Lewes’ BBM Sustainable Architects have been using a local Sussex wood, chestnut on many of their recent buildings. Here Annular profile some BBM buildings and explain the wood’s various advantages
“Why Sweet Chestnut? asks one half of Sussex sustainable practice, Baker-Brown McKay’s (BBM sustainable design) Duncan Baker Brown rhetorically. “Because” he continues, “it answers many questions, the most pressing of which is that it is a replenishable product. That’s the biggest deal.”
BBM have been ahead of the curve with specifying sweet chestnut cladding, (as well as glue-laminated chestnut beams), for a series of buildings which have been completed in the last nine months. These include two eye-catching homes, one in Brighton suburbia, the other for Baker Brown and family in county town Lewes. Both are being heralded as demonstrating a new ecological modernism built from mid-range affordable materials. Along the coast in Hastings, the Bridge, a community centre up in one of the towns estates, has just opened its doors; and a new media centre refreshing the downtown Hastings streetscape is also recently occupied. Now, with the RIBA specification list featuring sweet chestnut cladding among its one hundred best products, architects are looking at the material with interest and curiosity.
Some of this is to do with BBM being a committed sustainable practice and sweet chestnut being a local resource, readily available in the South East, with about 21, 000 hectares planted. It also has much to do with the efforts of the Flimwell Woodland Enterprise Centre, and a local company, In Wood Developments Ltd, who, working together, have put time, money and effort into making the low grade, small lengths coppiced sweet chestnut produces economically viable again. Once a mainstay material of Kent and Wealden Sussex’s now defunct historic hop picking industry, sweet chestnut has seen its uses declining. And although today there is some continuing use as fencing, it has mainly been used as pulp wood for packaging. However, with the development of finger-jointing chestnut timber BBM and a growing number of other local architects are proving there is a contemporary use for the wood. Its quick growth cycle, with trees being harvested within twenty to twenty five years, means it is up to four times as replenishable as other trees, such as oak and larch, which need between fifty and a hundred years to grow to harvest readiness. The reason for the quick growth is that there is very little sapwood in the trunk; so that the 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 biodiverse environment as well as enabling oak and other species to be left to grow to their full size.
Along with replenishability, the wider question sweet chestnut cladding addresses is performance, specifically environmental performance. As wood, it is low in embodied energy; It’s natural durability performance is also good, no additional treatments being required. This makes it an effective and locally attractive competitor to western red cedar, where specifiers do not need to use preservatives, as long as they are prepared to accept the woods natural colour changes to a silvery-grey. Most of those who have used it find the changed colour very appealing. There is also no requirement to use surface treatments reducing maintenance, which makes it a potentially effective alternative to less environmentally benign systems, such as aluminium and PVC’s. And in terms of life cycle costs, at the end of its life, being wood, sweet chestnut does not produce toxic waste.
With regard to downsides as with most durable timbers chestnut suffers water staining and tannin leakage; this can seem unsightly if allowed to come into contact with white render and concrete surfaces. Use of iron or steel fixings will not only result in blue-staining, corrosion of the metal will occur rapidly so any fixings must be stainless steel or other non-ferrous material.
In-Wood produce sweet chestnut cladding in lengths of six metres with standard strips of ninety by twenty-two millimetres, here 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 and knots which ensures that the cladding will provide a knot-free facade remarkably free from distortion, and results in a stable, clean timber, which continues to retain its stability as it weathers.
The Bridge’s main hall with its chestnut glulam post and beams developed by In-Wood Developments
BBM have used sweet chestnut cladding on the most recent of their practices buildings, adapting a system Baker Brown has used since the early days of his architectural career. The cladding is only the outer layer of the buildings breathing skin, derived from the mid-seventies continental Baubiologie experimental tradition of ‘breathing’ buildings, which Baker Brown uncovered in eco-architect’s David Pearson’s ground-breaking eighties, Natural House Book. The latest version of the breathing wall system the practice has recently adapted, is used at the Hastings Bridge building. Immediately behind the chestnut rainscreen cladding are timber battens with 80 mm wood fibre Pavatex Pavatherm insulation, sourced from Natural Building Technologies. Behind this is the Timber framework, with sheeps wool insulation positioned between the studs. Lastly, there is a 9 mm sheathing ply fixed to ensure the timber doesn’t rack or twist. Breathing wall systems allows any build up of condensation or moisture vapour to escape through the wall. The resulting U levels are about 0.16, exceeding the BRE standard. The technique also removes the risk of rotting, a problem which permeated a previous generation of nineteen sixties timber frame buildings.
Connectors for the Bridge’s chestnut glulam system
With many of BBM’s recent buildings the sweet chestnut rainscreen cladding is mitre-jointed at the buildings corners, in effect an indication of Baker Brown’s confidence in the stability of the wood not to distort. Such detailing shows BBM applying the qualities of this local wood to good effect. In so doing this use of cladding has become part of the vocabulary for renewing the language of local vernacular, while assisting the local economy and showing ways a contemporary ecological design aesthetic can emerge.
A first version of this piece appeared in the Architectural Journal technical section in 2006
Also , for the origins of sweet chestnut as a building material see the Annular piece on the Flimwell Woodland Enterprise Centre