Insulation

Cavity wall insulation slumping

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

You notice that the upper parts of your walls feel much colder than the lower parts. You may see damp patches or black mould forming specifically at the top of the wall (near the ceiling) or around window heads, even though the rest of the wall is dry.

What’s normal vs not normal

Normal:

  • A slight difference in temperature between the top and bottom of a wall (hot air rises).
  • Visible “injection holes” in your mortar that have been filled and matched to your brickwork.

Not normal:

  • Large, defined cold patches at the top of walls.
  • The sound of “hollow” brickwork when tapped at the top of the house, while the bottom sounds “solid.”
  • A sudden increase in heating bills a few years after the insulation was installed.

Likely causes (ranked)

  1. Poor installation: The insulation wasn’t injected at the correct pressure or density, allowing it to settle over time.
  2. Wet insulation: A leak (e.g. from a leaking gutter or roof) has made the insulation heavy, causing it to clump and slump to the bottom of the cavity.
  3. Debris in the cavity: “Snots” of mortar or rubble inside the cavity are catching the insulation and preventing it from filling the space evenly.
  4. Inappropriate material: Using older materials like loose wool or urea-formaldehyde foam that is prone to shrinkage or settling.
  5. Vibration: Living near a very busy road or railway line can sometimes accelerate the settling of loose-fill materials.

Quick checks you can do safely

  • Check the gutters: Ensure your gutters aren’t overflowing onto the walls, as wet insulation is the leading cause of slumping.
  • Thermal imaging: If you have access to a thermal camera (or a cheap phone attachment), look at your walls from the inside on a cold day. Gaps at the top will show as dark blue/purple areas.
  • Tap the wall: Gently tap the wall with your knuckles. A hollow sound at the top of the wall suggests a void.
  • Check for damp: Use a simple damp meter to see if the cold spots are also wet.

When to call an installer

  • If you find significant voids using a thermal camera.
  • If you have persistent black mould at the top of your walls that returns after cleaning.
  • If your insulation was installed recently (within 25 years), as it should be covered by a CIGA (Cavity Insulation Guarantee Agency) guarantee.

Questions to ask your installer

  • “Is the insulation covered by a CIGA or similar 25-year guarantee?”
  • “Can you perform a borescope inspection to see the condition of the insulation inside the cavity?”
  • “If the insulation has slumped, can the voids be topped up, or does it need to be extracted and replaced?”

Related NetZeroNow tools & guides

FAQs

Can I top up the insulation? Yes, in many cases a technician can drill new holes and “top up” the voids with modern bonded bead insulation, which is less prone to slumping. How do I know if I have a guarantee? Most UK cavity wall installations since the 1990s were registered with CIGA. You can contact them to see if a certificate exists for your address. Is slumping common? It was more common with early “loose wool” or foam products. Modern “bonded beads” use a glue that sticks the beads together, making slumping almost impossible. Will it cause damp? Yes. If the insulation slumps, the temperature of the wall at the top drops. When warm, moist air inside your house hits that cold spot, it turns into water (condensation), leading to mould. How much does extraction cost? If the insulation is wet and must be removed, it can be expensive (£2,000–£4,000). Always check if this can be claimed under your guarantee first.\n

Written by NetZeroNow | Last updated on 2026-03-02