Build it with

various types of claddings, as well as how to close external wall cavities at required locations while still providing the necessary drainage and ventilation.

Cavity barrier choices

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.

Timber cavity barriers

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.

Mineral wool cavity barriers

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.

Intumescent seal cavity barriers

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.

Merging acoustics with fire safety in 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.

Collaborating with timber frame specialists

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 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 makes the buildings of tomorrow safer than todays.

More at  www.bmtrada.com/timber-services