Strategies for connecting whole-building LCA to the low-carbon design process

Decarbonization is essential to meeting urgent climate goals. With the building sector in the United States accounting for 35% of total U.S. carbon emissions, reducing environmental impacts within the built environment is critical. Whole-building life cycle analysis (WBLCA) quantifies the impacts of a building throughout its life cycle. Despite being a powerful tool, WBLCA is not standard practice in the integrated design process. When WBLCA is used, it is typically either speculative and based on early design information or conducted only after design completion as an accounting measure, with virtually no opportunity to impact the actual design. This work proposes a workflow for fully incorporating WBLCA into the building design process in an iterative, recursive manner, where design decisions impact the WBLCA, which in turn informs future design decisions. We use the example of a negative-operational carbon modular building seeking negative upfront embodied carbon using bio-based materials for carbon sequestration as a case study for demonstrating the utility of the framework. Key contributions of this work include a framework of computational processes for conducting iterative WBLCA, using a combination of an existing building WBLCA tool (Tally) within the building information modeling superstructure (Revit) and a custom script (in R) for materials, life cycle stages, and workflows not available in the WBLCA tool. Additionally, we provide strategies for harmonizing the environmental impacts of novel materials or processes from various life cycle inventory sources with materials or processes in existing building WBLCA tool repositories. These strategies are useful for those involved in building design with an interest in reducing their environmental impact. For example, this framework would be useful for researchers who are conducting WBLCAs on projects that include new or unusual materials and for design teams who want to integrate WBLCA more fully into their design process in order to ensure the building materials are consciously chosen to advance climate goals, while still ensuring best performance by traditional measures.


Introduction
With high embodied and operational carbon relative to other industries [1,2], the building sector has seen increased demand for transparency in accounting for life cycle emissions.Life cycle assessment (LCA) is a process used to account for the environmental impacts of a product or activity and whole-building LCA (WBLCA), a subset of LCA, considers the environmental impacts of the materials and processes (e.g.transportation, construction) that are used to create buildings, both residential and commercial.This paper focuses on WBLCA.The term LCA may be used within this paper to describe some WBLCA processes or when speaking about LCA in a broader sense, such as describing current practices and standards.
Often, the LCA is performed after the design is completed, which provides valuable information about the completed design.While this approach provides data which can inform future designs, it wastes the opportunity for the LCA to analyze the impacts of design tradeoffs and impact the carbon contributions of a building through direct involvement during the design process.To enact real change and meet climate goals, LCA needs to be an integrated part of the design process so selecting the more favorable option or better yet, benchmarking against climate goals becomes the standard [3,4].Many common building materials have known impacts and high embodied carbon.New materials and processes with lower or even negative carbon impacts are emerging within the building sector; however, these newer or less common materials are often missing from WBLCA software tools, especially if the materials are just emerging in the construction market.For example, uncommon insulation materials such as vacuum-insulated panels, or wood fiber insulation are not commonplace in the construction industry and thus are not included in all WBLCA tools.This lack of data within existing WBLCA tools can result in misleading generalizations and makes the WBLCA process onerous, especially for teams that have not historically integrated LCA into their design process.This lack of reliable and readily available LCA data and processes is one of the reasons that the full LCA process is rarely used to inform decisions throughout building design and construction.
There is a need for innovative, iterative processes for harmonizing external information with existing WBLCA tools to shape the building design, consistent with the integrated design process (IDP) endorsed by the U.S. General Services Administration and adopted by Architectural/Engineering firms and others to eliminate waste and streamline design development to achieve optimal building performance across all disciplines [5].This need highlights the criticality of finding usable strategies for connecting WBLCA to the low-carbon design process.This work presents a procedural framework for providing iterative LCA feedback on a whole building design by using existing WBLCA tools in tandem with external sources for uncommon materials.This paper discusses the LCA within the context of IDP by asking the following research questions: 1. How can WBLCA be integrated into an interdisciplinary design process to optimize design iterations? 2. Which tools, workflows, and visualizations support the assessment of low-carbon strategies within such process/framework?
To answer these questions, we used the case study of the Circular Home, a modular, negative-carbon residence designed to support the circular economy through disassembly and reuse.This work is part of a Department of Energy Advanced Research Projects Agency-Energy (ARPA-E HESTIA) project aimed at developing novel materials and building concepts for decarbonizing the built environment [6].The paper discusses our strategies for conducting a whole building LCA for this project that utilizes some emerging, uncommon materials, in this case new bio-based, recycled, or low-carbon materials and processes that are not available in typical WBLCA tools.Optimizing material selection and use in building design can have a significant impact on our ability to meet climate goals [7], so expanding beyond the current limitations of WBLCA tools is essential.We discuss strategies for iterative design using LCA hotspot analysis techniques and define a feedback framework between the LCA team and the other design team disciplines involved in a design and construction project.These strategies and workflows are applied to the Circular Home, with examples and lessons learned from of applying the framework to this case study presented.

