Definition and regulations

A cavity barrier is a partition constructed for the purpose of closing a concealed space to protect the space from the penetration and/or restrict the movement of flame and smoke within the space and adjoining areas.

Cavity barriers are covered in the Building egulations under Technical Guidance Document B. All cavity barriers need to conform to I.S. EN1364 or BS476-22:1978 and each barrier should be clearly marked with the cavity widths that each barrier is suitable for use in, its fire rating, and the manufacturer’s name.

Installation

For installation:

Cavity barriers should be secured to the timber frame studwork through the internal flanges by austenitic stainless steel staples or another approved fixing method.
Cavity barriers should not be installed in such a manner that the barrier itself is bent or flattened as this may cause gaps within the barrier. (e.g. around corners, flues, etc.)
Cavity barriers should be lapped a minimum of 150 mm at horizontal and vertical joints when the cavity barrier laps itself.
Cavity barriers should only be installed by competent personnel as per the manufacturer’s guidelines.

Types and gaps

Polythene sleeved mineral fibre is the most common type of cavity barrier used, however other varieties are available. Timber cavity barriers are another form that can be used; they are constructed of solid timber that has been pretreated with preservatives. Their thickness varies in relation to the width of the external cavity but should be no less than 38 mm. They should be secured to the timber frame studwork using austenitic stainless steel nails in conjunction with suitable DPC (minimum 1000 gauge polythene can be used).

No gaps should be present between the cavity barrier and the masonry leaf mortar should be used as a filler material to seal any gaps between the cavity barrier and the masonry leaf on typical types of cavity barrier. For timber cavity barriers, the timber should be cut to size to fit correctly with no gaps and with exposed cut ends treated on-site with preservative to add protection.