3.1 Life cycle climate impacts (GWP) of original cases
In a first step, the environmental performance of the original case study buildings be2226 and SolarHouse are analyzed, starting with a focus on life cycle GHG emissions, based on the Global Warming Potential (GWP) indicator. Figure 4 presents these life cycle GWP results for both case study buildings, showing the contribution from different life cycle stages in stacked bar charts.
Concerning the be2226 building, the life cycle GWP is around 1221 kgCO2e/m²UFA. The most contributing life cycle module is the production stage (A1-A3), contributing about 497 kgCO2e/ m²UFA, or 41% of the life cycle emissions. Almost half of the whole life carbon emissions (45%) arise upfront, i.e. from the production and construction process stages (A1-A3 and A4- A5), which add up to 555 kgCO2e/ m²UFA. In a similar magnitude, the second largest contributor to GWP is the operational energy use (B6) with about 476 kgCO2e/ m²UFA or 39% of whole life cycle carbon emissions. The third most important life cycle module is the replacement of work sections and elements (B4.1 and B4.2) with about 100 kgCO2e/ m²UFA. In comparison, the results for the SolarHouse show that life cycle GWP is 399 kgCO2e/ m²UFA, not even 1/3 of be2226 results. Similar to the be2226, the most contributing life cycle module is the production (A1-A3). However, for the SolarHouse, the production contributes only about 221 kgCO2e/ m²UFA of the life cycle GWP, which in this case is a share of 56%. Upfront GHG emissions (A1-A3, A4, and A5) add up to 246 kgCO2e/ m²UFA (62% of life cycle GWP). The second largest contributor is, again, the operational energy use (B6), yet this time with a mere 90 kgCO2e/ m²UFA (23%). The third most important module for SolarHouse is cleaning and maintenance (B2.1, B2.2, and B2.3), which contributes almost 32 kgCO2e/ m²UFA (8%). Interestingly, replacement (B4) does not rank amongst the top 3 life cycle stages for the SolarHouse.
The first and rather striking observation in this side-by-side comparison (Fig. 4) is that, from a life cycle perspective GWP is more than three times as high for the be2226 building as for the N11 SolarHouse. Even tough similar life cycle modules are identified as the most contributing ones to GWP, this large difference is important to keep in mind when interpreting the relative contributions of different life cycle modules – or building elements, as will be done in the next section. While in relative terms, the upfront carbon emissions contribution is roughly around half of life cycle emissions for both buildings – 45% and 56% for be2226 and SolarHouse, respectively – a remarkable difference is noticed in absolute terms. Absolute upfront carbon emissions of the be2226 building and the SolarHouse show to be 555 kgCO2e/m²UFA and 247 kgCO2e/m²UFA, respectively, suggesting that the upfront carbon emissions of the SolarHouse are less than half of those from the be2226. For both buildings, B6 is the second most contributing life cycle module, albeit with 476 and 90 kgCO2e/ m²UFA, respectively. Here again, for be2226, this is 39% of whole life cycle GWP, while for SolarHouse it is just 23%. GHG emissions from replacement of work sections and elements (B4.1, B4.2) is remarkably higher for be2226, both in absolute and relative terms, and shows values more than five times higher than for SolarHouse. As we will see in more detail in the hotspot analysis, these differences are strongly influenced by the finishing materials and related replacement rates of either building. When focusing on material-related, embodied GWP across the life cycle (i.e., excluding operational energy use (B6)), the embodied GWP of massive brick be2226 is around 2.4 times that of the mass timber SolarHouse, with embodied GHG emissions mostly driven by the big difference in the impact of the production stage.
3.2 Environmental hotspot analysis of original cases
Most important indicators
In the second step, we go beyond GHG emissions and investigate how conclusions might be affected when considering potential trade-offs across different environmental impact indicators. To this end, we expand the scope of the analysis and investigate the results from a comprehensive life cycle assessment, considering 18 environmental indicators, as described in Methods and materials. To enable comparison of the contribution of different environmental indicators, an aggregated environmental score of environmental cost [30] is calculated, as described in Methods and materials. We identify the most important indicators as those that are cumulatively contributing 80% or more to the total aggregated score. The results for all environmental impact indicators are also provided as Supplementary Data.
