Revolutionary Accoya® wood has been used as an innovative and sustainable alternative to concrete in the construction of Dunsmore House in the Scottish Highlands.
Manufactured by Accsys Technologies, Accoya is one of the most advanced modified wood products on the market using the latest in wood acetylation technology to deliver outstanding levels of performance, stability and durability. Providing the same high-quality and aesthetic attributes of tropical hardwoods, it also boasts excellent environmental credentials by using wood sourced from FSC certified and sustainable forests – while being 100% non-toxic.
Dunsmore House boasts a two storey platform frame supported on eight shallow pad foundations. The design called for laminated timber beams to span the pad foundations and serve as a base for the platform build. The nature of this installation required the laminated beams to perform in a zone where there are significant dimensional instability and durability challenges, due to moisture. Napier University Centre for Offsite Construction and Innovative Structures worked with Neil Sutherland Architects to evaluate timber material options and settled on Accoya on the basis of proven durability and decay resistance in this type of use class. For highly reduced levels of both embodied energy (equivalent to a few years of operating energy) and carbon (energy required to form the product) it is advantageous to use a timber offsite closed panel system that is externally timber clad with a durable species, as is the case with Dunsmore House.
However, research has shown that if a traditional concrete slab foundation is used it will account for over 40% of the embodied carbon of the dwelling. The design strategy of this project reduces the use of cement substantially through the use of a lower embodied carbon alternative in the form of Accoya GluLam ground beams. As well as reducing both embodied energy and carbon compared to traditional concrete foundations, this approach also led to savings in cost and construction time for Dunsmore House.
Neil Sutherland, from Neil Sutherland Architects, commented: “Until now, using wood in damp external conditions for structural applications has been a real challenge, chiefly because of the risk of fungal attack and dimensional instability which affects the critical air tightness of the building envelope. The most effective preservatives are by definition toxic and so have environmental issues both in use and disposal. While there are a few species of wood with good natural resistance to fungal attack, they are often difficult to glue and offer very limited benefit with respect to dimensional stability. Additionally, such species are often of tropical origin and sustainably sourced supplies are becoming increasingly limited.
“With Accoya, all of these issues are addressed and resolved. Accoya wood is guaranteed against fungal decay for at least 50 years above ground and 25 years in ground.”
Accoya was supplied by International Timber for this project. Steve Rogers, Product Director for International Timber added “Timberframe is an important part of the Grangemouth business and this project extends the scope of material supply to timber frame further in terms of value and volume.”
The project has been such a success that it has been showcased in the newly published Accoya’s Structural Design Guide, co produced by ARUP and Timbersolve which provides structural design data based on extensive testing undertaken at Edinburgh Napier University, SHR Timber Research Netherlands, the University of Göttingen and the University of Brighton.
The Guide, which is available online at www.accoya.com, enables the design of both solid and laminated Accoya® members to Eurocode 5. It also discusses the environmental credentials of the material and provides advice on design life, maintenance, connection detailing and specification.
Launching the Guide, Bryan Crennell, Director of Sales and Marketing from Accsys Technologies said: “It’s a major breakthrough to be able to offer architects, contractors and builders the innovative use of Accoya wood in a wide range of structural applications demanding the highest levels of durability and dimensional stability. Its structural durability and reliability means that this is the perfect choice of material for many outdoor projects."
The full range of Accoya material properties and characteristic design values (C24 equivalent) are presented in Accoya’s Structural Design Guide to Eurocode 5 now available online at www.accoya.com