Cavity wall insulation

Around 33% of all heat loss in an uninsulated home escapes through the walls. For homes built between 1930 and 1990, the most likely solution is cavity wall insulation. This involves filling the gap between the inner and outer leaf of your home’s brickwork with insulating material.

Do I have cavity walls?

You can usually tell the type of walls you have by looking at the brick pattern:

  • Cavity walls: If the bricks are all laid lengthwise (stretchers), you likely have a cavity.
  • Solid walls: If the bricks have an alternating pattern of long and short ends (headers and stretchers), the wall is solid and cannot be filled.
  • Wall thickness: If you measure the thickness of the wall at a door or window and it is more than 260mm, it is likely a cavity wall.

The installation process

Cavity wall insulation is a quick, mess-free process that must be carried out by a professional installer.

  1. Survey: An installer will use a borescope (a small camera) to inspect the cavity and ensure it is clear of debris.
  2. Drilling: Small holes (about 22mm) are drilled into the mortar between bricks at regular intervals.
  3. Filling: The insulating material is blown into the holes until the cavity is full.
  4. Finishing: The holes are filled with mortar that matches your existing brickwork, making the installation almost invisible.

Materials used

  • Mineral wool: A fire-resistant fibre that is common and effective.
  • EPS beads: Small polystyrene beads that are often mixed with a light adhesive. They are excellent for narrower or uneven cavities.
  • Urea-formaldehyde foam: An older method no longer commonly used due to potential shrinkage and health concerns.

Benefits

  • Cost savings: A typical semi-detached home can save around £280 per year on energy bills.
  • Fast payback: Often pays for itself in less than 3 years.
  • Quick install: Usually completed in less than half a day.
  • Comfort: Reduces cold spots and draughts on external walls.

Note on Damp: Cavity wall insulation is generally safe, but if your home is in a high-exposure area (like a coastal cliff) or has existing cracks in the outer brickwork, moisture can travel across the insulation and cause damp inside. Always ensure your installer is registered with CIGA (the Cavity Insulation Guarantee Agency).

Grants

Many households are eligible for free cavity wall insulation through government schemes. Check the Great British Insulation Scheme for eligibility.