Energy Efficiency Measure Guide: Cavity Wall Insulation (CWI)

Note: All installations must comply with current regulations, which are subject to change.

Key points to consider

• Someone must be present during installation — either you or someone you trust.
• Survey and installation drill holes will be filled afterward, but may require painting over if the installers cannot match the existing colour perfectly
• Ventilation upgrades are likely required alongside this installation.

See our ventilation guide for more information. Ventilation | YES Energy Solutions

What is cavity wall insulation?

Cavity Wall Insulation involves filling the gap between the inner and outer walls of your home with insulating material. This reduces heat loss, making your home warmer in winter and cooler in summer. It also improves energy efficiency and can lower heating bills.

Common materials used:

• Mineral wool
• Polystyrene beads
• Foam

It’s a popular and cost-effective method for improving comfort and saving energy in many UK homes.

Common misconceptions

“Every house can have CWI.”

• Not true — homes with solid walls or unsuitable conditions may not qualify. A professional survey is required.

“CWI causes damp.”

• Only if installed improperly or in unsuitable properties. Professionally installed CWI in a suitable home will not cause damp.

“Insulation is always free.”

• Some grants may cover the full cost, but eligibility varies.

“It won’t save much money.”

• A properly insulated home can save up to £300/year on heating bills, depending on property size and energy use.

FAQs

Is my home suitable?

• A qualified surveyor will assess cavity size, wall condition, and exposure to weather.

How long does it last?

• Typically 25+ years. Most installations come with a 25-year CIGA guarantee.

The installation process

Before installation

A survey will confirm suitability and required works.

A borescope inspection is conducted:

• A small hole is drilled in the mortar joint.
• A borescope (small camera) is used to inspect the cavity.
• Typically, 3 photos are taken per wall (e.g., front, back, side of a semi-detached house).

The survey checks for:

• Presence and quality of insulation
• Wall condition
• Water penetration or other issues

You’ll be advised to clear any areas — this is your responsibility. If needed, contractors can assist.

During installation

If old insulation needs removal:

• Bricks at the bottom of the wall are removed.
• High-pressure air dislodges insulation, which is vacuumed out.
• Borescope confirms removal; bricks are reinstalled and sealed.
• Installers dispose of old insulation.

Steps:

• Small holes drilled into mortar joints (outside of property)
• Holes spaced ~1 meter apart, with extras around windows/lintels
• Insulation material chosen based on property specifics
• Material pumped into cavity using specialised equipment
• Holes refilled — colour match may vary; redecorating is not grant-funded

Typical duration: Half a day to a full day.

Maintenance recommendations

• Regularly check external walls for cracks or damaged mortar
• Keep vents and air bricks clear
• Report any signs of damp promptly

Handover documents

• After completion and sign-off, you’ll receive:
• Trustmark Certificate
• Insurance Backed Guarantees
• Claim of Compliance
• Workmanship Guarantee
• Manufacturer’s Warranty

Keep these documents safe — they may be needed for future home sales or insurance.