Timber technology Timber frame
The importance of cavity barriers Robin Lancashire explains the key role of cavity barriers in timber frame construction and how they can limit the spread of fire.
W
hile the importance of cavity barriers is increasingly recognised, the construction industry still needs to concentrate on ensuring that they are installed correctly, so that in the event of a fire they can perform as intended.
Cavity barrier performance Recent fire events in various building types have turned attention to the performance of cavity barriers and how they are installed in buildings. The cavity barrier plays a critical role in preventing fire and smoke from travelling unseen through what can be an extensive network of connected spaces in a building. Acting much like a chimney, a cavity can allow fire and smoke to travel fast and unseen, making it difficult to fight fires, causing extensive damage and risking lives. National building regulations tell us where cavity barriers should be installed, what they should be constructed of and how they should be fitted. There are some regulatory differences between the nations that make up the UK, both in terminology and specific requirements, but the principles are all the same – limit the spread of fire in a cavity.
Balancing ventilation and fire safety When building with timber frame, it is important to accommodate the requirement for cavity barriers along with the need to provide an environment where timber elements can remain dry and below the decay threshold (a moisture content of 20% or less). Timber frame construction needs a drained and vented external wall cavity behind all types of cladding. This provides a space where moisture vapour that travels though from the warm side of the wall to the cold side can ventilate away, without forming as damaging interstitial condensation. The external wall cavity is also a line of defence from the outside: it prevents wind-driven rain or leaks through the cladding from directly wetting the timber frame structure by letting moisture drain away freely. The challenge therefore is to provide cavity barriers where required, while allowing the timber frame structure to drain and vent. www.bmtrada.com
Timber cavity barrier around a window
Timber Frame Construction (5th edition) is the go-to publication for those designing and building with this increasingly popular material. The cladding chapter of the book provides detailed sections of various cladding types, along with information on how external wall cavities are closed at required locations, while still providing the necessary drainage and ventilation.
Fire resistance It may be surprising to learn that timber is listed in the building regulations as a material that can be used to provide the necessary fire resistance of a cavity barrier. However, it is the material of choice for cavity barriers around window and door openings, and with claddings that are supported by the timber frame structure. In the event of a fire, the timber slowly chars at a predictable rate so it can provide the required period of fire resistance. It can be installed in continuous lengths, is reasonably robust during construction and fulfils other roles while acting as a cavity barrier. >> Timber 2022
| 37
