Cavity walls explained: how they work and why we use them

What a cavity wall is

A cavity wall is really two walls built side by side with a gap between them.

• The outer leaf is the part you see, often brick, stone or blockwork with render.
• The cavity is the gap. It may be empty, partly filled or completely filled with insulation.
• The inner leaf is usually blockwork, a timber frame or a steel frame. It carries the floors and roof, and takes the plaster finish inside.
• Wall ties are small metal or plastic fixings that join the two leaves so they act together.

Why we use a gap

Rain is driven into the outer leaf, and brick and mortar absorb some of it. The cavity means that this water cannot reach the inner leaf: it runs down the inside face of the outer leaf and out through weep holes at the bottom. The gap also gives room for insulation, which makes the wall warmer.

A cavity wall is really two walls built side by side with a gap between them.

The details that make it work

A cavity wall is only as good as its details.

• Damp-proof course (DPC) stops water rising from the ground into the wall.
• Cavity trays are placed above openings and at other interruptions to catch water in the cavity and send it outwards.
• Weep holes (often open vertical joints) let the water out at the base and above trays.
• Cavity closers and lintels are detailed around windows and doors, so insulation and water-proofing are continuous.
• Wall ties must be the right type, number and spacing for the wall and the exposure.
• Keeping the cavity clean. Mortar droppings blocking the cavity can carry water across, so builders must keep it clear.

Insulation: partial fill and full fill

• Partial fill leaves a clear gap between the insulation and the outer leaf, which gives a drainage space.
• Full fill fills the whole cavity with insulation that is made to resist water crossing it. It gives a thinner wall for the same performance, but it needs products, details and workmanship that suit the exposure.

Which one is suitable depends on the climate, the exposure, the product's certificates and the building regulations in force, so it should be decided with the design team and the manufacturer.

Thermal bridges

Where something solid crosses the wall, such as a lintel, a wall tie or a slab edge, heat can escape and surfaces can get cold. We detail these places carefully. See Thermal bridging: what it is and how to avoid it.

Cavity wall insulation in existing houses

Many older houses had insulation added to their cavities after they were built. It can work well, but if the wall is exposed, the insulation is unsuitable, or the installation is poor, it can lead to damp. Anyone thinking of adding or removing it should get an independent survey from a qualified person.

Is it right for every project?

No. Cavity walls are common in the UK and parts of Europe. Other methods, such as timber frame, insulated concrete formwork or solid masonry, suit other projects. See ICF vs. cavity wall vs. timber frame.

In short

• A cavity wall is two leaves with a gap, joined by ties.
• The gap keeps rain from the inside and holds insulation.
• DPCs, trays, weep holes and good workmanship make it work.
• Choose the insulation type to suit the exposure, with professional advice.

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