Functional unit vs declared unit: why getting it right matters

One of the most-confused pairs in LCA literature is "functional unit" vs "declared unit". The distinction looks small but directly impacts whether one EPD can be compared with another — the wrong unit yields the wrong conclusion.

Definitions

The functional unit expresses the service the product provides quantitatively. ISO 14044 mandates four elements: qualitative dimension (what it does), quantitative dimension (how much), duration (how many years), performance level (to what standard). Example: "1 m² of wall insulation at R=2.5 m²K/W thermal resistance, 50-year service life". A declared unit is only physical quantity: "1 kg of product" or "1 m² of 60 mm insulation". It carries no performance information.

When to use which

Functional units are mandatory for comparative LCA — without holding performance constant, no fair comparison is possible. Declared units are acceptable for single-product EPDs, especially when the product is part of a larger system (e.g. cement per ton, then used quantitatively in a building-level LCA). EN 15804+A2 recognises both, with the choice governed by the relevant PCR Part B.

Typical functional units by sector

• Insulation — 1 m², R=1 m²K/W, 50 years (alt: fixed thickness at given λ in W/m·K)
• Cement — 1 ton of a given CEM type (declared unit; used as a scaler in building LCA)
• Concrete — 1 m³ at strength class C25/30 / C30/37 / C40/50, 50 years
• Structural steel — 1 ton at yield strength S355 or S420, 60 years
• Window — 1 window unit, 1.23×1.48 m, U=1.0 W/m²K, 30 years
• Paint — 1 m² of surface coverage, specified opacity level
• Lighting — 1000 lumen-hours, specified colour temperature

Common mistake: "1 kg of product" as a functional unit

A frequent manufacturer mistake is presenting a mass-based unit like "1 kg" as a functional unit. This is wrong because mass tells you nothing about performance. Comparing glass wool to EPS insulation by mass appears to favour glass wool — but 1 kg of glass wool can have much lower thermal resistance than 1 kg of EPS. The fair comparison is "for the same R-value". An EN 15804+A2 verifier will reject this mistake.

Conversion between functional and declared units

Suppose your EPD is published with a declared unit — "1 m² of 80 mm glass wool". For a building-level LCA it must be converted to a functional unit. The conversion: 80 mm × 0.034 W/m·K (typical glass wool λ) = R=2.35 m²K/W. Stating this conversion explicitly (thickness, λ, mass/m²) lets the certification team use your EPD smoothly. Missing conversion data effectively reduces the EPD's value.