WBLCA tools
A variety of tools exist for conducting LCAs of products and processes, including tools specifically designed for conducting WBLCAs.Prominent WBLCA tools that consider the entire life cycle include Tally, Athena Impact Estimator for Buildings, and One Click LCA.Each of these tools has been used in recent LCA literature [8][9][10].These three tools use different background data and have different underlying assumptions, leading to potential incompatibility between the outputs of each tool [11].Comparative reviews of WBLCA tools show that the tools differ in important ways, such as data sourcing and regionality, and building information modeling (BIM) integration [11][12][13].
While WBLCA tools streamline the LCA process for buildings, they are constrained by the materials and processes defined in their databases, which often lack more novel or experimental materials and even some basic or common but necessary building materials such as mechanical, electrical and plumbing components.Understandably, WBLCA tools scope their databases to typical construction materials, so to perform a WBLCA on a building that uses experimental materials necessitates outside sourcing of environmental impacts for atypical or emerging materials.Even external to the tools, some materials may lack abundant information regarding their life cycle impacts, which leaves the LCA researcher responsible for finding and assessing the reliability of available information.Some guidance on evaluating the usability of sources of environmental impacts exists in literature, like Carbon Leadership Forum's (CLF) recommendations on assessing the reliability of Environmental Product Declarations (EPDs) [14].However, there is an outstanding need for examples of how to take external sources such as EPDs or LCAs from literature and identify and execute appropriate adaptations to ensure that novel materials are reliably compatible with the assumptions underlying an existing WBLCA tool.

The computational structure of LCA
ISO 14040:2006 defines the process for conducting a WBLCA, including the following steps: goal and scope definition, inventory analysis, impact assessment, and interpretation [15].ISO 14044:2006 provides practical guidelines for implementing these steps.These steps involve the systematic collection of environmental impacts for each material and process involved in the construction of the whole building.Mathematically, the computational LCA structure uses matrices of flows and environmental impacts [16], which are inverted and processed based on a demand vector defined by the functional unit of the product, in this case a building or system.The ISO standards for LCA emphasize the need for iteration in the LCA process (see figure 1).Ultimately, this computational method outlined in the standard produces a report of the life cycle environmental impacts of a building.
An analysis of more than a dozen publications since 2013 that identify technological workflows for WBLCA (see table 1) reveals that few have intended or successfully accomplished an IDP process that includes LCA in a fully integrated way.Most have restricted their efforts to a limited timeframe, such as early stage, early stage + detailed design, design completion, design option selection, or at discrete stages scattered throughout the design process.In our work we have expanded that iterative approach to include all stages of IDP for the entire building, ensuring the opportunity to optimize the balance between environmental impacts and all other performance targets (see figure 1).

