As issues around climate change and carbon become increasingly important, the carbon cost of building materials is set to become a common design consideration. How it is calculated and what is included in the calculations is currently the subject of much discussion, locally and globally. This has important implications for timber, one of the world's oldest building materials. Timber's inherent qualities - carbon storage, low embodied energy, high thermal efficiency, light weight, availability - plus modern manufacturing technology make it an attractive option for some surprising applications - such as multi-storey residential buildings.
A landmark project is Waugh Thistelton Architects Stadthaus, a 9 storey, 29 apartment building, in London's East Side. A note of interest to developers is that all the available apartments were sold within an hour and a half of coming on to the market! The tower is constructed using cross laminated timber (CLT) slabs arranged in a honeycomb pattern around the central wooden core. Timber panels were the main building material in every aspect of construction including the stair core and elevator shafts. The building is clad with 5000 individual timber-composite tiles (manufactured mainly from waste timber) shaped to form a pixellated shadow portrait of the surrounding environment.
Using a structural system pioneered by KLH Austria, total construction time was an impressive 49 weeks, down from the originally estimated 72 (for a traditional concrete construction). Erecting the timber panels took four carpenters 27 days and the electricians' schedule was drastically reduced - from eight weeks to four days. When compared to an identical design using steel and concrete, the benefits of timber become obvious: 400 per cent reduction in weight, 70 per cent reduction in the foundation as well as a much shorter construction period led to a cost saving of 15 per cent; not including the environmental benefits.
The concept of the Stadthaus was developed as a response to London's world leading climate change policy. The London Plan is the first of its kind to impose a statutory carbon reduction of 10 per cent in all new buildings that must be achieved through the use of onsite renewable energy. Waugh Thistelton's aim was to demonstrate that carbon reduction could extend beyond energy efficiency improvements to include embodied carbon. At 300 tonnes in weight, the tower stores 181 tonnes of carbon and a further 125 tonnes was offset by avoiding steel and concrete components. This saving of 300 tonnes of carbon is the amount that the building is projected to emit for 21 years of operation, thus equivalent to meeting London's 10% reduction target for 210 years.
These impressive environmental and financial benefits are possible mainly because of the use of Cross Laminated Timber solid panels (CLT) developed by KLH. The panels are manufactured using thin softwood boards stacked together at right angles and glued over their entire surface in alternating layers (similar to thick plywood). Boards come in three, five or seven layers, and once stacked and bonded, form a solid unit that transfers loads to all sides (unlike a beam or a column that only transfers load in one direction) - a genuine plate and sheet action. The panels are produced, cut to size and shape (door and window penetrations are factory pre-cut) and delivered direct to the site for easy assembly. The timber for the boards is sourced from PEFC certified Austrian plantations, and is bonded with a solvent and formaldehyde-free adhesive and produced using a waste-free process where the shavings, off cuts and sawdust are used as biomass to power the production plant.
To meet UK fire resistance requirements, the CLT panels were designed to allow charring on the outside, thereby protect the strength of the inner core, 5 layers were utilised instead of three and this combined with the plasterboard covering, meant that the panels are able to achieve fire resistance ratings of 60 and 90 minutes. The effects of shrinkage and expansion were minimised by the CLT process of high pressure cross-wise gluing. Meeting the acoustic building code requirements proved to be more of a challenge, but this was overcome with creative use of compressed insulation and plasterboard.
In Australia projects including Grocon's Delta "Passive House", a ten storey, 50 apartment, timber building, part of the redevelopment of the Carlton United Breweries site. The innovative Delta building will not only be carbon negative (including transport carbon costs) but have a gasification plant to provide electricity from waste wood so the residents will be independent of the energy grid.
With CLT and other engineered wood products offering new design flexibility and conatruction adsvantages, wood, one of our oldest building materials, is set to make a comeback - or in the words of award-winning Canadian architect, Michael Green, "Wood is set to become the concrete of the 21st century"