Defence can be a passive activity

Materials World magazine
,
1 Dec 2016

Peter Wilson examines the role of cross-laminated timber in the protection and thermal insulation of buildings. 

In our highly volatile modern world, the word ‘defence’ has, for many people, immediate connotations with military activity and a somewhat-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 they are intended to preserve. Such is the case with cross-laminated timber (CLT), a product that has been on the market since the early 1990s, although its full potential remains in the very early stages of exploration in construction. However, cities around the world are now competing to deliver ever taller solid timber structures in some of their more densely packed urban areas, such as schools, supermarkets, office buildings and health facilities, all of which are increasingly taking advantage of the product’s low embodied carbon credentials and other environmental benefits. The number of building contractors who recognise the speed and ease that CLT construction can be created is expanding. Until recently, however, little research and development had been carried out to determine the capacity of very thick CLT panels to provide major benefits in terms of airtightness and passive control of internal temperature and relative humidity. 

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 20th Century approach to these challenges generally involved extensive use of expensively run mechanical systems to deliver a specific and strictly monitored internal environmental quality. All too often, these systems were introduced to mitigate problems created by design that demanded the use of materials inappropriate for the building’s actual purpose and functional effectiveness. As such, many of the solutions applied can nowadays be seen not only as insufficiently protective to the materials they are there to environmentally defend, but in some cases, entirely contrary to modern conservation practice.  

Thermal protection

In this context, therefore, what difference can CLT 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 resistance and the required U-values. But, in the UK, the walls are invariably also lined internally to provide a non-timber surface finish, which completely negates the hygroscopic benefits of the material. Additionally, this build-up of different materials may not deliver the levels of thermal performance demanded by curators, thus necessitating the introduction of mechanical air handling systems. This, might be described as an advanced building material in a wall system that simply replicates traditional construction techniques. 

An alternative and highly innovative approach was taken recently in the unlikely location of a, farmyard in Somerset, UK, where the owner sought to house his collection of historic and contemporary architectural drawings. Hugh Strange Architects (whose own ‘Strange House’ in London hads won multiple design awards for its innovative use of CLT) was 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 sophisticated new thinking by the architect about the physics of the new building. Recognising the inherent but hitherto under-used thermal potential of solid timber construction, Hugh Strange elected to use CLT panels of 300–420mm thickness for the walls, more than twice the usual depth. At around 3.6 tonnes each, these were also considerably heavier than normal, with some panels on the absolute limit of the lifting crane’s capacity. 

Using thick CLT 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 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 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. 

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 structural performance meant that the CLT roof panels could 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, creating simple internal volumes without the interruption of additional load bearing walls. 

Exposure to the elements

CLT is, however, formed from softwood strips arranged in perpendicular layers and is generally classified as non-durable when exposed to the elements. A number of measures have been applied to protect it from rain. The concrete foundation slab has upstand beams that effectively raise the panels well above ground level and splashes of rainwater that invariably causes the bottom of wall timbers to rot, while deep eaves to the fibre cement roof shield the top of the walls from driving rain. Remnants of the old stonewalls protect much of the remaining external timber surfaces, which have, in any case, been painted with a water-repellant that also provides UV protection. Anyone who has worked with wood will know that the end grain is the area most susceptible to water damage. To protect door and window openings where end grain is exposed, the architect has fixed the doors, windows and roof lights to the outside face of the timber, thereby also providing deep, useable reveals to the interior spaces.  

It should also be mentioned that CLT is fabricated using multiple axis CNC machines that deliver astonishingly accurate 3D 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 CLT 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. 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 using CLT 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 CLT 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. The architectural archive effectively demonstrates how two words often given a negative interpretation can, in combination, form a positive. 

Peter Wilson is an architect and managing director of Timber Design Initiatives Ltd. The company aims to deliver new, Europe-wide approaches to education, innovation and demonstration of best practice in the use of wood in architecture, design and construction through applied research collaborations between academia and industry