Cross-laminated timber (CLT) structures are gaining popularity as a sustainable alternative to traditional building materials. To assess their true environmental impact, it is crucial to analyze greenhouse gas (GHG) emissions throughout the life cycle – from from timber harvesting, through production, assembly and use, to end of life.

Life cycle stages and emissions:

1. Mining and production of slats (A1–A3): Emissions in this initial phase include the harvesting of wood, its processing into lamellas and the production of CLT panels themselves.
   • Logging and wood processing consume relatively little fossil fuels, for example the CORRIM study reports around 62 kg CO₂eq/m³ for wood handling and processing (19 kg from logging + 43 kg from board production). Other sources report low emissions for logging and board production (A1-A2) in the range of around 19–32 kgCO₂/m³.
   • Production of CLT panels (pressing, drying, gluing) is however the most important phase, typically contributes an additional 96–117 kg CO₂eq/m³. A lot of energy (electricity, gas) and adhesives are consumed in this phase. The adhesive itself (polyurethane or MF resin) can account for up to approximately 30 % of total global warming potential (GWP) of production.

   It is important to note that regional and technological differences have a huge impact on emissions. For example, a manufacturer in Scandinavia (with a clean energy grid) may only report around 34–60 kgCO₂/m³ for phases A1–A3, while in Australia (with a coal-dependent energy grid) this may be up to around 447 kgCO₂/m³. European conditions, such as Stora Enso’s Austrian production, show a lower A1–A3 balance of just 53.1 kgCO₂/m³. These differences are mainly due to the cleanliness of the electrical grid, the rate of heat recycling in the plant and the ratio of energy-intensive resin.

2. Transport of materials (A4): Transporting materials adds tens of kg of CO₂ per cubic metre. For example, transporting CLT from Europe to the UK can be 0.119–0.173 kgCO₂/kg (approximately 57–83 kg/m³). The European EPD states around 25.9 kg CO₂/m³ for transport to the site. Emissions depend on the distance and mode of transport: sea transport is relatively economical per 1000 km (approximately 0.08 kgCO₂/t·km), while road transport by truck is slightly higher (approximately 0.075 kgCO₂/t·km per 1000 km), and rail transport has a GWP in between (approximately 0.077 kg/t·1000km).
3. Assembly and installation (A5): This stage only adds units of kg/m³. In Stora Enso's EPD, for example, it was 5.4 kg CO₂/m³.
4. Use and maintenance (B): During use, the panels almost they do not require any energy or maintenanceImportantly, thanks to its structured form sequester carbon from trees, which remains bound. 1 m³ of CLT can store approximately 985 kg of CO₂.
5. End of life (C): At the end of its life, carbon is released through combustion or decomposition. If CLT is burned, approximately 95% of the stored carbon (equivalent to approximately 985 kgCO₂/m³) is released as CO₂. However, from an LCA perspective, emissions from wood combustion are often considered to be climate neutral (according to the IPCC, they are addressed in the LULUCF sector, not in the production phase). It is much more advantageous recycling scenario, with the literature mainly recommending reuse and recycling before incineration, with minimization of landfilling.

Comparison with concrete and steel:

Compared to traditional materials, CLT stands out in terms of emissions much better.

• For 1 m³ of regular concrete, approximately 100–300 kg CO₂ (depending on the cement content).
• Steel is extremely emission intensive, with approximately 1.1–2.5 kgCO₂/kg of steel, which at a density of approximately 7850 kg/m³ means approximately 8600–19600 kg CO₂/m³.
• CLT has a very low weight (400–500 kg/m³) and relatively low production emissions (approximately 0.25–0.5 kgCO₂/kg) with comparable strength. In addition, CLT offsets some of the other emissions thanks to substitution: each 1 m³ of CLT replaces approximately 1 m³ of concrete or steel panels. Meta-studies show that timber buildings can have about 30–60 % lower carbon footprint than concrete/steel objects.

The emission intensity of CLT production is in the order of tens to hundreds of kg CO₂eq/m³ (excluding biogenic carbon). With a detailed LCA with careful selection of raw materials and clean energy, the A1–A3 balance can be around 50–150 kgCO₂/m³. Transportation makes a big difference a collection of trees (type and density). CLT has a much more favourable carbon footprint compared to standard materials, especially when the amount of biogenic carbon stored is taken into account. It is also crucial to treat CLT at the end of its life in accordance with the principles of a circular economy, where reuse or recycling will significantly reduce emissions more than conventional incineration or landfilling. JRi