A cavity wall consists of two masonry walls (known as the inner and outer leaves or wythes) tied together with wall ties. The outer leaf acts as a ‘protective skin’ against the elements, principally driving rain, and the inner leaf transfers the load of the building to the foundations.
The cavity serves to keep the two leaves apart, preventing damp from passing through from the outside to the inside of the wall. It also allows for the evaporation of any condensation or rainwater which penetrates the outer leaf and ensures a more even temperature inside the building.
Wall ties that are built into the cavity walls installed before 1981 are prone to corrosion.
Cavity Wall Tie Failure can be identified by horizontal cracking caused by the expansion of the corroded wall ties, normally at approximately 450mm vertically apart, or in severe cases, bulging of the brickwork caused by the snapped wall ties which have completely corroded.
Thermal cracking can occur when inadequate provision of movement joints has not been incorporated in a wall. Although thermal cracking is not considered to be of any structural significance, on-going maintenance will be required. The cracking will vary at different times during the year due to seasonal temperature changes.
An example of Cavity Wall Insulation installed to a property not suitable for Retro Fit Cavity Wall Insulation.
The Cavity Wall Insulation which has been retrospectively installed has become completely saturated due to porous brickwork and defective pointing. The insulation may dry out eventually but there is a very high chance that it will have slumped or compressed which will reduce its insulating properties and cause, what is termed, a ‘cold bridge’.
A cold bridge is a weak spot in the insulation surrounding a house. Cold bridges (also known as thermal bridges) occur whenever there is a break in the continuity, or a penetration of, the insulation. Examples of cold bridges include:
- Junctions between walls and floors, and walls and roofs.
- Reveals around windows and doors.
- Holes made by pipes and cables.
- Studwork in timber frame walls (interrupting the insulation).
- Steel wall ties in masonry construction.
Before a cavity wall has Retro-filled cavity wall insulation a thorough survey of the wall should be completed that includes:
- The width of the cavity and condition of the cavity wall ties.
- The condition of the walls and mortar joints.
- The exposure of the walls.
- The suitability of the wall to receive the Retro installation.
Cavity wall insulation requires Building Regulations approval but this is usually met by what is referred to as a “type approval”. The installation procedure including checking the walls is part of this type approval. The installer of an approved system then simply has to register each installation with the Building Control Department of the local authority.