Embodied carbon and life cycle assessment

Mei Takahashi, the life cycle assessment consultant the students met in Building Materials and Assemblies, arrives with a spreadsheet of more than two thousand rows. 'This is the hub,' she says. 'Every beam, every panel, every bag of mortar, every pane of glass, the carpet in the daycare and the solar panels on the roof, each with its quantity, its environmental product declaration, the distance it travels to site, how often it will be replaced, and what happens to it at the end. Last term you compared floor systems. This term we count the whole building.' Reinholt adds a challenge: 'By the end of this lesson I want you to tell me what share of the hub's carbon over sixty years comes from its materials, in your city. You may be surprised by the answer, and by how much it depends on the grid.'

From materials to whole buildings

Building Materials and Assemblies introduced embodied carbon at the scale of products and assemblies: the carbon in a cubic meter of concrete or a ton of steel, and the comparison of floor systems. A whole-building life cycle assessment (WBLCA) applies the same method to an entire building over its whole life. It is the only way to know which parts of a building matter most, to compare design options fairly, and to show that a building meets a carbon limit.

A life cycle assessment follows the international standards for environmental assessment, ISO 14040 and 14044, and, for buildings, the European standard EN 15978, which most North American tools and guidelines also follow. In the United States, the American Institute of Architects and the Carbon Leadership Forum have published practice guides that adapt these standards to American practice. The method has four steps: define the goal and scope, build an inventory of materials and energy, assess their impacts, and interpret the results.

Life cycle stages

EN 15978 divides a building's life into modules, labeled by letters and numbers. Every whole-building LCA should state which modules it includes.

• Product stage, A1 to A3: extracting raw materials, transporting them to the factory and manufacturing products. This is the cradle-to-gate carbon reported in most environmental product declarations, and it is usually the largest share of embodied carbon.
• Construction stage, A4 and A5: transporting products to site and the energy, waste and temporary works of construction.
• Use stage, B1 to B7: emissions from materials in use (such as refrigerant leaks or concrete absorbing carbon dioxide), maintenance, repair, replacement of components that wear out, refurbishment, operational energy (B6) and operational water (B7).
• End of life, C1 to C4: demolition, transport of waste, processing and disposal.
• Beyond the system boundary, module D: potential benefits or loads from reusing, recycling or recovering energy from materials after the building's life. Because these benefits happen in the future and depend on decisions not yet made, module D is reported separately and not subtracted from the total.

Several terms group these modules. Upfront carbon is A1 to A5, the carbon emitted before the building opens. Embodied carbon usually means everything except operational energy and water: A1 to A5, B1 to B5 and C1 to C4. Whole-life carbon adds B6 and B7. The distinction matters because upfront carbon is emitted now, at the moment the climate budget is tightest, and cannot be reduced later.

Building the inventory

The inventory lists every material in the building with its quantity, usually taken from the BIM model the students learned to build in Digital Design and BIM. Each material is matched with an environmental product declaration (EPD), a verified document that reports its impacts per unit, or with a generic value from a database when no EPD exists. Product-specific EPDs, from a particular plant, are more accurate than industry-average EPDs, which in turn are better than generic database values. Takahashi's rule is to use the best data available for the materials that matter most, and to say clearly where generic values were used.

Tools make the work manageable. Tally, which works inside Revit, Athena Impact Estimator and One Click LCA are widely used. The free Embodied Carbon in Construction Calculator (EC3), developed by the Carbon Leadership Forum, holds tens of thousands of EPDs and helps designers choose low-carbon products during specification. Whatever the tool, the scope must be stated: which building elements are counted (structure, envelope, interiors, services, site), which modules, and over how many years. Most guidelines use a reference study period of sixty years.

Takahashi warns of three common traps. First, missing elements: an LCA that counts only the structure may miss half the embodied carbon, because envelopes, interiors and services add a great deal, especially through replacements. Second, double counting or mixed units, which a careful quantity takeoff avoids. Third, biogenic carbon: wood stores carbon absorbed from the atmosphere while the tree grew. Standards require that stored biogenic carbon be reported separately from fossil emissions, not simply subtracted, because it returns to the atmosphere if the wood burns or rots, and because forests must be managed sustainably for the storage to be real.

What the hub's LCA shows

Takahashi's assessment of the hub covers structure, envelope, interiors and services over sixty years. For the mass timber scheme the students designed, upfront carbon (A1 to A5) is about 21 kg CO₂e per ft² of floor area, about a third lower than a concrete version of the same building. Concrete foundations and the slab on grade, insulation, glazing and the mechanical systems each contribute a large share; the timber itself is not the main item. Replacements over sixty years, of roofing, windows, finishes, heat pumps and solar panels, add about a quarter again.

Then she adds operational energy. Reinholt asks the students to compute the hub's whole-life carbon for three structural options, using the grid of their own city.