LCA in the design process
Although the use is increasing, LCA analyses are still not a standard part of the design and construction process and are even less prevalent throughout the full IDP.Table 1 presents a selection of whole-building LCA literature, showing the design stage in which LCA is used and the method presented in the paper, which could include the development of an LCA framework/workflow, or a case study of a building, or both.Information regarding the LCA tools, life cycle stages, and impact categories are also shown.
Some literature emphasizes the value of LCA during early stages of design (table 1-'Early stage'), noting that the ability to influence design is greatest at the beginning [17][18][19][20][21]34].It is important to note that early LCAs may be less accurate as they are limited to predictive estimates, since design details material selections are subject to significant change as various disciplines work together on a design.Another strategy for LCA and by far the most common is including a full analysis after design completion (table 1-'Design completion') as a post-design calculation report or conclusion [22][23][24][25][26][27].Recent literature review of LCA in design revealed that most LCAs are reactive (after design completion), but that proactive methods have better potential to effect actual change, a factor acknowledged by those innovating in LCA capabilities for early design [34][35][36].Of the literature that incorporates LCA at multiple points throughout design (table 1-'Throughout'), most are unclear about how the LCA influences the design and are presented more as tracking mechanisms used at discrete points in the process rather than as a continuously evolving, iterative analysis, as part of an IDP that informs all disciplines [28][29][30][31][32][33].
Additionally, this previous work on LCA throughout the design process has often provided workflows for only select parts of the building life cycle, with many excluding use phases, which represent significant resource consumption [37,38].Others include few impact categories (e.g.only Global Warming Potential [GWP]) relative to the typical, complete LCA.A recent literature review of BIM-integrated LCA looked in detail at the methods researchers are using for WBLCA when a BIM is available [39].They found that most papers considered GWP, with only a small percentage looking at primary energy demand or other impact categories.It also found that A1-A3 were considered in a majority of papers, but that the other life cycle stages varied in coverage in the literature, which can result in missing critical parts of the process that have significant environmental impacts [40,41].
Overall, there is an established need for a flexible workflow that enables iterative LCA feedback during design in a way that proactively impacts the design as it develops in real time rather than just predicting or reporting [29].The method proposed here (final row of table 1) is intended to add WBLCA to the IDP as a A framework is defined here as a process or tool for conducting LCA.
b Conventions for reporting life cycle stages vary, so the closest possible match was selected, where cradle-to-gate = A1 through A3, gate-to-site = A1 through A5, cradle-to-grave = A1 through C4, cradle-to-cradle = A1 through D. c Conventions for reporting impact categories vary, so the closest possible match within the TRACI 2.1 methodology was selected.For example, 'life cycle carbon emissions' were generalized as GWP.Abbreviations include global warming potential (GWP), acidification potential (AP), eutrophication potential (EP), ozone depletion potential (ODP), smog formation potential (SFP), photochemical oxidant creation potential (POCP), abiotic depletion potential-elements (ADPE), abiotic depletion potential-fossil fuels (ADPF), primary energy demand (PED), primary non-renewable energy demand (PEDnr), and primary renewable energy demand (PEDr).first-order member of the design team, with impact in all stages of design, and improved accuracy using post-processing methods to more fully populate the life cycle stages and impact categories for a complete and reliable analysis.The literature outlined in table 1 involves WBLCA in the building design process to a limited extent; however, a fully integrated and iterative process throughout design remains an important gap in the literature.The framework presented in this work is unique in the following ways: • Integrates iteratively with the design model throughout design, with continuous feedback in both directions between LCA and all other disciplinary team leads • Enables integration of novel building materials that are not available in WBLCA tools • Enables hotspot analysis for any impact category available in the WBLCA software (including, but not limited to GWP, as shown in table 1)

• Enables flexible incorporation of any life cycle phase (see table 1)
• Creates underlying workflows needed for complete LCA upon project completion

Traditional project delivery
Traditional approaches to design and construction often separate the builders from the designers, using methods such as design-bid-build, where architects and engineers complete the building design entirely before handing it off to the construction team [42].In recent decades, there has been a push to include all members of a project team in design conversations from the beginning.The MacLeamy curve (figure 2) shows the shift in team involvement and effort between traditional project delivery and integrated project delivery.This curve-credited to Patrick MacLeamy, architect and former chairman of HOK, a global architecture, engineering and planning firm-is a concept that has seen decades of discussion in design and construction, encouraging shifting interdisciplinary collaboration and constructability review earlier in the project, where the ability to influence the cost and performance of a building is highest and design changes are less costly [43].Clearly, the forward-shifted effort curve allows greater design influence at a lower cost, which is advantageous to project teams.However, when looking at this workflow with a low-carbon construction lens, it is rarely implemented this way in practice since project teams still typically have no requirements for LCA implementation in U.S. construction, especially if the project is not seeking any type of environmental impact certification.Additionally, most project teams lack a defined workflow for integrating LCA into project processes.

Scope of current work
This work describes a framework for integrating WBLCA into the building design process in an iterative, interactive manner, where design and WBLCA inform each other throughout the entire design process.The process involves utilizing a WBLCA tool in conjunction with additional sources of life cycle impacts, external to the WBLCA tool, for materials that are not included in the existing tools.These sources can include EPDs, LCAs from literature, or life cycle inventory (LCI) databases.To demonstrate the utility of the framework, a case study involving a negative carbon, modular, cross-laminated timber (CLT)-based building designed for disassembly and reuse is presented.

