Timber frame The importance of cavity barriers

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.

While 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. 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.

Timber cavity barrier around a window

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. >>

The typically red polythene covered strips of mineral wool that adorn many timber frame buildings under construction are a common sight: these perform well as a cavity barrier if they are fitted in a continuous line and are sized to be installed under compression to fully close the cavity. Remember that in the event of a fire, the polythene sleeve quickly burns, providing no support to keep it in place. Precise sizing to ensure a compression fit of the mineral wool core is therefore critical to stop these strips falling down the cavity and failing. They can also be easily damaged or dislodged during the construction phase.

Although more expensive than timber or mineral wool, intumescent cavity barriers are gaining popularity. In the early stages of a fire and exposure to heat, they are designed to swell up, closing the cavity against further fire and smoke. In their inactive state they can contribute to good drainage and ventilation through a clear cavity. This can simplify detailing and reduce the need for what can be unsightly drainage slots.

Party walls

Cavities are not only found in external walls. The other key area where they occur is in party walls. Timber frame buildings rely on cavities to reduce acoustic transfer between areas of the same building. These cavities need closing at compartment lines to prevent fire and smoke having a direct route between them. Hard materials cannot be used as cavity barriers here as these would provide a route for acoustic transfer, so wire reinforced mineral wool or polythene sleeved mineral wool cavity barriers tend to be used in these locations. It is critical that they are fixed and sized to close the cavity and remain in place at compartment lines.

While most cavity barriers are installed at edges of cavities and along compartment lines, there are other locations where they are required by national building regulations. The relevant statutory documents should be consulted. n

Cavity barrier: Typically proprietary third party approved sleeved mineral wool cavity barriers Junction of element firestopped with plasterboard linings Party wall

Flexible cavity barrier of wire reinforced mineral wool or approved cavity barrier

Sheathing with breather membrane over Sill detail

BM TRADA frameCHECK

BM TRADA’s frameCHECK team specialises in providing consultancy on the design and construction of timber frame buildings. Its work assists all those involved in timber frame construction by helping to ensure that buildings can be detailed and constructed to best practice. Typical consultancy work involves evaluating drawing details and visiting sites under construction to provide specific advice. Whether you are an architect, contractor, surveyor or building owner, consider using the frameCHECK service, which has been helping raise the standards of timber frame construction for more than 25 years.

About the author

Robin Lancashire Senior Timber Frame Consultant BM TRADA

Further information

For technical advice or consultancy services, contact the BM TRADA technical timber team on +44 (0) 1494 569601 or visit www.bmtrada.com/advisory-services

Further reading

• Timber frame construction, 5th edition, ISBN 978-1-900510820, BM TRADA, 2011 • WIS 0-3 Introduction to timber frame construction,

BM TRADA, 2020