CLT’s environmental qualities came under scrutiny during World Green Building Week. Jon Astbury reports
It has been heralded as a material of the future, spearheading a timber renaissance that will be key to sustainable urban growth – but how good is cross-laminated timber? This was the question posed in a discussion at Nicholas Hare Architects, organised during World Green Building Week 2017. The underwhelming, albeit predictable, answer was: it depends.
With Waugh Thistleton recently completing what is for now the world’s largest CLT structure at Dalston Lane, and Groupwork + Amin Taha’s CLT Barrett’s Grove apartments shortlisted for the Stirling Prize, CLT is in the spotlight. But the discussion, with speakers from the worlds of construction, engineering and carbon profiling interrogating CLT’s entire life cycle, put paid to a few myths about the material that have arisen following some overexcited speculation about its benefits.
Dalston lane daniel shearing
Source: Daniel Shearing
Setting the scene was Qian Li, associate at Sturgis Carbon Profiling, who introduced the issues surrounding CLT’s environmental performance. Its overwhelming advantage lies in carbon sequestration, the timber acting as a carbon store and beginning life as a carbon-negative material (-900kg of embodied CO2 per tonne, compared with about 115kg for reinforced concrete, it is claimed). Taking into account material, transport, site work and end-of-life, a steel-framed CLT building generates nearly half the CO2 emissions per square metre as a reinforced concrete one. The overarching problem here is actually introducing this data effectively into performance measurement. Li made this abundantly clear by comparing Hopkins’ Olympic velodrome with Zaha Hadid’s Aquatics Centre; both were BREEAM Excellent, but the Aquatics Centre generated vastly more carbon at a material level. Since BREEAM 2014, embodied carbon has become far more important. Generic information exists, and complex life cycle assessments (LCAs) can be used to improve ratings, but it remains too easy to make claims for materials that do not take into account their overall impact.
Many residential developers are starting to be won over by the speed of delivering a CLT project
A myth Li was keen to dispel surrounds CLT’s alternative nickname: ‘Consumes a Lot of Timber’. The biomass required for 6,500m3 of CLT – almost twice that used at Dalston Lane – would take a mere 22 minutes to regrow at current rates. Not only this, but the EU is currently harvesting only two thirds of its annual timber growth, only a small amount of which currently ends up in construction. This would, of course, quickly change, were CLT to be taken up as a mainstream construction material but, as Jonathan Fovargue of Eurban explained, the largest bottleneck remains at the processing stage: while some companies will produce standard panels, the creation of CLT components such as entire wall sections is usually a bespoke one. BIM is streamlining this, but for now it remains something of an obstacle for rolling out CLT en masse. The penny is beginning to drop, however, particularly for medium-sized builds, where CLT begins to be more competitive in terms of cost. Many residential developers are starting to be won over by the speed of delivering a CLT project, as well as by its overall lightness, which requires less foundation work.
Mechanically ventilate a CLT build and it will cancel out the carbon negativity that made it worthwhile in the first place
Which takes us to the question of how CLT actually performs, tackled by Tom Bentham from Max Fordham. The main concerns here are thermal comfort, acoustics and light. While CLT is usually pitched as having a higher thermal mass than a timber frame, it is still well below concrete, which is able to store some 2.5 times the amount of heat. These fluctuations are easier to deal with in residential builds, but for offices used throughout the day, this demands a carefully considered ventilation strategy. Mechanically ventilate a CLT build and within its lifetime it will cancel out the carbon negativity that made it worthwhile in the first place. The answer here is often to simply provide large openings, but Bentham also showed examples where mass has been added back into floors and walls, a process that also chips away at carbon savings.
That being said, its low thermal conductivity makes detailing to avoid thermal bridging simpler, as does its air tightness. Another myth crops up here: that timber is inherently more sound-absorbent than other materials – Bentham pointed out that it was essentially the same as brick and concrete, and is really more suited to projects with less onerous sound requirements. Finally, while CLT is self-finishing with a long lifespan, there is also the issue of its low light reflectivity – far below that of white paint. While Bentham acknowledged it was ‘anathema’ for many to paint or cover CLT, it is worth bearing in mind the impact bare CLT will have on light levels.
160809 Amin Taha Barrett s Grove 389
Source: Timothy Soar
As for its ‘death’, which it fell to Integral Engineering Design’s Tim Mander to discuss, CLT’s use at a large scale remains too recent to tell how defunct buildings will be dealt with. Again, the lack of sufficient data is an issue. Mander pointed to the life-cycle table in the BSI’s Sustainability of Construction Works, in which, after construction, use and end-of-life stages comes the crucial Section D: ‘Beyond building life cycle’. This aspect remains something of a moot point for CLT. Incineration would be costly in terms of carbon; while in landfill the wood would rot and release methane. Ideally, CLT buildings will be salvaged and the panels put to use elsewhere; but how successfully this can be achieved in practice remains uncertain.
While the jury is still out on CLT specifically, the theme that ran through each presentation was that of data. The event shed light on the blind spots in measuring environmental impact and, while the material itself may not be a panacea, the focus it is encouraging on life-cycle assessment could well have an impact on sustainability across construction, whatever the material.
This article first appeared in AJ Specification