Theoretical approach
To establish a workflow involving continuous feedback throughout design, the team considered traditional LCA approaches and how the structure of interactions in this work would differ.Figure 3 shows a modified version of the MacLeamy curve, applied specifically to WBLCA effort/involvement in the building design and construction process.The modified workflow diagram includes a line showing the practical reliability of BIM for conducting LCA as the model develops.Introducing LCA at the conceptual design stage and committing to ongoing, iterative feedback throughout design development, as with all other design considerations and performance targets, ensures that LCA can become more accurate as the building design approaches final development.As a result, LCA becomes a useful driver rather than a disassociated afterthought.As the building design becomes more developed and certain, the LCA can become more accurate.The traditional LCA approaches are shown in shades of grey, with the predictive model on the left during preliminary design where ability to impact performance is high but model reliability is low.The reactive model shown on the right is based on LCA conducted only after substantial design completion, where the model accuracy is higher, but the ability to impact performance is lower.The framework presented in this work proposes to utilize LCA from early design through design completion in an integrated way, where LCA results are generated frequently as the design develops and are used to inform design changes.A point of departure for this work is the continuous nature of the feedback.Rather than a calculation performed at prescribed points in the process based on level of model development [21], this method encourages frequent iteration at any design change or proposed design change.

Interdisciplinary collaboration workflow
The first step in developing an iterative design workflow is to define a feedback structure between all disciplines of the design team: architectural, LCA, structural, mechanical, energy/moisture, and techno-economic, with constructability considerations intentionally interwoven throughout.Figure 4 diagrams the interactions between team members as the design is created/revised by the architectural team and reviewed/analyzed by the other disciplines.Weekly meetings provide a forum where each discipline shares updates/concerns that may have an impact on the building design, which in turn impacts the other disciplines.Additionally, a digital file sharing structure is essential for model sharing and collaborating on other supporting documentation.These tools enable the team to see disciplinary updates and build a communication structure as the model develops.Since the Circular Home case study is a research study intended as a proof of concept, the team utilizes a simplified version of model sharing via Autodesk Docs; in a more typical IDP, the team might utilize a more fully integrated BIM structure with real-time interdisciplinary model integration.For this project, the architectural team has ownership of the architectural model and any updates that are incorporated into it, while the other subject matter experts react to the design updates and provide discipline-specific feedback.In turn, the architectural team, the BIM curators in this project, receive the feedback and incorporate revisions into the model where necessary  (figure 4).In projects where there are discipline-specific models within the BIM, each discipline lead would be designated as a 'BIM curator' and would iteratively update the model, as shown in figure 4. LCA targets should be included as performance goals of equal importance to the traditional targets of functionality, cost, and energy efficiency.Because this project's funder mandated carbon negative performance cradle-to-grave, the feedback from the LCA team is given significant weight.For project teams without standing LCA leads, it will be necessary to designate personnel and resources to complete this work.With a net negative carbon goal, when immutable structural or construction requirements negatively impact LCA, other disciplines must be enlisted to reconcile the difference.This can only be accomplished through an IDP with LCA weighted as a full partner in the design process.University of Washington's CLF provided advice to the LCA team through webinars, quarterly one-on-one meetings, and a framework document outlining WBLCA best practices.Although they did not directly influence the design iterations, the guidance they provided helped frame the scope and interpretation of the LCA content, which in turn informed the design iterations.For other projects that seek to incorporate this model, consultation and feedback from third party verification can be a beneficial way to periodically evaluate the LCA results as they develop.Additional specialty industry partners can add critical advice and clarity as needed; for example, the Circular Home is designed for disassembly and reuse, so one of the specialty partners we use on the project is a fastener manufacturer familiar with the capabilities and limitations of heavy-duty structural fasteners.

Model sharing workflow
After establishing the interactive workflow between the disciplines involved, a framework for model development and sharing was established.Figure 5 shows the interconnected relationships of the discipline-specific models, with the LCA component expanded and outlined to show the LCA sub-processes involved.All design disciplines shown have similar sub-processes, but LCA is emphasized since it is not traditionally included in IDP.At the core of the model sharing is the BIM, which contains the architectural design and embedded information such as materials and dimensions.The BIM serves as a central source of information about the design, providing necessary information for each disciplinary model.For each of the disciplines, receiving an updated BIM impacts the performance, necessitating continuous feedback in both directions.For the LCA model, besides receiving an updated BIM, the model also relied on the outputs of the energy model ('Building Energy Use' in figure 5), specifically for module B6, operational energy.Just as the LCA process must follow ISO 14044:2006, the energy model must use ANSI/ASHRAE Standard 228-2023, Standard Method of Evaluating Zero Net Energy and Zero Net Carbon Building Performance [44].Energy performance results were provided to the team frequently; each time the energy model was optimized using parametric simulation, the team would provide new estimates of the operational energy to determine the potential magnitude of the impact, keeping in mind that quantities of insulation and equipment efficiencies were continually in flux, since they were additionally subject to constructability and cost feedback.