Figure 5.a presents the results of the aggregated environmental single score for the original case study buildings be2226 and SolarHouse, respectively. In Table 3 we present the most important indicators with their absolute, relative, and cumulative contribution to the single score for be2226 and SolarHouse, respectively. We see that for both buildings, GWP has the highest contribution to the environmental single score – 45% and 27%, respectively. Second most important indicator is Particulate Matter (PM), which contributes 16% and almost 20% to aggregated environmental impacts, respectively. The next most important indicators are related to Human Toxicity (HT), where we consider both cancerous (HTc) and non-cancerous effects (HTnc). The HT indicators contribute 13% (both HTc and HTnc) for be2226, and 11% (HTnc) and 18% (HTc) for SolarHouse. For the SolarHouse we further find that Eutrophication Potential (EP) is amongst the most important indicators, contributing around 11% to the aggregated environmental score. For be2226, EP contributes around 8% but is not part of the most important indicators (those cumulatively contributing 80% to the environmental single score). So also in this analysis, the impacts related to GWP are identified as most important, highlighting the relevant of the findings presents in the previous section.
Table 3
Most important environmental impact indicators for both original cases, based on the environmental single score (environmental cost per m²UFA).
| Environmental single score: be2226 | | | Environmental single score: SolarHouse |
| Absolute | Relative [%] | Cumulative [%] | | | Absolute | Relative [%] | Cumlative [%] |
GWP | 61.04 | 44.59% | 44.59% | | GWP | 19.84 | 27.16% | 27.16% |
PM | 22.42 | 16.38% | 60.97% | | PM | 14.48 | 19.82% | 46.97% |
HTnc | 18.08 | 13.21% | 74.18% | | HTnc | 12.80 | 17.52% | 64.49% |
HTc | 17.79 | 12.99% | 87.18% | | HTc | 8.30 | 11.36% | 75.86% |
EP | 10.37 | 7.57% | 94.75% | | EP | 8.15 | 11.15% | 87.00% |
Most important life cycle stages
As a next step, we investigate which life cycle stages are the most important, based on the most important indicators. We analyze the contribution of individual life cycle stages to identify those that cumulatively contribute at least 80% to full life cycle impacts for a given indicator.
Figure 5.b presents the contribution of different life cycle stages to life cycle impacts for the identified most important environmental indicators: GWP, PM, HTc/HTnc, and EP, each with their original unit, starting with the most important indicator on top. The detailed results for the cumulative contribution (% values) of different life cycle stages are provided in Table 4 and Table 5. Further analyses are available in Supplementary Materials. Figure 5.b shows that the most important life cycle stage across all indicators is the production stage (A1-3). The production stage contributes between 41% (GWP) to 64% (HTc) and 39% (EP) to 72% (PM) across the most important indicators for be2226 and SolarHouse, respectively.
Table 4
be2226 (CS1) - cumulative share of impact from the different life cycle stages
GWP | PM | HTc | HTnc | EP |
A1-3 | 40.65% | A1-3 | 45.65% | A1-3 | 64.16% | A1-3 | 42.11% | A1-3 | 42.47% |
B6 | 79.65% | C1 | 62.98% | B6 | 87.13% | B6 | 80.12% | B6 | 75.11% |
B4.1 | 85.41% | B4.2 | 71.32% | | | B4.2 | 85.73% | B4.1 | 81.34% |
| | B6 | 86.38% | | | | | B4.2 | 85.53% |
Table 5
SolarHouse (CS2) - cumulative share of impact from the different life cycle stages
GWP | PM | HTc | HTnc | EP |
A1-3 | 55.55% | A1-3 | 72.22% | A1-3 | 67.73% | A1-3 | 37.95% | A1-3 | 39.00% |
B6 | 78.24% | C1 | 78.97% | B6 | 77.06% | B2.2 | 74.37% | B2.2 | 76.46% |
B2.1 | 83.32% | B6 | 83.40% | B2.2 | 85.43% | B6 | 84.56% | B6 | 84.34% |
For be2226, the second most important life cycle module that shows up across all indicators is operational energy use (B6) with a contribution from 15% (PM) to 39% (GWP). Other important life cycle stages for be2226 are the deconstruction and demolition process (C1) and replacement of work sections and elements (B4.1, B4.2) contributing 17% (PM) and up to 6% (EP, B4.1) and 8% (PM, B4.2), respectively. For SolarHouse, as well, operational energy use (B6) is amongst the most important life cycle stages, across all indicators. Its contribution ranges from 4% (PM) to 23% (GWP). Other important life cycle stages for SolarHouse are cleaning and maintenance (B2.1, B2.2) which contribute 5% (GWP, B2.1) and up to 36% (HTnc, B2.2) or even 37% (EP, B2.2). Again, the deconstruction process (C1) ranks amongst the most important indicators for PM, contributing almost 7% to life cycle impacts on that indicator.
