How to Install Cavity Wall Insulation — Step-by-Step UK Guide

Understanding the Process

Cavity wall insulation is a professional installation that involves injecting insulating material into the gap (cavity) between the inner and outer leaves of an external wall. While you cannot install this yourself, understanding the process helps you choose the right installer and avoid problems.

Step-by-Step Process

Step 1: Suitability Survey

A registered installer will inspect your property to check: wall construction type, cavity width (using a borescope), condition of the outer leaf (pointing, cracks), exposure zone (sheltered, moderate or severe), and whether there is any existing insulation. They will also check for obstructions such as wall ties, debris or rubble in the cavity.

Step 2: Choose the Insulation Type

EPS beads: Polystyrene beads coated with adhesive, blown into the cavity. Good all-round choice, BBA-certified, suitable for most properties. PU foam: Sprayed as a liquid that expands to fill the cavity. Excellent thermal performance (lowest U-value) but more expensive and harder to remove if problems occur. Mineral fibre: Blown rockwool or glass wool. Breathable and moisture-tolerant, suits older properties and exposed locations.

Step 3: Preparation

The installer will protect windows, doors and any vulnerable areas. They will check and repair any damaged pointing or cracks in the outer leaf — these must be sealed before injection to prevent insulation escaping or moisture entering.

Step 4: Drilling Injection Holes

Holes (typically 22-25mm diameter) are drilled through the outer leaf of brickwork at approximately 1m centres in a grid pattern. The holes are usually drilled into mortar joints to minimise visual impact. On rendered walls, the render is cut and refilled after injection.

Step 5: Injection

The insulation material is injected through each hole using specialist pumping equipment. The installer works systematically across the wall, starting at the bottom and working up. They monitor the injection pressure and flow rate to ensure the cavity is fully filled without over-packing.

Step 6: Sealing and Making Good

Each drill hole is plugged with a colour-matched mortar or plastic plug. On rendered walls, the holes are patched with matching render. The finish should be neat and barely noticeable.

Step 7: Certification and Guarantee

The installer should provide a BBA (British Board of Agrément) certificate for the insulation material and a CIGA (Cavity Insulation Guarantee Agency) 25-year guarantee. Keep these documents safe — they are needed for house sales and any future claims. The work should also be notified to your local authority building control department.

Choosing an Installer

• Use a CIGA-registered installer — this provides a 25-year guarantee backed by an independent body.
• Check the installer holds current BBA certification for the materials they use.
• Ask for references and check reviews from previous customers.
• Get at least three quotes and compare the survey detail — not just price.
• Check whether you qualify for an ECO grant, which could cover part or all of the cost.

Common Problems to Watch For

• Damp patches appearing after installation: This may indicate the cavity has been bridged by debris or the wall was not suitable. Report it to your installer immediately — it is covered by the CIGA guarantee.
• Cold spots remaining: If certain areas are still cold, the cavity may not have been fully filled. The installer should re-inspect and top up at no additional cost.
• Insulation escaping: If you see insulation material coming out of air bricks or weep holes, the installer has over-filled or not sealed properly. Air bricks must remain clear for ventilation.

Cost Estimate (2026 UK Prices)

Use our Cavity Wall Insulation Calculator and U-Value Calculator to estimate costs and thermal performance for your property.