Cavity Closers in Timber Frame Construction

Expansion and Contraction Provision.

Cavity closers are now available to accommodate cavities up to 200mm wide. When such cavity widths are deployed within timber frame construction, the designer should considerer the effect a wider cavity width has on the relationship between the exterior skin of masonry and the internal skin of timber.

The co-efficient of expansion of both skins differs. The introduction of a wider cavity results in the axis point (or pivot point) between skins increasing, and this affects the ‘see-saw’ behaviour when movement takes place. (The movement of one skin will directly affect the substance between it and its opposite extreme). This manifests at reveals.

Whilst closers cannot banish movement at reveals, selecting specific types of closer can help. The WCA Cavicloser is an example of a closer that permits its inward portion to be screw fixed to the timber frame, and its exterior portion to be restrained to the masonry via bespoke wall ties. The main body has a friction link moulding offering some vertical movement provision. This type of feature is more likely to be sympathetic to expansion and contraction.

Type WCA may be used in traditional construction also. Ties connect to both skins in such applications

Differential movement between frame and masonry / cladding needs to be allowed for at a number of points in typical construction. An example adjacent to the reveal closer is where a masonry / tile windowsill spans from exterior to interior skin. Failure to provide for movement can result in shrinkage / expansion / contraction instigating rotation of the sill at the pivot point. The presence of a movement gap with a flexible soft infill where the sill terminates below (not touching) the frame is recommended. Always ensure the appropriate relationship exists where the undersill dpc tray interfaces with the reveal closer.

Suggested reading:

Closing Cavity Walls – Closers Stops & Barriers volume 19

Cavicloser Type WCA

Cavicloser Cavi 60 Type WCA (Fire rated version offering one hour rated integrity)

Cavity Trays Ltd, (main manual) Damp Proofing Products

Type U undersill tray