An issue that has been causing much confusion among developers at the moment is the requirement for the prevention of fire spreading within cavity walls.

The ultimate question being - when do cavity closers need to have fire resistance?

Let’s start with the basics.

What is the difference between a cavity closer and a cavity barrier?

The key difference between the two is that a cavity closer does not provide fire resistance. Cavity closers usually consist of a proprietary insulated closer or trim with no fire resistance whereas a cavity barrier is made up of a material designed to resist the passage of flame as outlined in the legislation.

When do you need to install cavity barriers?

According to the guidelines cavity barriers need to be installed around ALL openings in cavity walls.

The only exception to this rule is when walls have two leaves of masonry or concrete that has a minimum thickness of 75mm. In this case cavity closers need to be installed at the top and around the openings.

When fitting meter cupboards in brick cladding a maximum of 2 cavity barriers should be installed and the service penetrations through the inner leaf should be fire-stopped.

What materials can be used for cavity barriers?

  • Steel with minimum thickness of 0.5mm
  • Timber with minimum thickness of 38mm
  • Polythene-sleeved mineral wool/mineral slab which is compressed when installed in the cavity
  • Calcium silicate, cement-based or gypsum-based boards with minimum thickness of 12mm
  • Proprietary closers that have been tested and certified

Cavity barriers do not need to be installed where windows or door frames are constructed of steel or timber of the minimum thickness outlined above as the cavity barrier is already provided as part of the frame.

For more detailed information on installing cavity barriers to inhibit the spread of fire and smoke within concealed spaces check out the guidance in Approved Document B.

By Anna Symington