Properties of Timber Clad Walls
The issues involved in the acoustic, thermal, tie down and bracing performance of timber clad walls, and the behaviour of wails exposed to fire, are very complex. Much research has been carried out on these subjects and a large amount of detailed information is available.
The main points are summarised below, but more information is available in other datafiles and from local Timber Advisory Services.
Timber clad walls may be required to absorb airborne sound (prevent reflection) or reduce sound transmission from one side to the other.
Sound transmission loss through a timber clad wall is influenced both by the mass of the material in the wall, and by the type of construction. At most frequencies the sound transmission loss is directly proportional to the mass per unit area of the wall, and timber performs well when compared to other types of wall cladding.
Specialised sound control materials are available. Plywood with lead on the faces or in the core acts as an excellent barrier to sound.
Discontinuous construction techniques and the use of porous, sound absorbent materials in cavities will also reduce sound transmission.
In enclosed courtyards or playgrounds the sound absorbency of a wall may be important. Timber claddings generally have a fairly high sound absorption factor when compared to other materials, and the thoughtful use of profiles and broken surfaces will further reduce reflected sound levels.
Whenever there is a temperature difference between the inside and outside of a building, heat will be transferred through walls, floors, ceilings, windows and doors.
The rate of heat transfer through a wall will be related to the difference in temperature on either side, and the resistance to heat flow of the materials in between.
Heat is transferred through timber clad walls by
(1) direct conduction through solid materials,
(2) as radiant heat through cavities, or
(3) by the movement of air within the cavities.
But whilst timber is itself a good insulator, the final thermal performance of a complete wall is dependent on much more than just the timber cladding.
The finished colour of a wall could play an important role. If walls are a dark colour and subject to direct sunlight, summer surface temperatures will be very high, increasing temperature gradients across the wall and thus increasing the rate of heat transfer.
Sheet cladding or the use of sarking behind other cladding materials stops air movement into the cavity of the wall.
Reflective foil insulation will greatly reduce the transmission of radiant heat through the wall. Care must be taken not to create condensation problems by placing the foil membrane in the wrong part of the wall.
Still air is a good insulator and fibre insulation in wall cavities acts by preventing the formation of convection currents within the wall.
Heat movement is affected by more than just the timber clad wall. The thermal stability and comfort of rooms within a building is often affected as much-if not more-by the position and size of windows, doors and other openings.
Tie-down and Bracing
Where plywood exterior cladding will also be used to provide structural bracing and hold-down resistance to roofs, careful attention must be given to design and construction details.
To provide tie-down, external walls must be clad so that top and bottom plates are connected by plywood sheets acting in tension. The connection between floor joists and bottom plate, and top plate and rafter or truss must also be designed to complete the tie-down "chain".
Note that where plywood cladding is used to give structural bracing on external walls, additional bracing may still be required in internal walls.