Most important building elements
Building on the analysis of most important life cycle modules, which revealed the production (A1-A3) as the single most important contributor to life cycle impacts, this section now investigated the most important processes, i.e. building elements. The findings on the production stage again highlight the importance of embodied environmental impacts of buildings and confirm their crucial role as drivers of life cycle impacts, also in our low-tech, passive case studies. To gain a better understanding of the drivers of these material-related embodied impacts, we now focus on embodied impacts only, i.e. excluding operational energy use (B6) from the analysis. In the following, we analyze the most important building elements and their contribution to total embodied impacts for the most important indicators.
Figure 5.c presents the comparison of most important building elements and their relative contribution to total embodied impacts on the most important environmental indicators for be2226 and SolarHouse, respectively. Both items show the contribution of the main building elements classes, which individually show a substantial contribution to embodied impacts, namely: external walls (EW); internal walls (IW); internal floors (IF); flat roof (FR);pitched roof (PR), and ground floor (GF); as well as “other” element classes, which are relevant but have less contribution individually (this includes foundation, stairs, windows, external doors, internal doors, DHW system, PV systems, elevators). Analysis of the relative contribution of different building element classes (Fig. 5.c) reveals that the highest contribution across indicators and for both be2226 and SolarHouse stems from internal floors (IF). For be2226 and SolarHouse, IFs contribute between 27% (GWP) to 48% (HTc) and 25% (PM) to 54% (HTc), respectively. Second most important building element class in our analysis are external walls (EW), where contribution strongly depends on the environmental indicator and also varies between the two cases. EWs contribute 6% (HTc) to 26% (GWP) and 9% (HTc) to 37% (PM) for be2226 and SolarHouse, respectively. The third most important element class is less clear as it depends on both the environmental impact indicator as well as it differs across the two case studies. For be2226, internal walls (IW), flat roof (FR), and ground floor (GF) show impacts of similar magnitude across most indicators, each contributing between 3% (HTc for IW) to 12% (GWP, HTc for FR). The aggregated contribution of “other” element classes makes up 15% (GWP) to 29% (PM, HTnc) for the be2226. A relevant contributor within this aggregated category are the pile foundations (from reinforced concrete) of the be2226 building, which were required due to the specific ground conditions on the construction site of be2226. For SolarHouse, the third most important element class is the ground floor (GF) with a contribution of 11% (PM) to 24% (GWP). The pitched roof (PR) and internal walls (IW) show contributions of 4% (EP) to 10% (PM) and 1% (HTc) to 7% (PM), respectively. The aggregated contribution of “other” elements amounts to about 10% across each of the environmental indicators.
However, while the analysis of the relative contribution of the different building elements already revealed relevant differences between the two case studies, the vast difference in environmental performance only really becomes clear when analyzing the absolute values. Table 6 presents these absolute values for embodied environmental impacts of different building elements in detail. The table shows that, for example, within the most important element class of internal walls (IW), the GWP impacts on building level are twice as high for IW in be2226 (207 kgCO2e/m²UFA) than for SolarHouse (103 kgCO2e/m²UFA). Even more striking is the difference for external walls (EW), where we observe GWP impacts three times as high for be2226 than for SolarHouse, with 195 kgCO2e/m²UFA and 64 kgCO2e/m²UFA, respectively. This variation in absolute environmental impacts across the two case studies is very likely driven by the different materials applied in either case study. The question that arises, is whether embodied impacts could be effectively reduced by changing the buildings’ materialization strategy. To answer this question, we investigate in the next section, how the application of different bio-based construction material alternatives would influence, and potentially improve, the life cycle environmental impact results of the two case studies.