LCA framework
After the initial project scope was determined, team collaboration established, and an initial building information model developed, there was a need for quantified feedback from each discipline regarding the performance of the initial model.A subsequent critical step in the LCA process was to create a workflow that would enable iteration as design changes were implemented and materials were selected.We assessed the usability of WBLCA tools for this project using existing literature and trial versions of the software, where available.Tally was selected for initial assignment of LCI information to the materials and processes involved in the Circular Home.Tally integrates datasets for LCA modules A1-A3, A4, B2-B5, C2-C4, and module D. Data is consistently sourced from the GaBi dataset which facilitates the integration of additional data from ecoinvent and GaBi (shared data source).As a Revit plugin, Tally enables seamless integration with the building information model.Additionally, Tally is among the WBLCA tools that have the advantage of providing transparency about the LCI source for each material and the process-based defaults, such as end-of-life assumptions.It can output a spreadsheet that includes a line item for each unit process with the associated quantities for each impact factor.Tally uses a process-based approach to data sourcing, which is often considered the most reliable method [13].Additionally, since its underlying assumptions are geographically specific to the U.S., Tally is well-suited to U.S.-based projects.The Circular Home is being designed for specific, representative locations in the U.S., so the team benefitted from the accuracy and precision of Tally's default inventories.
Figure 6 shows the technology workflow used in the iterative LCA.The materials available in Tally were inventoried and compared to the initial list of materials that would likely be used the home design.Each building element that had a material match in Tally was assigned, and details such as material finish and take-off method were selected.As the model geometry changed, the information in Tally remained tied to the material, so updates to the embedded LCA information were automatic.Many of the materials used in the design were available in Tally, but several were not, such as foam glass gravel foundation insulation, rigid wood fibre wall and roof insulation, and the photovoltaic panels for the roof.To enable the addition of these materials into the LCA, it was necessary to transition to a different tool.A spreadsheet of quantities and environmental impacts for each material and life cycle stage was exported from Tally.This spreadsheet was then imported into R to enable the addition of new materials.R is an open-source computational tool used widely for statistical analysis [29], though any tool that enables data input, manipulation, and matrix inversion can be used, such as Python or MATLAB.A custom script was authored in R to enable the integration of the new materials in an organized and iterative fashion.Some materials can be modeled in Revit but may not have a perfect match in Tally.For example, a layer of wall insulation can be properly described in Revit, but Tally does not have a rigid wood fibre insulation option.For situations like this, it is possible to assign a placeholder material in Tally to retain an accurate volume output.In post-processing, the environmental impacts of this placeholder must be swapped out for those from an external source, making sure to match the functional unit.Some materials may not typically be geometrically modeled in Revit, and these components must be added manually in R. For example, EPDs for photovoltaic panels and associated system capacities are not always easily modeled in Revit nor do they have available material information in Tally, so the associated information can be added as a custom line item in the programming script based on external calculations.It is important to note that the analyses conducted by other disciplines, such as the techno-economic assessment, are conducted simultaneous to the WBLCA, but with different software workflows, appropriate to each discipline.All disciplines operate under the same assumptions regarding the materials and processes involved in the building design and construction, shared through the model, team meetings and communications.
One of the inherent challenges with performing an LCA of a structure that incorporates novel materials is finding and harmonizing the necessary environmental impact information from various sources.To obtain the environmental impacts for each life cycle stage for these externally sourced materials, the team used manufacturer EPDs.Other sources that could be considered are industry average EPDs, LCAs from literature, and other LCI databases such as the Federal LCA Commons [30].When sourcing external information, it is important to ensure compatibility with the default dataset, with considerations such as which life cycle stages and impact categories are included, and underlying assumptions, defined by product category rules [45].Considerations like data quality, specificity, transparency, and comparability should drive source selection, as different assumptions can lead to significantly different results [14,46,47].Taking careful consideration in selecting a compatible source limits the need for significant adaptation, and the impacts can simply be added into the matrix, where the formatting becomes fully integrated into the existing WBLCA information and can be used for calculations and visualizations.Sometimes complete information for newer products is not available, which requires additional sourcing.For example, some EPDs do not include information on the use and maintenance impacts for their particular product, so these life cycle phases must be filled in using other compatible information, always taking care to note the data source and underlying assumptions.For the Circular Home, only ∼10% of the total number of materials were not in Tally and required a separate source of information, such as an EPD.Even if the missing data is for a small number of components, if those materials-chosen specifically for their carbon impact-make up a large proportion of the building by weight, they become driving factors, and accuracy is vital.This makes the post-WBLCA tool calculation an extremely important aspect of the workflow.As WBLCA tools continue to develop in both material availability and flexibility of user-input, the exact workflows will evolve, but the principles of data integration will remain consistent.
From these processes, a dataframe of unit processes is produced that has a row for each material instance and life cycle stage pair.This information can be used to create a hotspot analysis that shows which materials and processes contribute most to the carbon footprint and other environmental impacts of the building.This hotspot analysis was performed using the plotly package in R [48].To obtain overall values for each environmental impact based on the functional unit selected for the building, matrix inversion was used to scale the materials and processes.A matrix of unit processes (A) was created with a rows and a columns, with a representing the number of unit processes and corresponding quantities/units.A corresponding environmental matrix (B) was created with b rows and a columns, with b representing the number of environmental impact categories being studied.A final demand vector (f ) was defined based on the functional unit using the computational methodology of Heijungs and Suh [16].Scaling factors were solved for by inverting the technology matrix and multiplying by the final demand vector, using the solve() function in R, part of the base package [49].Then, the result vector (g) was produced by multiplying B by the scaling factors.The result of this matrix inversion is a vector of environmental values (g) that correspond to the total impacts for the functional unit across all life cycle stages.For the Circular Home, the global warming potential (GWP) was particularly important, as negative carbon was an overarching goal throughout the project.The environmental impact categories that can be visualized and analyzed using this process include: • Smog formation potential [kgO 3 eq], • Primary energy demand • Primary non-renewable energy demand • Primary renewable energy demand These are the impact factors native to Tally and are based on the Tool for Reduction and Assessment of Chemicals and Other Environmental Impacts (TRACI) 2.1 characterization scheme, which was developed by the US Environmental Protection Agency (EPA) and is the standard format for North American LCAs [50].

