Custom impact factors, custom methods and weighting: flexibility without leaving the standard

In an EN 15804+A2 EPD the characterisation set is not negotiable: EF v3.1. But an LCA platform's use goes beyond EPDs — R&D comparisons, academic papers, customer-specific reporting and internal decision support carry different methodological needs. The right architecture separates the standard's territory from the flexible territory: verified output stays locked to the standard, the exploratory layer flexes.

Custom impact factors: characterising missing flows

Standard methods do not cover every flow. A new chemical, a sector-specific emission or a recent characterisation factor from literature (say a GWP update in a new IPCC assessment) may not yet be in the official set. Defining a custom impact factor binds that flow to your chosen category — provided you document the factor's source. Peer reviewers and verifiers will ask the same question: where does this number come from?

Custom LCIA methods: your own indicator set

Some projects need a full custom method: a client's corporate reporting template, a research project's reduced indicator set, or the specific category combination a tender demands. A custom method assembles a new set from existing categories and factors; the engine runs it with the same mechanics as EF v3.1 or CML v4.8. The critical discipline: reports produced with a custom method state the method name and version explicitly — which factor set ran under the heading "GWP" is never ambiguous.

Custom emission sets: turning supplier data into an activity

If your supplier hands you their own EPD or measured data, that data is more representative than the ecoinvent activity. A custom emission set (custom activity) turns it into a first-class activity on the platform: selectable in the inventory like an ecoinvent line, contributing identically in calculation, visible with its source in the process tree. Moving from generic to supplier-specific data directly improves your EPD's data quality rating (DQR).

Weighting: the path to a single score and its limits

Weighting multiplies normalised category results by importance coefficients and reduces them to a single score. The EF weighting set (climate highest at 21.06%) is the de facto standard; but for internal decisions, sets reflecting company priorities (raising the water weight in a water-scarce region, say) are legitimate. The limit is sharp: ISO 14044 prohibits weighting in comparative assertions disclosed to the public; an EN 15804 EPD has no single score. Weighted results are for internal reports and design comparison.

• EPD / verified output → EF v3.1, no deviation
• R&D design comparison → custom method + weighting set allowed
• Academic paper → allowed, provided method and factor sources are documented
• Supplier data → custom emission set; DQR improves
• Public comparative assertion → weighting prohibited (ISO 14044)