Wood: a carbon neutral material

The NZ Wood TVC – Framing and Cladding – makes the claim that the wood used for the framing is “carbon neutral”. Carbon neutral in this case means that no more carbon is emitted in its production and whole life cycle than is absorbed from the atmosphere when it is growing. This is the same as saying that wood’s “carbon footprint” is zero. With respect to wood, this claim is very conservative, and in terms of conventional CO2 measures, significantly under-represents wood’s carbon credentials.

Carbon footprint calculations

The carbon footprint is calculated by employing Life Cycle Assessment (LCA) principles and current methodologies applied to the production and manufacture of the building materials and components of building designs (often referred to as ‘cradle to gate’ assessment). 

The principal source of CO₂ associated with the production of building materials is the combustion of fossil fuels used, at all stages, to make those materials, together with the release of any CO₂ from the source materials (often referred to as ‘embodied CO2’). 

Applying known CO₂ emission rates for various fuels and processes provides embodied CO2 coefficients (g CO2 /kg) for building materials.  These details are contained in a NZ-specific study authored by Andrew Alcorn of Victoria University. (Alcorn, A, 2003).

Under this methodology, when a tonne (1000 kg) of processed, finished timber leaves the sawmill, it is calculated to have removed a net total of around 1644 kg CO₂ from the atmosphere.¹ This is net of all CO₂ related to its growth, harvesting and production.

Whole life cycle analysis

Note: the claim of carbon neutrality relates to wood as a building material, not the building itself nor the construction process.

A whole life cycle analysis must also account for CO₂ associated with disposal at end of life. In the case of wood, disposal options typically include recycling, disposal in a landfill or burning.

In the worst case scenario, burning will return the same amount of carbon to the atmosphere that was originally absorbed during the growth process (thus achieving virtual carbon neutrality). However, burning accounts for less than three percent of disposal of construction timber in New Zealand (Wood Processors Association figures).

Recycling/reuse of course retains the carbon within the wood. Disposal in a landfill (the predominant method of disposal) has been shown to release no more than three to eight percent of the carbon back into the atmosphere² .

This means that wood disposed of in a landfill is around 70% better than carbon neutral. (Ie. wood has absorbed 1.7 times more CO₂ than it has emitted over the full course of its life cycle from growth through to decomposition.)

Given the current mix of disposal methods employed for wood construction materials today, the claim of carbon neutrality is still likely to significantly under-estimate woods role in permanently removing CO₂ from the atmosphere.

(It should also be noted however, that we are talking about current methods and technologies for disposal, which may have little relevance at the time in 50 to 100 plus years hence when today’s wood products will reach their end of service life. Technologies for the draw-off of landfill gases can be expected to improve significantly in this timeframe.)

1.  Alcorn, A., 2003. Embodied Energy and CO₂ coefficients for NZ building materials.  Centre for Building Performance Research, Victoria University of Wellington.

2.  Micales J A, Skog K E, 1997. The Decomposition of Forest Products in Landfills. International Diodeterioration and Biogradation journal, Vol 39, No 2-3, pp 145-158.