Cavity Barrier Calculator

In Approved Document B Volume 1 (dwellings) cavity barriers are required to divide cavities and to close their edges, at three locations (paragraph 5.18 for houses, 8.3 for flats): at the edges of cavities, including around openings such as windows, doors and service entry points; at the junction between an external cavity wall and every compartment floor and compartment wall; and at the junction between an internal cavity wall and every compartment floor, compartment wall or other fire-resisting wall or door assembly. Volume 1 does not set a maximum cavity dimension. Note that cavity barriers (Section 5 for houses, Section 8 for flats) are deliberately distinct from fire-stopping, which is Section 9.

The 20m maximum cavity dimension is a Volume 2 rule for buildings other than dwellings (AD B Volume 2, Table 9.1): cavities are limited to 20m, reduced to 10m where exposed cavity surfaces are worse than class C-s3,d2, and there is a 40m limit on the line enclosing a room. It does NOT apply to dwellings. AD B Volume 1 (houses and flats) has no maximum cavity dimension — barriers are required only at edges, junctions and around openings. For a house or flat, you do not subdivide a long cavity at 20m intervals.

AD B Volume 1 paragraph 5.21 lists acceptable forms for a cavity barrier in a stud wall, partition or around an opening: steel at least 0.5mm thick, timber at least 38mm thick, polythene-sleeved mineral wool (or a mineral wool slab) under compression, or a calcium silicate, cement-based or gypsum-based board at least 12mm thick. Crucially the same paragraph notes these forms "do not necessarily achieve" the E30/I15 performance required by paragraph 5.20 — so the tested classification of the specific product governs. A steel or timber window/door frame of the minimum thickness can itself act as the barrier around an opening. Above 18m, materials in the external wall must meet the limited-combustibility requirement of Regulation 7(2).

Tested from each side separately, a cavity barrier should provide a minimum of 30 minutes integrity (E30) AND 15 minutes insulation (I15) — this is AD B Volume 1 paragraph 5.20 for houses and 8.8 for flats. It is E30 plus I15, not 30 minutes of insulation. Performance is demonstrated by test to BS EN 1363-1 and classified to BS EN 13501-2. Proprietary products should carry third-party certification such as BBA or Certifire. For buildings over 18m the external-wall combustibility limits of Regulation 7(2) also apply.

Yes. Cavity barriers must close the edges of cavities, which expressly includes around openings such as windows and doors (AD B Volume 1 paragraph 5.18 for houses, 8.3 for flats), to stop fire spreading unseen within the cavity past the frame. Close the cavity at the head, jambs and cill of each opening; a steel or timber frame of the minimum thickness in paragraph 5.21 can itself form the barrier. This applies to all building heights and is especially important where the cavity contains combustible insulation.