Table 6
Embodied impacts of the different building elements, including external wall (EW), internal wall (IW), internal floor (IF), flat roof (FR), pitched roof (PR) and ground floor (GF), for the most important indicators, absolute.
| be2226 | SolarHouse |
| EW | IW | IF | FR | GF | Other | EW | IW | IF | PR | GF | Other |
GWP | 194.82 | 95.51 | 206.93 | 85.55 | 47.58 | 114.73 | 64.42 | 8.33 | 102.60 | 23.81 | 74.16 | 34.91 |
PM | 0.10 | 0.04 | 0.19 | 0.04 | 0.03 | 0.16 | 0.15 | 0.03 | 0.10 | 0.04 | 0.05 | 0.04 |
HTc | 1.24E-06 | 5.17E-07 | 9.89E-06 | 2.38E-06 | 9.48E-07 | 5.65E-06 | 9.98E-07 | 1.29E-07 | 6.11E-06 | 9.78E-07 | 1.87E-06 | 1.24E-06 |
HTnc | 1.04E-05 | 4.84E-06 | 2.98E-05 | 6.68E-06 | 3.94E-06 | 2.22E-05 | 1.02E-05 | 1.54E-06 | 4.19E-05 | 3.67E-06 | 1.41E-05 | 8.46E-06 |
EP | 0.09 | 0.03 | 0.11 | 0.03 | 0.02 | 0.07 | 0.05 | 0.01 | 0.18 | 0.02 | 0.08 | 0.03 |
3.3 Environmental potentials of bio-based alternatives
In order to investigate the potential improvement of the embodied environmental impacts, scenarios for the two case studies are modelled based on the three bio-based building element sets from Mouton, Allacker and Röck [Reference to article #1], as presented earlier (see Fig. 3). These building element sets include a timber-based variant (TIM), a straw-focused solution (STR), and an option applying hemp-based construction products (HEM). After the deep dive into potential environmental trade-offs and burden-shifting across environmental impact indicators and life cycle stages, the analysis in this section again focuses on GWP, the environmental indicator identified as most important in the hotspot analysis. The results for all environmental indicators are available in Supplementary Materials. Furthermore, an in-depth assessment and environmental hotspot analysis of the individual building elements and their impacts per m²BE is presented in the original article [Reference to article #1].
Figure 6 presents the results for embodied GWP for the case studies in their original materialization and when applying the three alternative bio-based element sets. At first glance, Fig. 6 shows substantial reduction potentials in life cycle embodied GWP achievable through the application of the bio-based element sets. This is particularly the case for the be2226 case study, which in its original version is uses thick brick walls and reinforced concrete slabs.
Investigating the potential reduction in embodied GWP in detail, we find that most reduction potentials arise in the production stage (A1-3). In our analysis, the embodied GWP from production can be reduced by 142 kgCO2e/m²UFA (HEM) to 315 kgCO2e/m²UFA (STR) for the be2226 case. Furthermore, impacts from replacement of work sections (B4.1) could be substantially reduced, by between 59 kgCO2e/m²UFA (HEM) to 68 kgCO2e/m²UFA (TIM). From a full life cycle perspective, embodied GWP could be reduced by as much as 254 kgCO2e/m²UFA (HEM) to 434 kgCO2e/m²UFA (STR), which correspond to a reduction of 34–58% compared to baseline embodied GWP of the original be2226 case study. For SolarHouse, again the most reduction potential is for embodied GWP from the production stage. For the production stage, the bio-based alternatives could offer a reduction of embodied GWP by 28 kgCO2e/m²UFA (TIM) up to 91 kgCO2e/m²UFA (STR). Bio-based alternatives applied in HEM even increase embodied GWP by 18 kgCO2e/m²UFA. Upon detailed analysis, we further find that the application of the bio-based alternatives increases embodied GWP impacts for the SolarHouse in several life cycle stages. For example, embodied GWP for replacement of work sections (B4.1) and waste disposal (C4) could increase by 23 kgCO2e/m²UFA (STR) and 32 kgCO2e/m²UFA (TIM), respectively. From a full life cycle perspective, embodied GWP of SolarHouse could either be reduced by up to 24% or 73 kgCO2e/m²UFA (STR), or even increase by 11% or 34 kgCO2e/m²UFA (HEM), depending on the bio-based alternative applied.
