The strong interest among architects in the use of timber for cladding is demonstrated not only in domestic construction, but also for much larger and more prestigious buildings. The ‘green’ qualities of timber as a building material are a prime reason for this, but it also reflects the design potential offered by the variety of species, range of sizes and configurations available.
The majority of timber cladding in the UK is in softwood. Species such as western red cedar, European larch or Douglas fir may be chosen for cladding because of their inherent durability, but less durable woods like European whitewood, treated with preservatives, can be equally suitable. Tropical hardwoods, such as iroko, are also used for cladding and there is an increasing interest in durable temperate hardwoods, like European oak. This has a long tradition of use in the UK; left unfinished and allowed to weather to a natural grey it has lasted for many decades without maintenance.
The increasing popularity of timber cladding has opened up another interesting market for timber – for louvres and brises-soleil (sunbreaks). Recent revisions to the Building Regulations (Approved Document Part L2 Conservation of fuel and power) not only require higher standards of insulation and air tightness – and they will get even tougher in 2005 – but also allow “solar gain” to be included in the calculation of the thermal performance of the building. Consequently, windows have generally increased in size, even to the extent of becoming fully glazed walls in some cases.
Solar gain
The downside is that solar gain, which may be of benefit in winter, can lead to excessive heat and glare in summer. Finding ways of benefiting from the sun when it is useful and excluding it when it is not, have led to the use of various types of horizontal louvres, angled to keep out high level sun, but allowing penetration of low angle winter sun. To reduce unwanted glare, similar louvres can be used vertically to exclude low level western sun, often in conjunction with horizontal louvres.
Louvres positioned directly across all or part of the glazed wall or window are the simplest form. These may be fixed or adjustable, in that they can be angled like an internal venetian blind. External sunbreaks have an advantage over any internal sun shading as they stop heat from entering the building.
External louvres can also provide some degree of visual privacy, or even security if this is required, although it may not be necessary for shading purposes to cover the whole glazed area. The louvres may also act as “light shelves”, designed to reflect light back into the room, increasing daylight penetration without the disadvantage of direct glare.
Wood louvres are often used in conjunction with wood clad buildings, or in conjunction with wooden balconies, screens or windows. They may even be incorporated as an extension of the wood cladding profile chosen for the unglazed parts of a building’s elevation. They can be left unfinished or can be stained or painted like any other external wood component.
Choice of species
Besides choosing the best profile and spacing to shade the building, wood louvres need a careful choice of species and suitably robust detailing to ensure sufficient strength, stiffness and durability. Not only should they be stiff enough to resist high winds, they should also be designed to shed water effectively.
For louvres, certain hardwoods are ideal because of their strength, stability and inherent durability, but good quality, durable or moderately durable softwoods can be used (providing sapwood is removed in all cases). Western red cedar is less suitable for louvres than for cladding, however, as it is less strong and may need more frequent fixing and support than other species.
If the wood cladding and louvres are to be left unfinished, the eventual weathered appearance will be very similar, even if different species are used for cladding and louvres. Whichever species is used, the timber must be of good quality, straight grained and, preferably, largely knot free.
