Cavity Wall

Cavity walls consist of two separate parallel walls or ‘leaves’ with a 50-90 mm gap left between them.  They are used for the external walls of buildings. 

The purpose of the gap is twofold:

  • It helps prevent moisture from penetrating through to the interior of the building
  • It helps provide heat insulation


The two leaves of a cavity wall are held together by ties which are inserted as the building is being erected. Ties were traditionally made of wrought iron or mild steel, and were sometimes unprotected or possibly coated in bitumen galvanised. Plastic or stainless steel ties are now used.

Some houses were built with Cavity walls in the late nineteenth century, but their use did not become common until the 1920’s and they have now become almost universal. It can be difficult to tell from visual in section whether an older property has cavity walls. If this is an issue then it may be necessary to drill a small hole into a wall to check.

The exterior leaf is often constructed of facing bricks, but may be constructed of common bricks or Aggregate blocks which are covered by cement render or other decorative finish (e.g. Pebble Dashing, weatherboarding) 

In earlier times the interior leaf was constructed of common bricks, but aggregate blocks of various types have often been used. Nowadays the inner leaves if cavity walls are usually constructed of lightweight aerated concrete blocks which provide better insulation.

Older properties may have had cavity wall insulation inserted in the gap. 

Potential problems with cavity walls 

  • Cavity wall tie failure due to corrosion or rusting 
  • Damp bridging the gap between the leaves and affecting the interior of the home – damp should not normally be able to cross the gap and penetrate the interior wall. However if the gap is bridged (e.g. by mortar being allowed to fall on the wall ties during building work, or faulty installation) then damp can cross and cause damp patches on the internal walls of the property


Your surveyor will report any suspected problems such as cracks in exterior walls or evidence of dampness on internal walls. Further investigation will be necessary to ascertain the cause. Further specialist surveys will then be required, together with an estimate of any necessary remedial works.

Post Author: Frances Traynor