Insulation

Solid wall insulation cracking

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)

Fine hairline cracks or larger fissures are appearing in the render on the outside of your house. You may also see “bulging” where the insulation boards seem to be pulling away from the wall, or discolouration around the cracks.

What’s normal vs not normal

Normal:

  • Tiny hairline cracks (less than 0.1mm) that are only visible when the wall is wet.
  • Minor colour variations in the render over time due to weathering.

Not normal:

  • Cracks wider than a 10p piece (approx. 2mm).
  • Horizontal or vertical cracks that follow the lines of the insulation boards.
  • “Spider-web” cracking over large areas.
  • Render that sounds hollow or “drummy” when tapped.

Likely causes (ranked)

  1. Thermal movement: The insulation boards are expanding and contracting in the sun, putting stress on the render if it wasn’t installed with enough reinforcement mesh.
  2. Poor fixing: The mechanical fixings (pins) that hold the boards to your house were either too few, too short, or not driven into solid material.
  3. Moisture ingress: Water has gotten behind the render (often around windows or gutters) and has frozen, forcing the render off the board (freeze-thaw action).
  4. Subsidence or settlement: The house itself is moving, and the rigid insulation system cannot flex enough to accommodate it.
  5. Incompatible materials: Using a cheap, non-breathable render on top of a breathable insulation board like wood fibre or mineral wool.

Quick checks you can do safely

  • The “Tap” test: Gently tap the area around a crack with a wooden spoon. If it sounds hollow, the insulation has detached from the wall.
  • Check the seals: Look at the sealant around your window frames and where the insulation meets the roof. Is the silicone cracked or missing?
  • Monitor the width: Use a “crack gauge” or just a pencil mark to see if the crack is getting wider over a few months.
  • Check the gutters: Ensure your gutters aren’t leaking directly onto a cracked area.

When to call an installer

  • If you see large chunks of render beginning to flake off.
  • If a crack is wider than 2mm and you can see the mesh or insulation board underneath.
  • If you have a 25-year SWIGA (Solid Wall Insulation Guarantee Agency) guarantee, as repairs should be covered.

Questions to ask your installer

  • “Was a reinforcement mesh used across the entire surface of the insulation?”
  • “Are the cracks caused by the insulation boards ‘bowing’ or by movement in the building’s structure?”
  • “Can the crack be repaired with a flexible patch, or does the entire elevation need to be re-rendered?”

Related NetZeroNow tools & guides

FAQs

Can it be patched? Yes, but it’s hard to make it invisible. Most modern renders are “through-coloured,” so a patch will often stand out unless the whole wall is painted with a dedicated silicone masonry paint afterwards. Will my house fall down? No. Solid wall insulation is not structural. However, if water gets behind it, it can eventually cause damp issues inside your home. Is it covered by a guarantee? If your installation was done through a government scheme (like ECO), it should have a 25-year SWIGA guarantee. Why do the board outlines show? This is called “pillowing” or “ghosting.” It happens if the boards weren’t sanded flat before the render was applied, or if the fixings are pulling too tight. Does it need regular maintenance? Yes. You should check the seals around windows and doors every year and replace any cracked silicone to prevent water from getting behind the system.\n

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