Much has been made of the ability to sequester carbon by using timber construction, but what does this really mean? Furthermore, how do we make the most of timber construction to reduce the impact of what we design on the environment?
Let’s address the first part of the question; Carbon is sequestered through trees- like all plant life which derives energy to grow through absorbing carbon dioxide- thus removing it from the atmosphere. Carbon is captured within the tree and accumulated throughout its growth cycle. It remains captured even when the tree is harvested and used in construction, which is only realised when the timber decays or is incinerated. Unlike steel, concrete and other materials requiring large amounts of energy in their manufacture, far less is required in harvesting and machining timber.
At construction stage, timber is shown to be far more beneficial to the environment, only realised if the timber isn’t incinerated or allowed to decay. At that point, stored carbon is released and the impact can be similar to steelwork and concrete.
While the absorbed carbon in timber is locked-in for the life of a building, the full benefits are over-stated and the results of the measurement seriously skewed. This is due to assuming the full benefit when assessing a building’s impact without a clear plan for retaining the carbon within the timber.
HOW SHOULD WE APPROACH CARBON SEQUESTRATION?
There are several ways to approach calculating how much carbon is generated by a timber buildings construction. Degrees of optimism vary, with many cases of ‘counting our acorns before they’ve germinated.’
The approach promoted by IStructE guidance seems sensible and assumes:
· sequestration is achieved through replanting after harvesting the timber used for construction. It accumulates over the life of the building.
· not measuring the carbon stored in the timber used in the building, described as a ‘forward-looking’ approach.
· a large carbon release is assumed at the end of life with 55% by mass assumed to be recycled, 44% incinerated with energy recovery and 1% given to landfill.
More optimistic assumptions involve the use of bioenergy with carbon capture and storage (BECCS). While this would achieve 90% capture of combustion emissions, this technology is in its infancy and not commercially available.
WHAT DOES THIS MEAN IN PRACTICE?
If sequestered through tree planting, carbon is generated through lifecycle stages A1 to A5 (extraction of raw materials through to practical completion). However, it is gradually recaptured through new tree growth, with potential for the building structure to become carbon neutral at around 40 years life. Assuming the trees planted for sequestration are harvested at 50 years life, no further carbon is captured but will be released during stages C1 to C4 (deconstruction, demolition and disposal).
On this basis, timber structures should generate less carbon than equivalent steel or concrete structures. It should be remembered that an inefficiently designed timber building with a short in-service life and poor end-of-life management can generate more carbon than a well-designed concrete-framed building.
HOW SHOULD WE DO THIS?
When specifying timber make sure it is sourced sustainably through managed forests which include replanting.
· Specify local sourced timber to cut down on transportation when possible (lifecycle stage A4).
· Design as economically as possible using regular grids, reasonable spans and the lowest imposed floor loads for the intended use.
· Minimise wastage through using simple sections and details.
· Design for as long a use as possible.
· Design for re-use of the building or building components at end of life.