Results and discussion
When following the process described above, several technological and personnel resources are needed.Table 2 outlines a task list of software and information tools that would be required to perform an iterative LCA throughout the design process, as described in the methodology.
The examples from table 2 are from the workflow used for the Circular Home; however, it should be noted that any software that fits the capabilities described can be used within this framework.For example, OneClick LCA is compatible with Revit and provides a detailed report in spreadsheet format that displays the impacts from a variety of impact categories, separated by material and life cycle stage.This could then be post-processed in Python using supplemental information from LCAs in published literature.This is one example of an alternative set of software that could be used to perform WBLCA within the framework described in this paper.

Key output: visualization and iterative design process
A key output from this work was the establishment of a workflow that enabled iterative design feedback.The team discovered that a carbon heatmap was a particularly useful tool in the design process, especially since  the Circular Home case study involved requirements for carbon negative embodied and operational performance.This dynamic carbon heat map color-codes each building component at each life cycle stage according to the magnitude and sign (±) of the carbon impact in the context of the current building design (see figure 7).The heat map method of visualization has seen effective use for LCAs in other disciplines, with occasional use in WBLCA [51].While GWP was selected as a primary metric of interest for this work, as in several of the other frameworks and case studies mentioned in table 1, this heatmap mechanism can be used for any of the impact categories available in the WBLCA tool.To understand the impacts each material and process have on the other environmental factors, the code would be modified for a heat map to display the impacts from another category, for example, AP, where the units would then be kgSO 2 eq.These additional impact category maps could be compared side by side with the other impact categories to see the relative contributions of each material and process to each impact category of interest.In addition to flexibility in which metric is displayed, this work utilized a scripted calculations approach to data processing after the WBLCA tool was used to the maximum extent possible.This flexible post-processing approach allows for additional dimensions to the analysis that are not typically afforded by WBLCA tools, such as dynamic LCA [52], changes to end-of-life assumptions, and additions of missing life cycle stages of materials.This can be particularly useful when incorporating emerging findings and understanding about the impacts of circularity in WBLCA [53].
In the heat map in figure 7, materials are ordered along the y-axis by magnitude of cumulative impact across all life cycle stages.This allows the design team to see which materials or components most impact the building performance and to focus efforts on these high-impact areas.By seeing a material or component's contribution in context of the whole structure, teams can maximize the impact of the material and process alternatives that they explore.Each design change produces a new heat map, which can be compared to the previous.
Operational energy (module B6) was excluded from figure 7 because the magnitude outweighs the other materials, obscuring the detail between components.On-site Construction (module A5) was excluded due to a lack of available data at the time of analysis.Both of these modules are included in the overall analysis of the Circular Home example and certainly impact trade-off decisions.
The Circular Home example is dominated by bio-based materials with negative carbon impacts, so much of the color scale is dominated by negative GWP values.
The whole-home sums of GWP are marked with arrows in figure 7, allowing for visual comparison of the difference a design change might make.This visual is slightly different than what would be seen within the software; the exact numerical values for the cumulative total are displayed above the heat maps for internal team use, allowing direct comparisons of the quantitative impact a design change would have.These exact numbers are excluded from figure 7 since these design iterations are taken from the middle of the iterative process and the authors want to avoid erroneous assumptions about the preliminary numbers.The arrows serve as a placeholder for visual comparison.In the example shown, by substituting expanded polystyrene (EPS) insulation (figure 7 left) with wood fibre insulation (figure 7 right), the team was able to note significant reduction in the total GWP for the whole home.Since the materials are sorted by magnitude of total impact, it is easy to see that the EPS is a high contributor (figure 7 left) and that it would be beneficial to the team's low-carbon goals to substitute a bio-based low-carbon material like wood fibre insulation, which becomes the second lowest carbon contributor in the home (figure 7 right).
