The short answer
Most external wall insulation problems come down to poor installation rather than the idea itself. The most common early sign is render cracking — hairline crazing, cracks at board joints, or stress cracks from corners where movement joints were missed. The more serious issue is damp: if water gets behind the system through cracks or poor detailing it can be trapped between the wall and the insulation, causing penetrating or interstitial damp and hidden damage. Quality matters enormously here — TrustMark audits have flagged widespread installation faults in grant-funded work, which is exactly why funded EWI must be done to PAS2035 standards by a TrustMark-registered installer with a retrofit coordinator. Choosing a registered installer and a properly detailed system is the main protection against these problems.
EWI is a sound measure when it is fitted well, and a source of trouble when it is not. Knowing the warning signs — and the quality framework that should sit behind the work — helps you avoid the common failures.
What to watch for
- Render cracksjoints, corners, crazing
- Penetrating dampwater through cracks/detailing
- Interstitial dampmoisture trapped behind boards
- Poor detailingaround windows, sills, eaves
- Main safeguardPAS2035 + TrustMark installer
The common failures and their signs
- Render cracking: horizontal cracks at board joints can mean joints were not staggered; diagonal cracks from corners suggest missing movement joints; fine crazing can mean render applied too thick or mixed wrong.
- Penetrating damp: damp patches inside after heavy rain can mean water is getting through the system at cracks or poor detailing.
- Interstitial damp: once water is behind the boards it can be trapped against the wall, causing hidden moisture damage over time.
- Detailing faults: poor work around windows, sills, eaves and downpipes is a frequent entry point for water.
How PAS2035 and TrustMark protect you
PAS2035 is the retrofit standard that sets out how insulation work should be assessed, designed and installed, including how to manage moisture and ventilation so the wall can cope with the change. TrustMark is the government-endorsed quality scheme; a registered installer and a retrofit coordinator overseeing the project are what turn a good system into a durable one. If your EWI is grant-funded these are normally mandatory, and even when paying privately, choosing that route is the clearest way to avoid the cracking and damp problems above.
| Safeguard | What it does |
|---|---|
| PAS2035 | Retrofit standard: assessment, design, moisture management |
| TrustMark registration | Government-endorsed quality assurance for the installer |
| Retrofit coordinator | Oversees the project against the standard |
| Correct detailing & joints | Prevents most cracking and water ingress |
General guidance on the quality framework. Sources: GOV.UK and Energy Saving Trust.
Want EWI done to the standard?
We'll match you with a PAS2035/TrustMark EWI installer who specifies correct detailing and movement joints, manages moisture, and works with a retrofit coordinator.
Frequently asked questions
What are the most common problems with external wall insulation?
Render cracking and damp, almost always caused by poor installation. Cracks appear at board joints or corners where movement joints were missed, and water getting behind the system can cause penetrating or interstitial damp. Good detailing and a PAS2035/TrustMark installer prevent most of these.
Can external wall insulation cause damp?
Poorly installed EWI can. If water gets behind the system through cracks or poor detailing it can be trapped between the wall and the insulation, leading to penetrating or interstitial damp. A correctly designed and installed system to PAS2035, which manages moisture and ventilation, is designed to avoid this.
How do I avoid a bad external wall insulation job?
Use a TrustMark-registered installer working to PAS2035 with a retrofit coordinator, on a system with correct movement joints and detailing around windows, sills and eaves. Compare quotes on the same specification rather than choosing the lowest-priced one that skips those steps.
Sources & further reading
- GOV.UK — what to do if you have poor quality wall insulation
- Energy Saving Trust — solid wall insulation
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.