Cavity wall is a double wall consisting of two separate walls, called “skins” or “leaves” of masonry separated by an air space and joined together by metal ties at suitable intervals.
These walls are generally exterior walls, although sometimes used as interior walls.
Advantages of cavity walls:
- There is no possibility of the moisture travelling from the outer wall to the inner wall.
- The layer of air in the cavity being non-conductor of heat, and reduces the transmission of heat from the external face to internal one.
- This acts as damp barrier, reduces the cooling cost of the building.
- Economy – A 275mm cavity wall costs less to construct than a 328mm solid wall.
- Cheaper than exterior or interior wall insulation.
- Maintains existing wall thickness.
- Minimal disruption to install.
- Can reduce condensation.
Disadvantages of cavity walls:
- Cavity walls were first built, in exposed coastal areas, in order to keep out wind-driven rain. Filling the cavity with insulation will always hold the risk that moisture will be able to find its way across to the inside, whatever the insulation material.
- There is also the possibility that the installation will leave unfilled air pockets –causing ‘cold spots’ on the inside walls which attract condensation.
- Another problem concerns wall-tie corrosion; cavity insulation makes the outer brick leaf colder, and therefore wetter, which can accelerate rusting of the wall ties. And if the ties then have to be replaced, there is no satisfactory way of refilling the holes in the insulation, whatever the material.
- Thermal bridging problems.
- Thickness of insulation is restricted by width of cavity.
- Significant parts of the UK are unsuitable due to their exposure.
- There are significant number of buildings with mortar droppings on ties within the cavity which result in penetrating dampness.
- Settlement and saturation of cavity-fill leading to cold bridging.