EN 15978 Life-Cycle Modules Explained: A1–A5, B, C and D

Ask two consultants for a building's embodied carbon and you can get answers that differ by a factor of two — without either being wrong. The reason is almost always scope: they counted different life-cycle modules. EN 15978 is the standard that gives those modules their names, and understanding the alphabet is the difference between comparing numbers and comparing nothing at all.

What EN 15978 is

EN 15978, Sustainability of construction works — Assessment of environmental performance of buildings — Calculation method, sets out how to assess a whole building across its life. Its companion, EN 15804, does the same job one level down for construction products and is the core rulebook behind EPDs. Together they let product-level data roll up into a building-level result.

The assessment runs over a reference study period — commonly 50 or 60 years — which matters enormously, because everything in the use stage scales with it.

A1–A3: the product stage

• A1 — Raw material supply: extraction and sourcing of the inputs;
• A2 — Transport: moving those raw materials to the factory;
• A3 — Manufacturing: converting them into the finished product.

Together these are "cradle to gate" — the figure an EPD headlines, and typically the single largest slice of a new building's embodied carbon. Concrete, steel and aluminium dominate here.

A4–A5: the construction stage

• A4 — Transport to site: getting products from the factory gate to the project;
• A5 — Construction and installation: site energy, temporary works, and — often overlooked — material wastage, which carries the full A1–A4 burden of everything skipped or damaged.

B1–B7: the use stage

The longest and most assumption-heavy stage:

• B1 — Use: emissions from installed materials in service, such as carbonation of concrete or refrigerant leakage;
• B2 Maintenance, B3 Repair, B4 Replacement and B5 Refurbishment: the cyclical burden of keeping the building serviceable. B4 is where short-life finishes and services quietly accumulate a large total across 60 years;
• B6 — Operational energy use: the regulated and unregulated energy the building consumes;
• B7 — Operational water use.

B6 is the bridge to the rest of our work: it comes from the energy model, not from the LCA tool, and the grid-decarbonisation assumption applied across 60 years can change it dramatically.

C1–C4: end of life

• C1 — Deconstruction / demolition and C2 — Transport to waste processing;
• C3 — Waste processing for reuse, recovery or recycling, and C4 — Disposal of what remains.

Module D: outside the boundary

Module D captures benefits and loads beyond the system boundary — the credit for materials that will be reused, recycled or recovered after this building's life ends. It is reported separately and must never be netted off against A–C to produce a flattering headline. Treat Module D as a signal about design for disassembly, not as a discount on today's emissions.

The three terms people mix up

Upfront carbon is the number that matters most for climate targets, because it is emitted now rather than spread across decades — and because it is the part still within your control at design stage.

Why scope discipline matters

• Two buildings are only comparable if the modules, reference study period and boundary all match;
• Rating systems differ in what they require — confirm before you model, not after;
• A result quoted without its modules is unusable. Always state them alongside the number.

The takeaway

Learn the alphabet and the arguments get simpler. A1–A3 is the product, A4–A5 gets it built, B is the long tail of operating and maintaining it, C is taking it down, and D is what happens afterwards — reported separately. State your modules and your study period with every number you publish, and insist that others do the same.

This article is general guidance and reflects information available at the time of writing. EN 15978 and EN 15804 requirements are defined by the published standards and are periodically revised — always confirm the current editions and any scheme-specific scope rules for your project.