A perspective on tools for assessing the building sector’s greenhouse gas emissions and beyond

Increasing impacts from anthropogenic climate change, coupled with the rising need to provide safe and healthy buildings in which people can live, work, and play, necessitates methods and tools for decarbonizing the building sector. Governments, industry, and others are interested in assessing both the embodied and operational greenhouse gas (GHG) emissions of buildings. Stakeholders have embraced whole building life-cycle assessment (WBLCA) as a framework for quantifying the life-cycle impacts of buildings, from raw material extraction to the building’s end of life. The purpose of this perspective is to offer an analysis on prominently used WBLCA tools, focusing on how well the tools are suited for assessing the embodied and operational GHG emissions from all phases of a building’s life cycle, and to suggest recommendations for improving the tools. Existing WBLCA tools can provide a detailed assessment of most materials used in the building’s core and shell but lack the capability to quantify impacts accurately and comprehensively from all building systems as well as from the construction, transportation, operation, and end-of-life phases. Suggested short term improvements for the tools include: (1) increased standardization among tools and environmental product declarations (EPDs) to allow for detailed comparison among different material options earlier in the design process; (2) incorporation of verified, local-manufacturer EPDs for all building materials, components, and systems and of specific on-site conditions; and (3) integration of tradeoffs between embodied and operational design decisions. We need to move beyond the prevailing approach of using WBLCA tools to select building materials that have the lowest embodied footprint. Future WBLCA tools need to be able to assess, in detail, how different design, construction, transportation, operation, and end-of-life decisions for a building not only affect GHG emissions, but other key sustainability goals including resilience to climate change, environmental justice, and human health of local communities.


Introduction
Increasing impacts from anthropogenic climate change, coupled with the rising need to provide safe and healthy buildings in which people can live, work, and play, necessitates methods and tools for decarbonizing the building sector.Greenhouse gas (GHG) emissions from the building sector comprise almost 40% of the world's annual footprint, with electricity and natural gas use in buildings accounting for the majority of emissions [1].Assessment of building operation impacts is a relatively advanced field, with efforts born out of the 1970s energy crisis aimed at improving energy efficiency and reducing energy consumption in buildings leading to the development of sophisticated modeling approaches [2].Recognition of the importance of mitigating embodied building sector emissions, especially within the context of rapidly decarbonizing energy and transportation sectors, is emerging [3,4].Governments, industry, and others are interested in assessing both the embodied and the operational GHG emissions from buildings.Stakeholders have embraced whole building life-cycle assessment (WBLCA) as a framework for quantifying the life-cycle impacts of buildings, from raw material extraction to the buildings' end of life.The framework follows the European Standard (EN) 15978:2011 [5], which divides GHG emissions, colloquially referred to as embodied and operational carbon, into life-cycle modules as depicted in figure 1.A recent update to EN 15643 [6] incorporates new stages not depicted in figure 1, including A0 (Pre-Construction Stage), B8 (Building User Activities), and D2 (Exported Utilities).
WBLCA tools, which are distinct from existing LCA software such as GaBi and SimaPro, follow figure 1's framework and rely on incorporation of environmental product declarations (EPDs).EPDs are standardized, third-party verified documents on the life-cycle environmental impacts of specific products [7].In theory, these tools and EPDs offer stakeholders the opportunity to efficiently quantify the whole carbon impacts from all life-cycle stages of buildings.Stakeholders are beginning to use WBLCA tools and EPDs for several specific reasons, including: • Compliance by contractors/project developers/owners with regulations and policies mandating reductions in embodied carbon in buildings and other projects (e.g., California's AB 2446 [8], the Federal Buy Clean Initiative [9], the European Union's level(s) framework [10]).• Comparing impacts between two different design strategies (e.g., tradeoffs between a more insulated building envelope compared to using reduced HVAC equipment and distribution).• Earning points towards certification in green building rating systems (e.g., LEED [11], Green Globes [12], BREEAM [13]).• Selecting the perceived 'best' environmental option by owners/developers/designers/contractors.• Driving the environmental footprint of products lower as markets compete for product share of the most environmentally friendly option.
Consideration must be given when selecting WBLCA tools and EPDs so that stakeholders can appropriately interpret results and understand the limitations tools have in answering questions about buildings' environmental and other impacts.This perspective aims to fill an important gap for building stakeholders by offering an overview of commonly used WBLCA tools and EPD databases, proposing action items for improving WBLCA and identifying overarching perspectives for managing the climate-change implications of the building sector.

