Building LCA under EN 15978: end-to-end from data collection to result

EN 15978 is the standard defining how environmental performance is assessed at building scale. Where EN 15804 works at product level, EN 15978 assembles those products into a building. In practice a building LCA is the work of matching the right EPDs to the right quantities and defining the right scenarios.

Step 1 — the bill of quantities

Everything starts with the bill of quantities, and the study's quality is decided there. It should be organised by building element: foundations, structure, envelope, internal partitions, services, finishes. Each line needs a material type, a mass or volume and an expected service life.

The most common omission is services and finishes. They are small by mass but short-lived, and are replaced several times over the building's life; their cumulative contribution in module B4 can rival the structure.

Step 2 — EPD matching and the data hierarchy

Every material line must be matched to a data source. The hierarchy is clear: first a product-specific, manufacturer-verified EPD; failing that a sector-average EPD; failing that a generic database record. Which line came from which source must be traceable in the report.

• Verify the declared unit matches the quantity unit — m², m³, kg, pieces
• Check the EPD's scope: A1-A3 only, or C and D included
• Geographic representativeness — a German EPD may not represent Turkish production
• Validity date — expired EPDs are not accepted by some certification schemes
• Waste factor — model construction site waste separately within A5

Step 3 — reference study period and scenarios

The reference study period is the time span the assessment covers, typically set at 50 or 60 years. That choice drives module B4 directly: a component with a 25-year expected life is replaced once over a 50-year period. Components outlasting the study period are never replaced.

Scenario definitions are the most debated and least standardised part of a building LCA. How much material is recycled at end-of-life, how much goes to energy recovery and how much to landfill rests on assumptions. Unless documented explicitly, the result is not comparable.

Step 4 — reading the result

Results are reported by module and by impact category. A single headline number is not what EN 15978 expects; keeping the modules visible shows which stage dominates and directs improvement work. Breaking A1-A3 down by element then tells you which building component to focus on.