IS CROSS LAMINATED TIMBER CONSTRUCTION A TROJAN HORSE?

A new report has questioned the assumed environmental and structural benefits of cross laminated timber (CLT) construction. The British Association of Reinforcement (BAR) report, ‘Is Cross Laminated Timber a potential Trojan Horse?’, calls for the advocated benefits of CLT not to be taken at face value. Instead, they should be closely examined.

In particular, the report explains that when it comes to sustainability the belief that ‘wood is good’ is naively simplistic. 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 often from several thousand miles away. In addition, there is the environmental impact of the necessary polyurethane glues, fire retardants, waterproofing and insect repellents. Furthermore, the industrial plantations grown to replace natural harvested forests are uniformed monocultures that bear little resemblance to the natural ecosystems that they have replaced.

Steve Elliott, BAR Chairman 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.”

Elliott also highlights the need to consider the operational CO2 impact of a timber building. “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. He explained: “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.”

Furthermore, the long-term structural performance should be considered. Elliot points to the Oregon State University school building built to showcase CLT whose floor collapsed after it became unglued. Whilst post-Grenfell, the potential fire risk of hi-rise timber buildings should not be under-estimated.

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

To download a copy of ‘Reinforced Insight: Is cross laminated timber construction a Trojan Horse?’ visit: https://bit.ly/3HCy3J4

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