While materials were still being selected, the team iterated with new design configurations and new materials to gauge whether a proposed idea would have a detrimental or beneficial impact to the overall carbon performance of the building.Some examples of changes to the initial design or materials that have been tested within this framework include: a modified window configuration, a modified roof truss configuration, various types of wall insulation, various envelope designs, photovoltaic panel materials on the roof, and preliminary modified end-of-life assumptions that maximize material reuse and avoid traditional landfill or incineration pathways for the CLT and wood fibre insulation used in the building design.
The heat map is used in the design process of the Circular Home to understand the impact that design choices would have on the carbon performance of the building.For example, the architecture team suggested reducing the overall thickness of wall insulation to improve constructability.The energy team determined that the reduced efficiency of thinner wall insulation could be offset in part with more efficient triple pane windows in place of double pane.However, additional window glass and framing materials increased net embodied carbon, as did reducing the quantity of carbon-sequestering wood fibre insulation.The LCA team was then able to confirm that the substantial carbon negativity of the structural CLT easily absorbed this change, maintaining net-negative embodied carbon.The LCA team also effectively calculated the impact of several other proposed design modifications, including alternate foundation designs to avoid putting carbon negativity at risk due to the extreme carbon density of traditional concrete, and the feasibility of remaining embodied carbon-negative despite maximizing photovoltaic panels on the roof.Understanding the magnitude of the embodied carbon penalty associated with the PV system was key to providing flexibility for insulation choices and equipment efficiencies to streamline construction and control costs.
As the model developed, the level of detail increased and the certainty of material choices increased, leading to increasingly greater confidence that the LCA results are indicative of potential real-world performance.
By providing a clear, dynamic visual that adapted with each design change, all disciplines could be aware of how even small design changes they made would impact the environmental performance of the building.Some literature has considered interactions between multiple disciplines, such as operational energy consumption trade-offs within envelope design [54].The workflow described in this paper emphasizes the need for communication not just between individual disciplines and the LCA team, but between all project participants.
While many of the previously mentioned frameworks have provided excellent examples of how to compare major design options in early stages (see table 1 'Early Design' literature), this work provides a process for understanding not only large decision dynamics but also subtle changes that occur as a design develops.These changes may or may not have significant impact on the balance of environmental impacts, but this framework allows the question of impact to be addressed quickly and efficiently.

Recommendations for data sharing
As we grappled with LCA data sourcing challenges, our team has also developed a key idea for sharing LCA results for other architecture and building teams to use.Due to ongoing challenges with sustainability data on building materials and components, we encourage research and design teams to publish a unit process appendix in a public forum.For example, table 3 shows what information a unit process table would provide, based on an example from the Circular Home.For a whole building, this would take the form of a matrix of information for each material assembly and life cycle stage.The purpose of this table is to make each unit process and impact results freely available for other teams to adapt.By sharing data this way, we are building an open and transparent resource for future researchers to utilize and enhance.transition to open-source and transparent models to enable ease and accuracy of LCA implementation.The process outlined in this work requires a basic understanding of coding, although most platforms provide sufficient functionality so knowing a particular language is not required.In fact, this process could also utilize spreadsheet-based computation, although lack of the automated functionalities of a coding script might slow the process down.Future work in this space could include a comprehensive analysis of WBLCA tools from literature, considering their benefits or drawbacks as part of an IDP.This process also requires an understanding of LCA standards and methods, which may require time investment for teams that do not already include this capability.
Finally, this process requires a building information model, which, while standard practice for building design in many firms, is still not always utilized and may not be available in conceptual design phases.Data management, transparency, and sharing within project teams is a noted limitation of some infrastructure projects for conducting LCA [55], so as these processes continually improve, so will the impact of LCA in IDP.
The authors acknowledge ongoing conversations about the separation of biogenic and fossil carbon as well as surrounding the forestry impacts and trade-offs of CLT.This work uses standard practices at the time of publication and the framework outlined in this work enables flexibility to incorporate new findings as they emerge.

