External wall insulation problems: what can go wrong and how to avoid them

The most common problems

1. Render cracking

Thin-coat render (the finish layer on EWI systems) can crack if:

• The base coat wasn't properly cured before the top coat went on.
• Insufficient reinforcement mesh at stress points (corners, window openings).
• Movement in the substrate (expansion/contraction of the insulation boards).
• Wrong render type for the wall (cement render on a soft brick wall causes stress cracking).

Fine hairline cracks (under 0.3mm) are normal and don't affect performance. Wider cracks (over 1mm) can let water in and should be repaired.¹

2. Damp penetration

EWI can cause damp if:

• The original wall had damp problems that weren't fixed before install.
• Weep vents or cavity trays were blocked during install.
• Detailing around windows, doors, and porches was done badly (water can track behind the insulation).
• Ground level was raised after install (soil or paving pushed up against the EWI, bridging the damp-proof course).

A proper survey checks for existing damp, and a good installer will refuse to insulate a damp wall.³

3. Poor window detailing

Windows sit in reveals (the recessed bit where the window meets the wall). When you add 100mm of external insulation, the reveals become very deep. If the installer doesn't extend the window cills and fit drip edges properly, rain can run down the reveal and soak into the edge of the insulation board.

Good detailing uses corner beads, stop beads, and extended cills to shed water away from the insulation.⁴

4. Fire performance concerns

After the Grenfell Tower fire (2017), there's been more scrutiny of EWI systems on tall buildings. Combustible insulation (EPS, phenolic foam) is now banned on residential buildings over 11m tall in Scotland, and restricted in England and Wales.⁵

For houses (under 11m), combustible insulation is still permitted if fire barriers are installed around windows and at each floor level. Your installer should follow the manufacturer's fire-safe installation guide.

5. Poor fixing

Insulation boards are fixed to the wall with adhesive and mechanical fixings (plastic or metal anchors). If the fixings are too short, too few, or badly positioned, the boards can sag or detach over time. This is most common on tall gable walls or where the substrate is weak (soft brickwork, crumbly mortar).¹

How to avoid problems

Choose an installer who:

• Is registered with SWIGA (Solid Wall Insulation Guarantee Agency) or has BBA certification.
• Offers an insurance-backed warranty (10 to 25 years).
• Does a proper pre-install survey (checking for damp, testing the substrate strength, measuring wall thickness).
• Uses a complete system from one manufacturer (insulation boards, adhesive, base coat, mesh, top coat all from the same brand). Mixing components voids the warranty.
• Follows the manufacturer's fixing pattern (number of fixings per board, edge distances, curing times).

Red flags:

• No survey before quoting (a proper survey takes 1 to 2 hours).
• Quote is much cheaper than competitors (corners are being cut).
• Installer isn't registered with a trade body.
• Warranty is from the installer's own company, not an insurance-backed scheme.

Can EWI be removed if it goes wrong?

Yes, but it's expensive and disruptive. You have to strip the render, remove the insulation boards, repair any damage to the substrate, and then either re-render the original wall or install a new EWI system correctly. Cost: £4,000 to £10,000 depending on house size.

This is why it's worth spending time choosing a good installer upfront.

Related reading

• Is solid wall insulation worth it? (full cost and savings breakdown)
• Is internal wall insulation worth it? (the alternative if EWI isn't suitable)
• Insulation board types explained (materials guide)