Scott Reid of Neil Sutherland Architects outlines work at Dunsmore Houses, Stitenham in Ardross, Scottish Highlands.
The two sustainable and affordable two-bedroom homes are located on a forested site and our aim was to build them almost entirely using local timber including Scots pine, Scottish larch, spruce and Douglas fir, to blend with the surroundings, reduce embodied carbon and give the contractor more control over quality.
The houses are raised two storeys above ground level on eight shallow pad foundations, with concrete posts projecting up from the pads to support the ground beams.
We had planned to use Scottish larch hardwood to create the ground beams, but we were concerned that it might be affected by seasonal shrinkage and swelling and potentially alter the level of the house. Any movement would have affected the fine tolerances we were working to, and perhaps damaged the critical air tightness of the envelope.
Tests carried out in collaboration with project partner Edinburgh Napier University identified Accoya as providing a similar durability to larch, but with superior stability, strength and stiffness.
Accoya is wood that has been chemically modified through acetylation, which prevents water from bonding onto the cell walls of molecules, therefore stopping the material from swelling. Strength is further increased through the glue lamination process, carried out by our Scottish joiner Nor-Build.
We had considered installing a concrete ring beam, or a steel sub-structure instead, but Accoya is 100% non-toxic and acetyl groups are naturally abundant in all wood species. The hardwood species used to create Accoya are also sourced from FSC certified and sustainable forests. Its superior strength also meant we were able to reduce the number of concrete posts in the foundations.
