Functional Unit or Declared Unit? Before You Compare Two EPDs

The confusion we meet most often, in files arriving for verification and in the questions we receive, is two EPDs’ GWP figures placed side by side and declared “this product is lower”. Most of the time the two documents are not measuring the same thing. The difference sits in the most basic concept of LCA: what is the result given per?

Two units, two questions

A functional unit makes the function the product delivers measurable. In ISO 14040/44 it is the reference unit for LCA results. Example: “insulating 1 m² of external wall for 50 years at a thermal transmittance of U = 0.20 W/m²K.” There is no product in that definition; there is a function. One material may deliver it at 8 cm, another at 14 cm. The functional unit is the only way to weigh the two on the same scale.

A declared unit is a quantity reference used without defining a function: 1 tonne of steel, 1 m³ of concrete, 1 m² of plasterboard. Construction product EPDs under EN 15804 mostly use a declared unit, for a simple reason: the manufacturer does not know which building, or which function, the product will serve. A cubic metre of concrete can go into a foundation or a column; the function only becomes known at building level.

The bridge between the two is the reference flow: the amount of product needed to deliver the functional unit. If — in a hypothetical example — “insulating 1 m² of wall for 50 years” takes 3.2 kg of one product and 4.9 kg of another, the comparison is made on those quantities, not on GWP per kilogram.

The common mistake, step by step

A hypothetical example (figures illustrative): a procurement team opens two insulation EPDs. The first declares per 1 kg, GWP-A1-A3 2.1 kg CO₂e. The second declares per 1 m² at a given thickness, GWP-A1-A3 6.8 kg CO₂e. The conclusion drawn is “the first is lower”. But:

1. The units differ — kg cannot be compared with m².
2. Even converted to the same unit, thickness and density differ — 1 kg of product does not give the same insulation performance.
3. Once performance is equalised (same U-value, same service life) the ranking can reverse.

As verifiers we do not reject this comparison as “wrong”; EPDs are not made for comparison in the first place, but to provide inputs to building-level calculations. EN 15804 says explicitly that comparing construction product EPDs is meaningful only in a building context, for the same function and under the same rules.

Where verification looks

For a declared unit, the first check in verification is conformity with the PCR: if the PCR says 1 tonne, 1 m³ cannot be declared. The second is that every conversion tied to the unit appears in the report: density, thickness, unit weight, packaging share. The third is that the declared unit is consistent across all modules — 1 tonne in A1-A3 and 1 m³ in the C modules does not work.

For studies using a functional unit (full LCA reports, comparative studies), the measurable definition of the function and the calculation of the reference flow are additionally required. If a “better than” claim is to be disclosed to the public, the ISO 14044 critical review panel comes into play; that is a separate process from EPD verification.

A practical checklist

Before comparing two EPDs:

• Is the declared/functional unit the same? If not, is conversion possible, and is the data needed for it (density, thickness) in both documents?
• Same PCR and same standard version (EN 15804+A1 and +A2 are not comparable)?
• Same module scope (A1-A3 and A-C are not comparable)?
• Same function — same performance, same reference service life?
• Are the scenarios (transport, end of life) comparable?

If the answer to all five is not “yes”, two figures side by side are not a comparison but two separate information cards. How to read an EPD is covered in a separate article, and the scope decisions of an LCA in the LCA guide.