Construction

Buildings can be carbon sinks. The Global Construction C-Sink Standard certifies the biomass-derived carbon stored in construction materials — from structural timber and straw insulation to biochar in concrete and asphalt — turning the built environment into verified, temporary and long-term carbon sinks.

The Global Construction C-Sink Standard was developed in 2025 and was the first certification standard for carbon sinks in the built environment. The standard verifies buildings and civil engineering works that contain biomass-derived carbon sink materials — wood, hemp, straw, biochar, compressed cellulose, pyrolysis oils in bitumen, and other organic carbon incorporated into construction. Each verified structure is registered as a C-Sink Unit in the Global C-Sink Registry.

The principle is straightforward: biomass removed CO₂ from the atmosphere during growth. When that biomass is incorporated into a building in a form that prevents biological decomposition — a wooden truss, straw fibre insulation, biochar in a concrete foundation — the carbon is stored for the lifetime of the construction and often beyond. The standard provides the methodology to quantify, verify, and register that storage.

The standard distinguishes between temporary and geological carbon sinks within the same building. Organic carbon in wood or straw forms a temporary C-sink that persists as long as the construction stands. Biochar incorporated into mineral-bound matrices — cement, lime, clay, or geopolymer-based materials — is fully protected from degradation during the building's lifetime, through demolition, and into end-of-life scenarios including landfill deposition, recycling as aggregate, or reuse in new materials. The geological persistent carbon (GPC) fraction of embedded biochar qualifies as a C-sink with persistence modelled beyond 1,000 years.

The first certified carbon sink construction, built by Openly in Widnau (Switzerland)

The standard requires rigorous chain-of-custody tracking of biomass-derived carbon from production to the construction site, full greenhouse gas accounting for all processing and transport steps, and third-party validation and verification. A building's C-sink can only be traded or used for insetting once all GHG emissions caused by the production and transportation of the embedded carbon sink materials have been offset.

The first building was validated and verified under the standard in Switzerland in 2025. The standard and its certification are operated by Carbon Standards International. The Ithaka Institute develops and curates the standard.

Link to Global Construction C-Sink Standard (PDF)

Link to Carbon Standards International for further information