The short answer
The render finish on external wall insulation (EWI) is the layer that takes all the weather, so it is where most visible problems appear: cracking (from missing mesh, absent movement joints or stress at openings), debonding or blowing (the render losing its bond and sounding hollow), algae and green staining on shaded damp elevations, discolouration and streaking below sills and copings, and water ingress at poorly detailed junctions that then risks the insulation behind. Most of these are specification and workmanship issues — a correctly reinforced basecoat, movement joints, the right finish for the exposure, and proper weather detailing at sills, eaves and penetrations prevent the great majority. Catching faults early, while they are cosmetic, stops them becoming structural.
EWI render protects everything behind it, so understanding its failure modes helps you spot trouble early. The detail below covers each common fault, its cause and its fix.
Render faults
- Crackingno mesh / movement joints
- Debonding / blowingpoor bond, hollow sound
- Algae / green stainingshaded, damp elevations
- Streakingpoor sill / coping detailing
- Water ingressbad junctions, risks insulation
Cracking and debonding
The two structural render faults are cracking and debonding:
- Cracking happens where movement concentrates at a weak point — a basecoat without enough reinforcing mesh, a missing movement joint, or the corners of windows and doors with no diagonal reinforcement. Defined cracks let water in.
- Debonding (or blowing) is the render losing its grip on the layer beneath, often heard as a hollow, drummy sound when tapped. It points to a poor bond, an unsuitable substrate, or moisture behind the render.
Both undermine the system's job of keeping water out and need repair before they spread.
Algae, staining and discolouration
Cosmetic faults are common and usually less serious, but they signal where water is lingering:
- Algae and green growth appear on north-facing or shaded, damp elevations where the render stays wet. Some finishes resist it better than others.
- Streaking and dark staining below window sills, copings and downpipes show that water is running across the face instead of being thrown clear — a detailing issue with sills and drips.
- Patchy discolouration can follow uneven application or repairs in different conditions.
| Fault | Severity | Usual cause |
|---|---|---|
| Hairline crazing | Cosmetic | thin / stressed top coat |
| Defined cracking | Serious | no mesh / movement joint |
| Debonding (hollow) | Serious | poor bond / moisture |
| Algae / streaking | Cosmetic | shaded / poor sill detailing |
Indicative guidance. Source: TrustMark / PAS 2035 retrofit guidance.
Water ingress at junctions
The most consequential render problem is water getting behind the system, because once water reaches the insulation it can degrade performance and, on a breathable wall, contribute to hidden damp. Ingress almost always starts at a junction:
- Window and door sills without a proper overhang and drip to throw water clear.
- Eaves and verges where the system meets the roof.
- Pipe and cable penetrations that aren't sealed.
- The base, where a missing or wrong starter detail lets water track up.
Spotting and fixing problems early
Because render protects the insulation and wall behind it, early action matters. Inspect periodically for cracks near openings, hollow-sounding areas, green growth on shaded walls, and damp or staining around junctions. Cosmetic crazing and algae can often be cleaned or over-coated; defined cracks and debonded patches should be cut out, reinforced and re-finished, with the underlying cause (a missing joint, a poor sill) corrected at the same time. Keeping the installer's system documentation and any guarantee makes repairs simpler. A well-specified, well-detailed render gives many years of service, and most of what 'goes wrong' is avoidable with good practice and light maintenance.
Frequently asked questions
Why is my EWI render cracking?
Usually because the basecoat lacks enough reinforcing mesh, a movement joint is missing, or the corners of openings weren't reinforced, so movement concentrates and cracks. Fine crazing can be cosmetic, but defined cracks let water in and should be repaired.
What does it mean if EWI render sounds hollow?
A hollow, drummy sound when tapped usually means the render has debonded — lost its grip on the layer beneath — often due to a poor bond or moisture behind it. Those areas typically need cutting out and re-rendering before water gets in.
How do I stop algae growing on EWI render?
Algae forms on shaded, damp elevations where the render stays wet. Choosing a finish with good algae resistance, ensuring sills and drips throw water clear, and occasional cleaning all help. It is usually cosmetic rather than structural.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.