Overview of WBLCA tools
There are multiple emerging tools and calculators used for assessing the total GHG emissionss of buildings.Three of the more fully developed, design-integrated tools are OneClick LCA [14], tallyLCA [15], and Athena Impact Estimator for Buildings [16].The Carbon Leadership Forum maintains a list of additional tools [17] that are intended for analysis of building assemblies and structural components; some of these additional tools can be used to conduct broad-based WBLCAs using general information such as building size and structural framing system but they are not as complete as the aforementioned three.The most prominent building materials and components EPD database is the Embodied Carbon in Construction Calculator (EC3) [18].While primarily a construction materials database of EPDs, EC3 also supports assessments of whole-building impacts and has expanded to include a building planner comparison tool where users can estimate impacts from A4 (Transportation), A5 (Construction), and B6 (Operational Energy Use) life-cycle stages.Additional databases that host relevant EPDs of building components, equipment, and technologies include: (1) the Sustainable Minds Transparency Catalog [19] (which also includes health product declarations), (2) the SPOT Sustainable Product Database from UL Solutions [20], (3) the International EPD System from EPD International AB [21], and (4) the Institut Bauen und Umwelt e.V., a German/European EPD database [22].Table 1 provides an overview of relevant criteria for assesing and comparing the WBLCA tools.do not yet account for the expanded life-cycle framework of EN 15643 and therefore do not explicitly address impacts related to grid-interactive buildings, which would be accounted for under the 'D2' life-cycle stage.A detailed comparison among the tools, including a discussion on the tools' limitations, is explored in section 3.

Current benefits and limitations of WBLCA tools
The WBLCA tools described in table 1 are designed, and currently best suited, for decisions around material selection for structural systems and building exteriors (e.g., facades, windows) and interiors (e.g., walls, floors).The tools are relatively easily integrated with Building Information Modeling (BIM), making it efficient to explore different design options and material choices upfront.
There are gaps within the WBLCA tools that should be filled.The tools are capable of assessing emissions from procurement (i.e., material selection), construction and transportation activities, and building utilization (i.e., operation, maintenance, decommissioning), but there is less customizability for users to accurately reflect a project's local conditions or the grid interactivity of the project.The WBLCA tools rely on average annual grid emission factors at regional or national scale.There is research indicating that annual average grid emission factors undercount GHG emissions compared to use of hourly grid emission factors in both attributional assessments [23] and consequential applications where building grid interaction matters [24].Additionally, there can be large differences in the carbon intensities of regional/statewide and local grids, which also have an impact on the accuracy of a building's operational GHG footprint.The tools would not be well suited for assessing differences in GHG emissions between onsite construction and prefabrication, or for modeling changes in how materials are delivered to a project.The tools do not account for every structural assembly, interior/exterior element, or system in buildings, as evident in the 'Components and Materials Considered' criterion in table 1. Users should be aware of the tools' reliance on averaged data and default assumptions and choices and how that impacts the uncertainty of results.
A key limitation of the WBLCA tools is the lack of integration between embodied and operational design decisions and their tradeoffs.Recent research indicates that building envelope design decisions have a simultaneous impact on the embodied and operational carbon emissions of a building [25].These decisions can become even more important throughout the building's life as operational energy sources decarbonize, but there is limited capability of existing WBLCA tools to assess tradeoffs between embodied and operational emissions under various design and future operating scenarios.Another key limitation is the lack of integration between design tradeoff interactions on whole building embodied carbon alone.For example, ideally, the tools would allow at least some first-order comparison of the impact of improved envelope on the required HVAC sizing (and associated embodied carbon impacts).In reality, such analysis would currently need to be done manually using two separate models.
EPDs are useful benchmarks for establishing the A1-A3 GHG emission footprint of materials.However, a significant limitation of the integration of EPDs into the WBLCA tools is the inability to compare different materials for a specific building assembly or component.For example, a user can compare A1-A3 emissions among different concrete mixes for a reinforced concrete framed structural system, but they would be unable to compare between a specific reinforced concrete framing and a specific mass timber framing system.The lack of harmonization among different material EPDs is due to variations in background data and system boundaries and is a problem that stakeholders are eager to address [26,27].The lack of number of EPDs for Table 1.Overview of commonly used WBLCA tools summarizing key tool factors including the life-cycle stages each tool calculates, the building components each tool is capable of assessing, and the background information used in estimating impacts.A brief description of the assumptions and methodological approaches for estimating impacts in each respective life-cycle stage (e.g., Materials A1-A3, End-of-Life C1-C4) is provided.

Athena impact estimator for buildings
One

Uses avoided burden approach
Not accounted for unless specified in a specific material's EPD key building systems (such as MEP systems), technologies, and appliances makes it difficult to accurately assess the total carbon footprint of a building.

