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SWEET SMELL OF SUCCESS

FOCUS ON: CLADDING

Sussex-based sustainable practice Baker-Brown McKay has specified sweet chestnut cladding and glue-laminated chestnut beams for a series of recent projects. These include two homes, one in Brighton and the other for Duncan Baker-Brown's own house in Lewes. Built from mid-range, affordable materials, both demonstrate a new ecological Modernism. With sweet chestnut cladding among the RIBA specification list's 100 best products, architects are looking at the material with increasing interest and curiosity.

Sweet chestnut is a local resource in the South East, where about 21,000ha are planted. Once a mainstay material of Kent and Wealden Sussex's now defunct historic hop-picking industry, sweet chestnut has seen its use decline.

Although today there is some continuing use as fencing, it has mainly been used as pulp wood for packaging. But the combined efforts of the Flimwell Woodland Enterprise Centre and local company InWood Developments have helped make low-grade, smalllength coppiced sweet chestnut production economically viable again. And with the development of finger-jointing chestnut timber, Baker-Brown McKay and a growing number of other local architects are proving there is a contemporary use for the wood.

Its short growth cycle, with trees being harvested within 20 to 25 years, means it is up to four times more replenishable than other trees such as oak and larch, which need 50 to 100 years to reach harvest readiness. The reason for the quick growth is that there is very little sapwood in the trunk, which means that most of it, excluding the outer rim of the grown tree, can be used. A good thing, says Baker-Brown, pointing out that coppicing sweet chestnut preserves a much richer and more biodiverse environment, as well as enabling other species to be left to grow to their full size.

PROS? Along with replenishment, the wider question sweet chestnut cladding addresses is environmental performance.

It is low in embodied energy and its natural durability performance is also good, with no additional treatments required. This makes it an effective and locally attractive competitor to western red cedar.

The use of preservatives is unnecessary, providing specifiers accept the wood's natural colour change over time to a silvery-grey. There is also no need to use surface treatments, therefore reducing maintenance, and making it an effective environmentally friendly alternative to aluminium and PVC.

? AND CONS As with most durable timbers, chestnut suffers water staining and tannin leakage, which can seem unsightly if allowed to come into contact with white render and concrete surfaces.

Use of iron or steel fixings will result in blue-staining, and metal corrosion will occur rapidly, so any fixings must be of stainless steel or other non-ferrous material.

InWood produces sweet chestnut cladding in lengths of 6m, with standard strips of 22 x 90mm, applying the same finger-jointing technology to European hardwoods which has been so successful with buildings such as the Weald and Downland Gridshell and Edinburgh's Scottish Parliament.

This process initially removes any defects, ensuring that the cladding will provide a knot- and distortion-free facade, and results in a stable timber that continues to retain that stability as it weathers.

BREATHING BUILDINGS Baker-Brown McKay has used sweet chestnut cladding on its most recent building, adapting a system Baker-Brown has used since the early days of his architectural career.

The cladding is only the outer layer of the building's breathing skin, and this process is derived from the mid-1970s continental Baubiologie experimental tradition of 'breathing' buildings, which Baker-Brown uncovered in eco-architect's David Pearce's Natural House Book.

The latest version of the breathing-wall system is used at the practice's Hastings Bridge building. Immediately behind the chestnut rainscreen cladding are timber battens with 80mm wood-fibre Pavatex Pavatherm insulation, from Natural Building Technologies.

Behind this is the timber framework, with sheep's wool insulation positioned between the studs. Lastly, there is 9mm sheathing ply-fixed to ensure the timber doesn't rack or twist.

Breathing-wall systems allow any build-up of condensation or moisture vapour to escape through the wall. The resulting U-levels are about 0.16, surpassing the BRE standard. The technique also removes the risk of rotting, a problem which permeated 1960s timber frame buildings.

With many of BakerBrown McKay's recent buildings, the sweet chestnut rainscreen cladding is mitrejointed at the building's corners, an indication of Baker-Brown's confidence in the stability of the wood not to distort.

Such detailing shows Baker-Brown McKay applying the qualities of this local wood to good effect. In so doing, this use of cladding has helped the local economy and shown how a contemporary ecological design aesthetic can emerge.

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