# Community hub whole-life carbon over 60 years, three structural options
# carbon in kg CO2e per ft2 of floor area; EUI and grid from Lesson 2
upfront = {"mass timber": 19.5, "concrete frame": 30.7, "steel frame": 27.9}   # A1-A3, structure + envelope + interiors + services
A4A5 = 0.10            # transport and construction, as a share of A1-A3
B4 = 5.6             # replacements over 60 years (roofing, windows, finishes, heat pumps, panels)
C = 1.4              # end of life
EUI = 19.7            # kBtu per ft2 per year
EF_NOW = 0.126          # kg CO2e per kBtu of electricity in 2027

def grid(year):
    """Emission factor in a given year: falling in a straight line to zero in 2040 under the state carbon-free law."""
    if False:
        return EF_NOW
    return max(0.0, EF_NOW * (2040 - year) / (2040 - 2027))

b6 = sum(EUI * grid(y) for y in range(2027, 2087))
b6_static = EUI * EF_NOW * 60
for name, a in upfront.items():
    embodied = a * (1 + A4A5) + B4 + C
    total = embodied + b6
    print(f"{name:14s} upfront {a * (1 + A4A5):5.1f}, embodied {embodied:5.1f}, operational {b6:5.1f},"
          f" whole life {total:5.1f}; materials are {embodied / total * 100:3.0f}% of the total")
print(f"operational carbon if the grid never improved: {b6_static:6.1f} kg CO2e per ft2")

mass timber    upfront  21.5, embodied  28.5, operational  17.4, whole life  45.8; materials are  62% of the total
concrete frame upfront  33.8, embodied  40.8, operational  17.4, whole life  58.1; materials are  70% of the total
steel frame    upfront  30.7, embodied  37.7, operational  17.4, whole life  55.1; materials are  68% of the total
operational carbon if the grid never improved:  148.9 kg CO2e per ft2

On the Minnesota grid, the answer depends on the grid's future. If the grid never improved, sixty years of operation would emit about 149 kg CO₂e per ft², several times the materials. If the grid reaches zero by 2040 as the law requires, operational carbon over the hub's life falls to about 17 kg per ft², and materials make up well over half of the whole-life total. Either way, the upfront carbon is emitted now and is certain, while the operational savings depend on future decisions.

The comparison between options is also instructive. The mass timber scheme saves more carbon than any plausible improvement in its energy performance could. That is why the course treats structure and material choice as climate decisions of the first order, and why Reinholt insists that a sustainable building be judged on its whole-life carbon, not its energy alone.

Strategies for reducing embodied carbon

Takahashi gives the students a list of strategies, roughly in order of effect.

• Build less: reuse existing buildings (Lesson 10), share spaces, design compact plans and avoid unneeded basements and parking.
• Build light: efficient structural grids and spans, optimized members, less concrete in foundations through lighter superstructures.
• Choose low-carbon materials: timber and other bio-based materials where appropriate, low-carbon concrete mixes with limestone cement and slag, recycled steel from electric arc furnaces, insulation with low-carbon blowing agents.
• Specify with EPDs and set carbon limits for key products in the specifications, as public owners increasingly do.
• Design for durability, adaptability and disassembly, so components last longer and can be reused.
• Reduce construction waste and transport distances.

Policy is starting to require these steps. Minnesota's Buy Clean Buy Fair law of 2023 requires environmental product declarations and carbon limits for key materials in large state buildings, beginning with concrete in January 2026 and extending to steel, rebar and asphalt in 2028. Federal Buy Clean procurement lost its policy basis when the executive order behind it was rescinded in January 2025, but several states continue their own programs.

Exercise 3: whole-building LCA of the hub

• Using the hub's BIM model, prepare a quantity takeoff of structure, envelope, interiors and services.
• Match each material with an EPD or a stated generic value, noting data quality.
• Calculate upfront, embodied and whole-life carbon over sixty years with your city's grid, and report biogenic carbon separately.
• Identify the five largest contributors and propose a reduction strategy for each, with an estimate of the saving.
• Write a one-paragraph statement of scope: elements, modules, period, tools and data sources.

Self-critique checklist

• Have I stated the scope clearly enough for someone else to reproduce the assessment?
• Have I included replacements and end of life, not only upfront carbon?
• Have I reported biogenic carbon separately?
• Have I tested how the result changes with the grid's future?

Uncertainty in the numbers

Takahashi ends with a caution about precision. Whole-building LCA results carry real uncertainty: generic data may differ from the actual products by a large margin, replacement intervals are estimates, and the future of the grid and of end-of-life practices is unknown. Two competent consultants assessing the same building with different tools and databases can reach results that differ noticeably. That does not make LCA useless. Its value lies in comparing options assessed with the same method and data, and in revealing which items dominate. Results should be reported with their assumptions and, where possible, with a range rather than a single figure. A design decision that holds across the range, such as choosing timber over concrete for the hub, is robust; one that depends on a narrow margin deserves a closer look.