How should WBLCA tools be improved?
Based upon the review of the existing WBLCA tools and external conversations with building stakeholders, there are active steps stakeholders can take right now to address the tools' limitations, including: (1) There needs to be increased standardization among tools and databases so that users can be assured that results from one tool are not wildly different from another tool.Detailed comparisons among specific materials (e.g., concrete versus wood flooring) would also allow for improved analysis earlier in the design process.(2) Stakeholders should aim to further incorporate local data and decision making into tools to better reflect the accuracy of results from building projects.This can be done by increasing verified, local-manufacturer EPDs for all building materials, components, and systems and by incorporating specific on-site (i.e., construction, transportation/supply chain) conditions for WBLCA tools.Generation of more EPDs needs to be driven by regulatory efforts; "buy clean" laws, which mandate the use of EPDs for materials such as structural steel or concrete, are likely responsible for the increased proliferation of EPDs for those materials.A similar mandate for EPDs of, for example, MEP systems would likely lead to proliferation of EPDs for HVAC components and appliances.(3) There must be better integration of assessing trade-offs between embodied and operational carbon.
Embodied and operational carbon trade-offs need to be explored at the earliest possible stage within the project process, necessitating an overhaul in how operational emissions are typically accounted for in WBLCA tools.Rather than relying on the admittedly quick and inexpensive method of estimating building energy consumption with energy performance benchmarking databases [28] or utility bills, users will need to be trained in operational modeling approaches such as building energy modeling.WBLCA tools will need to be improved so that they account for uncertainties related to estimating operational emissions.These uncertainties largely pertain to the timelines at which local electrical grids will decarbonize and to the temporal specificity of electricity emission factors (i.e., annual versus hourly and average versus marginal).Currently, there is a mismatch between the hourly annual weather data source used in energy models and the hourly annual weather data used to generate hourly grid carbon emission factors for future grid decarbonization scenarios, a factor which WBLCA tools will need to consider.

What should the future use of WBLCA tools look like?
We anticipate two general use cases for WBLCA tools as they become more prominent and readily available.The first use case reflects current practice where stakeholders (e.g., developers, designers, contractors, etc.) utilize WBLCA tools to efficiently explore and attempt to answer important questions about the environmental impacts from individual building projects.The more BIM-based tools (e.g., tallyLCA, One Click LCA) allow for explicit comparisons among different options for the same building element when a more detailed design of the project is complete.Other tools, not explicitly analyzed herein, are used by stakeholders much earlier during the conception phase of the project process to create ballpark estimates of embodied and operational carbon by general building typology and size.The second use case applies to potential, and likely, application of the WBLCA tools to support the creation of new building-related policies (e.g., mandating off-site fabrication for housing, implementing net-zero buildings) or to meet embodied carbon requirements in building code changes.We argue that neither of the general use cases of WBLCA tools are necessarily beneficial for managing and mitigating the GHG, and other environmental and human health, impacts of buildings.Regarding the first use case of the tools, there needs to be a paradigm shift when detailed embodied and operational GHG estimates occur for a building project and when WBLCA tools need to be adapted so that stakeholders can conduct these estimates.Buildings are 'one off ' local projects created with localized supply chains and operated under unique parameters, with distinct disposal and reuse options at the building's end of life.We need to move beyond the prevailing approach of primarily using WBLCA tools to quantify upfront carbon (i.e., life-cycle stages A1-A3).It should be the goal for project stakeholders to use WBLCA tools to assess in a detailed manner the expected impacts from every life-cycle stage at the earliest point possible in the design process (e.g., the effects of materials selection on operational energy use).Coming up with estimates for all the life-cycle impacts of a building project matter for reasons beyond the current scope of WBLCA tools, which are largely focused on GHG emission footprints.
We caution about the use of WBLCA tools to support the creation of new building-related policies or code changes aimed at mitigating the building sector's GHG footprint.Relying on these tools, as they currently stand, could lead to us failing to address interdependent impacts from buildings.Our existing and future building stock must change to meet a myriad of sustainability goals including climate mitigation and resilience, affordability, environmental justice, and improved human health of local communities.While it is enticing to consider existing WBLCA tools as immediate solutions for assessing and mitigating building impacts, more work must be done to make them truly effective decision-support tools.Future WBLCA tools need to be able to assess, in detail, how different design, construction, operation, and end-of-life decisions for a building affect not only GHG emissions, but also direct and indirect costs, quantitative indicators of environmental justice, and measures of human health of local communities.
Table 1 indicates the capabilities of Athena Impact Estimator for Buildings, One Click LCA, tallyLCA, and EC3 in supporting WBLCAs by describing the building components and systems each tool can assess and by providing descriptions of the assumptions and methodological approaches each tool uses to estimate impacts in each of the life-cycle stages from figure 1.It should be emphasized that the existing WBLCA tools