Embodied Carbon: Two Verified Product Numbers May Not Be Comparable
A verified environmental declaration reduces uncertainty but does not remove differences in functional unit, scope or accounting rules.
The Mechanism in One Sentence
An Environmental Product Declaration can be independently verified and still support a poor purchasing choice when the buyer treats different units, boundaries or calculation rules as equivalent.
Why It Matters
Purchasers, manufacturers and project owners increasingly rank materials by embodied-carbon figures. A lower number can result from a narrower accounting boundary rather than a cleaner process. Verification of a declaration does not automatically certify commercial or environmental superiority.
Explain It Simply
Two cars list fuel use as 5 litres per 100 km and 4 litres per 80 km. Both may be measured correctly, yet directly ranking the numbers is meaningless. Environmental declarations likewise need compatible units and comparable system boundaries.
How the Mechanism Works
A Type III EPD documents a product life-cycle assessment under declared Product Category Rules (PCR). Those rules define units, stages and modelling choices. An independent verifier can check compliance with the chosen rules but does not make competing methodologies equivalent.
Physical, Information and Money Flows
Material involves inputs, production, logistics, use and end-of-life stages depending on the declared scope. Information involves plant records, emission factors, allocation rules, PCR version and verification. Cash involves material price, proof costs, procurement eligibility and the cost of discovering incomparability too late.
Evidence Map
The U.S. GSA has used product-specific EPDs in concrete procurement and linked requirements to mix and strength categories. A historical 2022 specification requested declarations by concrete mix rather than one number for an entire manufacturer. Published research also documents weak comparability when PCRs, units and boundaries differ.
Economic Logic
Material A may claim less carbon per kilogram while requiring greater mass for the same function. Procurement must compare carbon per equivalent service while separately tracking price, strength, durability, availability and installation conditions. Otherwise, optimizing a product number can shift impacts to another part of the project.
Constraints and Boundaries
Fair comparison requires equivalent function, compatible units, aligned life-cycle boundaries, sufficiently harmonized PCRs and adequate data quality. Methods and databases evolve. An expired declaration or a different plant’s EPD may not represent the delivered batch.
What Most People Miss
The asymmetry is not merely that sellers know more than buyers. It can survive independent verification because parties are describing different reference objects. The blind spot is the shared language of comparison, not merely the honesty of the underlying data.
Critical View
EPDs are not useless: they organize evidence and can make well-defined comparisons possible. But one climate metric does not capture toxicity, biodiversity, water or durability. Declared advantage should be checked against the actual project service and delivered material.
Sidy’s Synthesis
I distinguish trust in a piece of evidence from comparability between pieces of evidence. The first depends on methods and verification; the second requires an explicit contract for the same unit of outcome. Verified data outside that comparison contract can lead to a wrong decision without being false.
Reducing asymmetry therefore calls for a bridge between declarations: comparable use, aligned boundaries, visible exceptions and a reasoned choice. The same rule applies to quality certificates, cost estimates and performance disclosures.
AI and Future Lens
Today, software can parse units, PCR versions and included stages to reject false comparisons. By roughly 2031, AI may pre-screen packages for mismatched plants, allocation rules or categories. By 2036, building models may compare installed functions when inputs are trusted. Longer term, the value will still depend on traceable assumptions, not visual polish.
The assistant must be allowed to return 'not comparable' rather than manufacture a ranking.
Build From This
Build an EPD comparability gate for procurement: product, intended use, functional or declared unit, PCR and version, included stages such as A1–A3, producer, plant, validity, verifier and carbon measure. Enable a numerical ranking only when the comparison contract passes.
For incompatible documents, issue a targeted clarification request and record ownership. Preserve a rejection ledger: preventing a wrong comparison can be more valuable than producing a prettier shortlist.
Actions
Select two EPDs; compare unit, PCR, version and boundary; verify equivalent function; request justified conversions; block invalid rankings; archive the reasoned decision.
Remember This
Verified does not mean comparable. Define the outcome unit before ranking. PCRs and boundaries matter. Evidence should match the delivered product. A useful system can say not comparable.
Primary sources
Facts, figures and quotations should be traceable to the sources below. Sidy's synthesis is labeled as synthesis and does not replace sourced facts.
- U.S. General Services Administration — Low Embodied Carbon Concrete Standard (2022) (GSA official explanation of EPD and criteria in concrete procurement)
- U.S. General Services Administration — Concrete EPD specification by mix and strength class (Historical 2022 specification illustrates declared products and comparison basis; not asserted current for every project)
- Gelowitz and McArthur — Comparison of Type III EPDs for construction products, 2017 (Peer-reviewed secondary critique of PCR harmonization and EPD comparability)
- EPD Portal — PCR Selection and EPD Comparability, August 2026 (Programme operator perspective, authored August 2026)