Clearing the Air on A4 Emissions: A Guide for Timber Construction

This article is not just about why A4 matters; it is a practical tool for making smarter early-stage choices based on accurate data. Using CLT as an example, project-specific assessments allow us to refine A4 estimates to reflect the true carbon impact.

There are two methods we can use to understand and assess the carbon emissions associated with transport which are briefly described below.

Energy Based

This calculates average emissions for a fleet and an average A4 emission per material delivered, using fuel consumption and government-issued emissions. However, tracking fuel use in material transport is challenging.

Activity Based

This widely used, simpler method, adopted by the Institute of Structural Engineers (IStructE) and Timber Development UK (TDUK), estimates a trip's carbon footprint based on distance and average vehicle emissions.

To understand the journey and related environmental impact of CLT, we need to look beyond the sawmill. The IStructE's Structural Carbon Tool gives us an insight into the travel-related carbon cost; it calculates an emission value of 0.161 kgCO2e for each kilogram of timber product carried over 1500 kilometres.

TDUK recently published their “2024 Embodied Carbon Data for Timber Products” that highlighted the emission factors for importing CLT. This value, originally published in kgCO2e/m3 but converted using 480kg/m3 for CLT, has a value of 0.173 kgCO2e/kg. This accounts for both sea and road travel from Europe, including a 20% addition to the figure to account for unladen lorry journeys once the CLT has been delivered at the port.

To understand the journey and related environmental impact of CLT, we need to look beyond the sawmill. The IStructE's Structural Carbon Tool gives us an insight into the travel-related carbon cost; it calculates an emission value of 0.161 kgCO2e for each kilogram of timber product carried over 1500 kilometres.

TDUK recently published their “2024 Embodied Carbon Data for Timber Products” that highlighted the emission factors for importing CLT. This value, originally published in kgCO2e/m3 but converted using 480kg/m3 for CLT, has a value of 0.173 kgCO2e/kg. This accounts for both sea and road travel from Europe, including a 20% addition to the figure to account for unladen lorry journeys once the CLT has been delivered at the port.

To better understand the specifics in CLT transport, we engaged with the industry and collected data on many of the factors that impact on the A4 emissions.

Using CLT transport to UK from mainland Europe as an example, most CLT is carried by articulated lorries (>33 tonnes), though the lorry's load capacity often exceeds the volume of timber carried, resulting in an average 76% load.

Timber transport involves road and sea routes. Most CLT reaches the Port of Rotterdam by road, then roll-on roll-off ferries transport it to a UK port for final delivery to construction sites.

Our research on typical transport modes, vehicle load, and distances has led to a recommended carbon factor of 0.119 kgCO2e/kg — 26% lower than the IStructE standard by accounting for actual load levels.

Although these figures are related to UK timber use, the concepts remain the same for mainland Europe, with the use of articulated lorries and roll-on roll-off ferries to reach its desired destination.

To get an accurate freighting profile, it can be broken down as follows:

For road trips:

• Articulated lorries (capacity for more than 33 tonnes) journey around 1100 km from the timber mill to the European port.
• The last leg from UK ports to construction sites is a shorter 200 km sprint.
• Lorries, on returning, travel lighter by around 50 km to the next commission.

For sea voyages:

• Roll-on roll-off ferries cover about 350 kilometres between European and UK ports.

When assessing total emissions for materials from production to on-site delivery (A1-A5), it is essential to look beyond production (A1-A3) for a complete view, particularly in early design stages.

Using this research for cross-laminated timber, the IStructE carbon calculator's standard delivery stage (A4) emissions can be lowered by 26%, resulting in a 10% reduction in combined A1-A5 emissions. As the construction industry works to cut production emissions (via low-emission steel, carbon capture, cement alternatives etc.), accurately measuring transport emissions will be vital for full embodied carbon assessments. This calls for greater data collection and knowledge sharing across the supply chain to better understand carbon sources.