KEY CONSIDERATIONS WHEN DESIGNING AND INSTALLING TIMBER FRAME CAVITY BARRIERS

With timber frame construction expected to continue growing in popularity, it’s critical that any building made from the material is adequately protected against the spread of fire. Robin Lancashire, Senior Timber Frame Consultant at BM TRADA, explains the key role cavity barriers have here and what to consider when designing these elements.

here’s no denying that timber is growing in popularity as a building material. According to a study by MTW Research, timber frame housebuilding was expected to see a £70m increase in 2022, with demand rising rapidly across both residential and commercial building sectors.

It’s not hard to see why either. Timber is a highly sustainable material and can capture carbon better, unlike other materials, such as steel and aluminium. Alongside this, it’s fast to build with, so the commercial benefits are significant for developers.

However, while the growth of timber construction is a good sign, safety remains vital. This is where the importance of cavity barriers comes in and, in the event of a fire, they must be able to perform as intended. There are some key factors around design, specification and installation that must be considered to ensure both buildings and the people within them are protected.

Recent fire events in various building types have highlighted the performance of cavity barriers and how they are installed in buildings. Cavities act much like a chimney, so without a properly designed barrier, fire and smoke can quickly travel unseen through what can be an extensive network of connected spaces in a building. National Building Regulations stipulate 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.

One of the first decisions to make is how to balance ventilation with fire safety. When building with timber frame, it is important to accommodate the requirement for cavity barriers while also ensuring timber elements 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 through from the warm side of the wall to the cold side can ventilate away, without forming 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 key challenge, therefore, is to provide cavity barriers where required, while allowing the timber frame structure to drain and vent.

For those designing and building with timber, the Timber Frame Construction (fifth edition) is the go-to publication to reference. The cladding chapter of the book provides detailed information on the various types of claddings, as well as how to close external wall cavities at required locations while still providing the necessary drainage and ventilation.

Knowing where to provide cavity barriers is just one part of the equation, as they must be tailored to the characteristics of timber frame buildings. There are a number of choices available, from using timber itself to mineral wool and intumescent seals.

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 cladding that is 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.

Next, there is mineral wool. Typically red in colour, polythene-covered strips of mineral wool are a common sight on many timber frame buildings under construction. The material performs well as a cavity barrier if it is fitted in a continuous line and is sized to be installed under compression to fully close the cavity.

When designing with this material, it is important to bear in mind 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 stopping 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 in popularity. In the early stages of a fire or heat exposure, 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.

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.

Cavity barrier: Typically proprietary third-partyapproved sleeved mineral wool cavity barriers

Junction of element firestopped with plasterboard linings

Party wall

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

Sheathing with breather membrane over

Seal

Allowance for differential movement

Wall ties as required

Ultimately, designing and building timber frame constructions needs appropriate due diligence to ensure that they are safe and durable. The accurate design and specification of cavity barriers is critical to this.

From the outset, it is highly recommended that designers and construction planners work with timber

Insulation

Window fixed to the timber frame

500 micron polythene DPC frame consultants to build according to best practice. Here, using services like BM TRADA’s frameCHECK allows specialists to evaluate drawing details and visiting sites under construction to provide specific advice. By following this approach, the standards of timber frame construction can be raised, and it helps make the buildings of tomorrow safer than today’s. www.bmtrada.com/timber-services