Conclusion
At the end of the case study, the authors reviewed the research questions in detail.
R1: how can WBLCA be integrated into an interdisciplinary design process to optimise design iterations?We found it is possible to create an iterative and flexible approach using several pieces of software and frequent communication.The IDP was used as a high-performance benchmark for all disciplines, with LCA treated as a key player in the design process.The feedback structures that teams already successfully implement can be simply augmented by the inclusion of an LCA lead early on and throughout the design process.
The authors emphasize the criticality of consistent communication between team members, a skill that seems obvious but is often lacking in interactions within technical fields [56].Weekly check-ins were essential to the process, where technical leads would present the impact of design changes on model performance within their discipline.Frequent between-meeting communication alerted all team members to potential areas of concern, such as if a design change had significant impacts on the carbon performance of the model.R2: which tools, workflows, and visualizations support the assessment of low-carbon strategies within such process/framework?A streamlined workflow required a BIM model connected directly to an LCA tool like Tally.The team also required a post-processing software tool like R to incorporate newer materials that did not yet exist in the WBLCA tools.By using a script to automate the post-processing steps, each new development represented only incremental effort in the calculations.We developed hotspot visualizations that allowed all members of the design team to review the relative impact of specific material choices quickly and efficiently.This quick iterative procedure within the LCA discipline allowed frequent feedback as the other members of the design team iterated within their own disciplines.By meeting weekly and sometimes more frequently, interdisciplinary results could build off each other, creating a robust and optimized design.The framework described in this paper has been extremely valuable for the success of the Circular Home as disciplines work together to meet requirements and produce the best performing design possible within the given constraints.
Having LCA input at every design discussion has led to increased confidence within the team that the building is meeting specifications as intended.Due to these successes, the authors are optimistic about similar processes being implemented within the design workflows for other building design teams, both in research and industry.
Overall, the process for conducting LCA described in this work is novel in its elevation of the LCA practitioner to full design team member by incorporating iterative, consistent LCA feedback throughout the design process.The case studied-a carbon negative CLT-based residence designed for disassembly and reuse-is also notable and while the scope of this paper is limited to describing the iterative LCA design process, future work will publish complete LCA results.

Figure 2 .
Figure 2. Traditional MacLeamy curve showing design/construction effort curves and where they fall within the design/construction stages for traditional project delivery (for example design-bid-build) and integrated project delivery, which involves all members of the design and construction teams early in the design stages.

Figure 3 .
Figure 3. Modified MacLeamy curve showing WBLCA effort curves based on traditional predictive and reactive LCA techniques, as well as the iterative method of LCA proposed in this work.

Figure 4 .
Figure 4. Interdisciplinary collaboration framework between project participants in IDP.

Figure 5 .
Figure 5. Interdisciplinary modelling workflow and boundaries of LCA.

Figure 6 .
Figure 6.Technological workflow of LCA design iteration process.

Figure 7 .
Figure 7.Example heat maps from the Circular Home showing relative impacts of the materials/assemblies involved in this design iteration by life cycle stage for a home using expanded polystyrene insulation (left) vs wood fibre insulation (right).Materials are ordered by cumulative impact across all life cycle stages (see far right column), with highest impact at the top of the y axis.An arrow shows the whole home total for visual comparison (in the software, this would be displayed as a numerical value, but exact values were removed here to avoid erroneous conclusions from mid-process snapshots).

Table 1 .
Selected literature showing whole-building LCA frameworks and case studies at various stages in the design process.

Table 2 .
Task list of planning items to consider for iterative LCA in IPD.

Table 3 .
Example of unit process table (rebar transport from the Circular Home's stem wall foundation) that should be included in whole-building LCAs when published.Full LCAs would include a matrix of all unit processes involved.Overall, while the process outlined within this work is flexible and robust, it is not without limitations.The WBLCA tool used in this work (Tally) is not open source, which limits usability to those whose means and priorities enable purchase.The authors urge software developers and LCI curators in the LCA space to