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Defence can be a passive activity


 

Volume 24,  No. 12 - December 2016

In our highly volatile modern world, the word ‘defence’ has, for many people, immediate connotations with military activity and a sometimes Orwellian inversion of its literal meaning. As a means of protecting something from attack, however, new materials can provide forms of defence that stand far outside the mechanics of conventional warfare, but which deliver successful, passive forms of protection to that which they are intended to preserve. 

 

Such is the case with cross laminated timber, a product that has been on the market since the early 1990s but the full potential of which as a component of modern construction technology might be said to be still in the very early stages of exploration. Yes, we now have cities around the world competing to deliver ever taller solid timber structures in some of their more densely packed urban areas; we have schools, supermarkets, office buildings and health facilities increasingly taking advantage of the product’s low embodied carbon credentials and other environmental benefits; and we have expanding numbers of building contractors who recognise the speed and ease with which new construction can be effected with its use. Until recently, however, little research and development had been carried out to determine the capacity of very thick panels of cross laminated timber to provide major benefits in terms of airtightness and passive control of internal temperature and relative humidity. 
Why might such research be important and have far reaching implications? Consider the modern art gallery, museum or archive collection in which rigorous control of environmental factors are applied to protect the artefacts on display or in storage. The twentieth century approach to these challenges generally involved extensive use of - and expensive to run - mechanical systems to deliver a specific and strictly monitored internal environmental quality. All too often, however, these systems were introduced to mitigate problems created by designs that demanded the use of materials inappropriate to the building’s actual purpose and functional effectiveness. As such, many of the solutions applied can nowadays be seen not only to be insufficiently protective to the materials they are there to environmentally defend, but which - albeit unintentionally - have sometimes also proven to have deleterious effects entirely contrary to modern conservation practice.  
 
In this context, therefore, what difference can cross laminated timber construction make? The first thing to consider is external wall thickness. The material is conventionally used in large scale panels of 100-150mm thickness which are then insulated and clad to achieve weather protection and the required U-values. In the UK the walls are invariably also lined internally to provide a non-timber surface finish, but which completely negates the hygroscopic benefits of the material. Additionally, this type of build-up of different materials may still not deliver the levels of thermal performance demanded by curators, thus necessitating the introduction of mechanical air handling systems. This then, might be described as the use of an advanced building material in wall systems that simply replicates traditional construction techniques. 
 
An alternative and highly innovative approach was taken recently in the unlikely location of a Somerset farmyard where the owner sought to house his collection of historic and contemporary architectural drawings. Hugh Strange Architects (whose own ‘Strange House’ in London had previously won multiple design awards for its innovative use of cross laminated timber) were commissioned to work within the wall remnants of a derelict farm structure, with the client’s brief as to the required environmental qualities resulting in some very sophisticated new thinking by the architect about the physics of the new building. Recognising the inherent, but hitherto under-utilised, thermal potential of solid timber construction, Hugh Strange elected to use cross laminated timber panels of 300-420mm thickness for the walls, i.e. more than twice the usual depth. At around 3.6 tonnes each, these were also considerably greater in weight than normal, with the heaviest panel on the absolute limit of the lifting crane’s capacity. Why so big therefore? 
Using thick cross laminated timber without any external wall cladding, insulation or internal wall lining obviates the need for other materials in the wall make up and the intention was thus for the panels themselves to achieve the necessary insulation and thermal mass values. This integral combination of properties has the added benefit of delivering the stable temperature and relative humidity conditions essential for the long-term protection of the valuable drawings collection. 
 
This may all seem to be relatively unexciting to those unfamiliar with the use - and potential - of solid timber components, but the concept of an uninsulated and unclad solid timber building envelope is, to all intents and purposes, unexplored territory, especially in the search for high performance as a thermal and moisture ‘sink’ for environmentally sensitive spaces.
And there are other design and construction benefits to be had from the building’s very thick timber walls. Being sized to achieve thermal efficiency rather than for their structural performance meant that the cross laminated timber roof panels are able to span length-wise and thus minimise the loads on the side walls as well as avoiding the need for a down stand beam at the apex of the roof pitch. An obvious solution perhaps, but in itself quite novel and, more importantly, making it possible to create simple internal volumes without the interruption of additional load bearing walls.
It should also be mentioned that cross laminated timber is fabricated using multiple axis computer numeric cutting (CNC) machines that deliver astonishingly accurate 3-D products from computer generated design drawings, no matter how complex the openings and junctions required. Panels arrive at site direct from the factory and are quickly assembled with tolerances of 2mm not uncommon. With openings pre-formed in the panels, there is usually little on-site joinery adaptation required and their precision manufacture permits airtight junctions to be achieved far more easily than with other types of construction. In the case of the architectural archive building, the sheer weight of the thicker panels meant reduced manoeuvrability and a consequent slower assembly. That said, once the panels were in position, the remainder of the works proceeded at the fast pace normally associated with cross laminated timber structures.
 
The main issue in this instance though was not speed of construction, but of creating an internal environment absolutely suited to the building’s proposed function and contents. In this respect the Architectural Archive is something of a pioneer - indeed, an experiment - in recognising the opportunity presented by an engineered timber product in which the properties of thermal mass, insulation and water retention are combined. In utilising the product to provide stable conditions suitable for the archiving of sensitive material, the design approach taken here provides an exemplar for innovative construction strategies to be applied in other small-to-medium size archive, gallery and museum buildings, particularly those with commensurate scale budgets. The use of cross laminated timber technology has also been advanced, since the building provides a basis for the establishment of guidelines for its use in the construction of environmentally sensitive spaces for which specific environmental control requirements exist. Achieving these conditions with an entirely passive solution can be regarded as something of a Golden Fleece in the curatorial world where the security and protection of artefacts is paramount. Defence: definitely; protection: certainly, with the Architectural Archive effectively demonstrating how two words often given a negative interpretation can, in combination, actually form a positive.