3.4 Comparison with carbon targets for buildings
In the final step of the analysis, we analyze the results obtained for both the original version of the case study buildings as well as when applying the bio-based building element alternatives with carbon targets for buildings published in scientific literature. An overview of the process of developing carbon budgets and related carbon targets for buildings was recently presented by Habert et al. [4], from where we selected target values of studies that apply a similar RSP (60 years) [37, 42]. For this step of the analysis we calculate annualized values and compare our results with target values for embodied and total life cycle-related (i.e. sum of embodied plus operational) GWP per square meter and year (m²UFA/a), respectively. The carbon target values for buildings identified range from established 9 and 11 kgCO2e/m²/a (SIA 2040, as applied in [2]) down to more ambitious 4.5 and 6 kgCO2e/m²/a (Hollberg et al. in [4]) for embodied (blue) and full life cycle targets (green), respectively.
Figure 7 presents the comparison of climate targets for embodied (blue) and life cycle (green) GWP with the results for the original case study buildings as well as when applying the bio-based material alternatives. It shows that in their original version, neither the embodied nor life cycle targets are met by be2226, SolarHouse meets the established targets, but not the ambitious targets. Applying the bio-based material alternatives, we find that in all bio-based variants be2226 now meets the established climate targets for embodied emissions, falling just slightly short of also meeting the ambitious embodied GWP target when applying the STR variant. SolarHouse already performs comparably well in its original version, confidently within the established targets and falling just short of the ambitious targets for both embodied and life cycle GWP, respectively. The climate performance further improves when applying the bio-based material alternatives. For SolarHouse, applying the STR variant, i.e. the bio-based element set focusing on straw-based solutions, enables the case study to actually meet even the ambitious climate targets on both embodied and life cycle GWP, respectively.
An important note to add is that the scientific literature further suggests that the use of bio-based materials in building envelopes (such as hempcrete or straw) can result in less energy consumption for air-conditioning because of their excellent thermal and moisture buffering capacity [20, 43]
3.5 Limitations of this study
Evolution of the concepts
In this study we analyze two building case studies analyzing similar concepts, based on their original implication. Particularly the concept of be2226 has been further developed and advertised. It is by now also being applied for residential building typologies as well as with different material solutions, including bio-based variants (See implementation overview on the 2226 website). The environmental performance on these recent implications of the 2226 concept is very likely different from what our analysis showed, particularly regarding embodied impacts. For SolarHouse, future steps in further developing and scaling the concept are unclear.
Focus on embodied impacts
Our analysis investigates the full life cycle and environmental impacts across different environmental impact indicators. For the investigation of potential improvements to the original cases, we focused on potential improvements to embodied impacts by example of the GWP when applying bio-based material solutions. The investigation of potential improvements to operational energy use and related environmental impacts was outside of the scope of this study. An in-depth analysis of operational energy use (B6) of the be2226 building is presented in Maierhofer et al. [36].
Regenerative design strategies
This study investigates the environmental potentials of using bio-based materials for building construction [Reference to article #1] as well as the application of low-tech, passive building concepts (this article). Recent research, including the most recent IPCC AR6 WG3 reports, suggest that besides (energy) efficiency and consistency strategies (such as the change to renewable energy systems and bio-based materials), sufficiency-based design approaches have to be leveraged for reducing environmental impacts of both existing and new building construction and operation [44–46].
[1] Website of the 2226 concept incl. overview of implementations: https://www.2226.eu/en/implementation/