensate released emissions during the production processes where the intensive energy is needed.Therefore, the substitution process towards low carbon alternatives is crucial.One of favorable solutions is to shift towards wooden based alternatives and presenting the real value of a product and showing the potential of a building as a "carbon sink".It should be also mentioned from energy carrier perspective, that supplying Dalarnas Villa house with Swedish electricity mix decreases around 88% operational carbon compared to European driven electricity.

Advantage of using CLT
The results of study done by Andersen et al. [11] where they compared wooden building and concrete building including biogenic carbon in their assessment, show the potential benefits by using CLT as the construction material.They concluded that increased use of CLT in new buildings is a good solution for decrease in GHG impacts per m 2 .Likewise, as CLT is a relatively new construction material, for further mitigation of climate impacts the technological solutions for fire protection and noise transportation should be developed [11].Considering these factors CLT will be more desirable structural material in future buildings.In the study by Häfliger et al. [51], wood building materials based on EPDs have shown biogenic carbon as negative emissions for (A1-A3) within system boundary (Cradle to Gate), moreover, carbon release was also considered at the disposal phase (C) and wood incineration emissions from module D. The benefits coming from module D for wooden based materials have shown relatively low climate impacts on a building level [51].In the study [52], the comparable analysis of mass timber, and concrete and steel as building materials have shown that mass timber decreases construction phase emissions by 69%.They also estimated that replacing wood for conventional building materials in half of new buildings will provide 9% reduction of GHG emissions and keep the global warming below 1.5 • C. By choosing sustainable alternative materials that reduce the climate impacts can influence significant contribution to the ambitious 2030 targets without large changes to consumption or policy [52].Cabeza et al. [53] identified great difference in wood and concrete materials, by concluding that timber needs special attention in the embodied carbon dataset.Carbon emissions stored in biomass product life can be taken into consideration as "sequestered carbon" [53], also depending on the study approach, embodied carbon can be seen as negative [54].

Future research: clarification and transparency of LCA life cycle substages
According to limited access to detailed LCA in previous studies, the future research in wooden buildings could discover different modules within the "cradle to grave" system boundary, specially within B stage when it comes to use, repair, maintenance, and refurbishment of building products in a long run.It is a challenge to calculate and estimate these emissions as they mostly depend on customer preferences and materials nature, while generic estimations in percentage could be an option for the coming future.As well going into deeper analysis of replacing conventional materials into more sustainable alternatives will mitigate the climate impacts on a broad level.Furthermore, in the future the whole LCA will be mandatory in the building sector including all substages and not only aggregated modules, therefore it would be beneficial to start with its investigation and estimation.Further, to provide more actual value of negative emissions and adapting consequential LCA method based on real case studies will provide more knowledge on biogenic timing flow of wooden products.However, elaborating research including climate impacts from reusing and recycling processes could be more developed and applied in different building types.

Conclusion
This paper presents LCA of a reference building vs. its improved building design in terms of GHG emissions and financial costs, expressed in CO 2 e and SEK.The analyzed GHG emissions between different scenarios for reference and improved building design show great difference.The lowest amount of GHG emissions was found for the scenario 4 ′ Cradle to Gate + Biogenic Carbon + D module' for improved building design, by considering only material assessment in the production stage and its carbon storage together with external benefits after its the end-oflife.The scenarios that present negative GHG emissions in total outcome significantly rely on wooden building materials that are recognized as a carbon storage during its service life.It can be concluded from our results that inclusion of biogenic carbon accounting and external benefits after the end-of-life of building products can result in significant reduction of GHG emissions.Findings in our study shown "real" value of using wooden based materials as the main construction materials for a building.Concluding our results, CLT elements for the foundation, wooden roof, solar PV panels manufactured in Sweden and other followed wooden base materials present very low final embodied GHG, in some cases even negative embodied GHG emissions.However, it can be noticed that sometimes low carbon materials are not the best profitable solutions.In case of foundation, CLT could cost even double more than concrete slabs, while on the other side, easier construction and light elements will reduce logistic costs and looked as a better solution in a long run.
By summarizing our study, it can be pointed out that wood-based solutions, including its great advantage of carbon storage and increased use of them after the end-of-life, could be seen as the winning scenario for mitigating climate in the building sector.Inclusion of biogenic carbon will support/incentivize the use of wooden-based materials [4].Hence, the broad analysis involving Cradle to Grave scenario  for a whole building would provide complete "picture" of the flow of GHG emissions.Different scenarios will influence future decision makers to include biogenic carbon and benefits from D module in their assessment for having more accurate outcome of carbon accounting.Furthermore, presenting and following costs would give larger perspective for decision makers in built environment.

Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Table 2
Building materials emissions: GWP fossil and GWP biogenic .

Table 3
LCA results for 50 RSP presented in kg CO 2 e.

Table 4
LCA results for 100 RSP presented in kg CO 2 e.