Designing and installing cavity barriers for timber frame buildings: What you need to know

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. Here, cavity barriers are imperative, and Robin Lancashire, Senior Timber Frame Consultant at BM TRADA, explains the key considerations when designing and installing these elements.

The sustainability agenda is leading a renewed interest in the use of timber in construction. Demand is rising exponentially across both residential and commercial building sectors, and a study by MTW Research found timber frame housebuilding was expected to rise by more than £150m by 2026.

Reasons for the increased usage is clear. Highly sustainable, timber captures carbon better in comparison to other materials such as steel and aluminium that capture none at all but use tonnes upon tonnes of carbon in their production. From a commercial perspective, one of the biggest benefits is that it’s fast to build with, making it more profitable for developers.

While it’s a good sign that timber construction is on an upward trend, safety is still key. In the event of a fire, cavity barriers are of fundamental importance, and they must be able to perform as intended. From design and specification through to installation, there are several key factors that must be considered so that buildings and the people within them are protected.

Preventing fire and smoke from tearing through

The performance and installation of cavity barriers has been flagged in recent fire events across different building types. Without a properly designed barrier, cavities act much like a chimney and can allow fire and smoke to quickly travel unseen through what can often 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. In the UK, there are some regulatory differences between the nations in terminology and specific requirements, but the principles are all the same – limit the spread of fire in a cavity.

Fire safety and ventilation must be balanced

When designing timber frame external walls, one of the earliest decisions that needs to be made is how to balance ventilation with fire safety.

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% of less). A drained and vented external wall cavity behind all types of cladding is required for any timber frame construction. This space allows moisture vapour that travels through from the warm side of the wall to the cold side to ventilate away, without forming damaging interstitial condensation.

The external wall cavity is a line of defence to protect the timber frame from external moisture.  It prevents wind-driven rain or leaks through the cladding from directly wetting the timber frame structure, by letting moisture drain away freely. Therefore, the decisive challenge is delivering cavity barriers where required, while still allowing the timber frame structure to drain and vent.

For those designing and building with timber, the Timber Frame Construction (5^th edition) is the go-to publication to reference. Information on various types of cladding can be found in the book’s cladding chapter, with details on how to close external wall cavities at required locations, while still providing the necessary drainage and ventilation.

Choices with cavity barriers

Understanding where to specify cavity barriers is just one part of the equation. The material they are made of must be tailored to the type of cladding specified. There are a number of choices available, from using timber itself, to mineral wool, and intumescent seals.

Timber cavity barriers

Surprising to some, timber is listed in the building regulations as a material that can be used to provide the necessary fire resistance of a cavity barrier. 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. Available for installation in continuous lengths, it is reasonably robust during construction, and fulfils other roles while acting as a cavity barrier.

Mineral wool cavity barriers

Mineral wool is another choice. Strips of mineral wool, typically protected in red polythene sleeves, can often be seen on many timber frame buildings under construction. As a cavity barrier, this material performs well 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 building product, bear in mind that in the event of a fire, the polythene sleeve quickly burns, meaning there is no support from it to keep the cavity barrier in place. Accurate sizing to ensure a compression fit of the mineral wool core is therefore critical to stopping these strips falling down the cavity and failing. It is important to note that they can also be easily damaged or dislodged during the construction phase, so care must be taken during installation.

Intumescent seal cavity barriers

Despite being more expensive than timber or mineral wool, the use of intumescent cavity barriers is expanding. 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 maintaining 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 – merging acoustics with fire safety

External walls are not the only place where cavities are found. The other main area where they occur is in party walls. Timber frame buildings rely on cavities to reduce acoustic transfer between areas of the same building. To prevent fire and smoke having a direct route between them, these cavities need closing at compartment lines.

Hard materials cannot be installed as cavity barriers here, as these would provide a route for acoustic transfer. Instead, wire-reinforced or polythene-sleeved mineral wool cavity barriers tend to be used in these locations. It is important that they are fixed and sized to close the cavity and remain in place at compartment lines.

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

Check with timber frame consultants

Designing and building timber frame constructions ultimately needs appropriate due diligence to ensure that the resulting structures are durable and safe. The accurate specification of cavity barriers is critical to this.

It is highly recommended that designers and construction planners work with timber frame consultants at the initial design stage to ensure the project adheres to best practice. Using services such as BM TRADA’s frameCHECK allows specialists to evaluate drawing details and visit sites under construction to provide specific advice.

By going down this route, the standard of timber frame construction can be raised while also helping to make the buildings of tomorrow safer than today’s.

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