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Sat June 25 2022

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Concrete lobby lays into cross laminated timber

5 days The concrete lobby has launched a war of words against the world of wood, with an attempted put-down of cross laminated timber.

Cross laminated timber was used for the roof of the Canary Wharf Crossrail (Elizabeth line) station
Cross laminated timber was used for the roof of the Canary Wharf Crossrail (Elizabeth line) station

A report from the British Association of Reinforcement (BAR) says that cross laminated timber CLT is ‘a  chemical cocktail’ that is not juts flammable but also environmentally damaging.

The report, titled Is Cross Laminated Timber a potential Trojan Horse?, suggests that specifiers should take a closer look at the claimed benefits of CLT.

The wood lobby dismissed the BAR report as “a desperate attempt to smear a sustainable construction material by those sectors which have a less than positive tale to tell about their environmental impact”.

The BAR report says that consideration should be given to the CO2 emissions resulting from timber harvesting and rotting tree stumps, the energy used for the industrial process to dry the timber and fabricate the CLT panels and the CO2 impact of timber transportation around the world.

“The CLT lumber is placed into a vacuum chamber where it is subjected to temperatures of up to 150°C and pressure from 700 to 6000 psi. Once this process is complete, the panels are then cut and glued to the required shape and size. All-in-all, quite an intensive industrial process that should be taken into account,” the report says.

It continues: “A major reason why harvesting forests for construction materials is not so green is the resultant monoculture of the industrial timber plantations that are planted as farmed replacements. Such plantations should not be viewed as forests. A plantation is a highly uniform monoculture geared to the production of a single raw material that replaces natural ecosystems and their rich biodiversity. Increased demand for CLT could well increase the prevalence of monoculture ‘green deserts’.

“Furthermore, whereas forests are recognised carbon sinks, the same should not be said of new forced plantations which, unlike forests allowed to regrow naturally, can actually be net emitters of carbon due to the disturbance to the soil and the degradation of the previous ecosystem.”

BAR chairman Steve Elliott said: “When you consider the destructive harvesting, industrial manufacturing process, additional chemicals and monoculture plantations it may be that too much credit has been given to timber being a green material. Indeed, it may better to keep the ‘wood’ alive rather than cut it down and build with it.”

He said: “The embodied CO2 used the make the materials is estimated to be 10 to 20% of a building, the rest, 80 to 90% results from its use and operation. CLT, compared to heavyweight concrete construction, has low thermal mass. This means it has limited ability to absorb heat and even out temperature fluctuations. Concrete’s heavyweight thermal mass can play an active role in reducing heating and air conditioning requirements. Lightweight construction is far more reliant on mechanical air conditioning.

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“Air condition a CLT building and, over its lifetime, its operational CO2 emissions will far surpass any supposed initial embodied savings. Then there are all the additional finishes and materials to provide what concrete inherently provides. The lightweight structure’s need for additional fire proofing, flood resilience, sound insulation all come with additional CO2 impacts.

Elliott said: “Cross laminated timber is a relatively new construction material having only been developed in the 1990s. The jury should, therefore, be out on its long-term performance just as the jury should also closely examine its so-called green credentials.”

Timber Development UK chief executive David Hopkins was having none of it. “This report from BAR really brings nothing new to the table other than a desperate attempt to smear a sustainable construction material by those sectors which have a less than positive tale to tell about their environmental impact,” he said.

“There are already fully verified environmental product declarations behind all timber construction products which consider the whole supply chain. This means the impacts of timber products are measured, assessed, and verified by independent experts from forest to factory to operational building – right through to the product’s end of life.

Hopkins continued: “The truth is that timber is a renewable material which comes from sustainably managed resources – growing throughout Europe – which absorbs and stores carbon and requires very low energy inputs to process into high-performance low-carbon construction products.

Concrete, on the other hand, requires materials such as sand – produced from dredging rivers and seabeds, destroying ecosystems and habitats in the process – and huge inputs of energy and water to manufacture. It’s a very high carbon material – and an issue which must be tackled to decarbonise construction.

“Rather than a constructive attempt to find solutions to the climate crisis, they would rather waste time dreaming up strawman arguments like this report because they know they cannot compete in a market with a greater focus on sustainability.” 

Citing a recent academic paper in the Buildings & Cities journal, Hopkins concluded: “With the concrete industry having been called out for the inaccuracy of their messaging on carbon just last month, they should instead focus their time on writing a realistic net-zero road map – rather than one based on wishful thinking, rhetoric, and the use of carbon capture and storage technologies which do not exist.”

To decide for yourself, download a copy of  Is cross laminated timber construction a Trojan Horse? at bit.ly/3HCy3J4

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