A crack in the render or a damp patch indoors does not automatically mean your external wall insulation has failed. Some marks are cosmetic, others are early warnings. Knowing the difference is what decides whether you are facing a small repair or a large one.
External wall insulation is a system, not a coating. Boards, adhesive, mechanical fixings, reinforcing mesh, basecoat, beads and the render finish all have to work together, and each layer depends on the one beneath it being right.
This is why the standards treat it as a specified system rather than a finish. PAS 2030:2023, the installation standard used for retrofit work in the UK, requires that installation methods follow those recommended by the system supplier and match the retrofit design. INCA, the trade association for insulated render and cladding, publishes guidance on the same principle.
In our remediation work, systems rarely fail because the wrong insulation was chosen. Far more often the cause sits in preparation, sequencing, or the detailing around openings, rooflines and junctions. Those are the parts of the job that are quickest to compress when a price is tight, and hardest to see once the scaffold comes down.
Cracking is the fault homeowners notice first. It is also the one where the pattern tells you far more than the presence of a crack on its own.
If you are unsure which category you are in, photograph the crack against a ruler and repeat it a few weeks later. Change over time is the single most useful piece of evidence you can bring to an inspection.
The common fear is that wrapping a solid wall seals moisture inside it. A correctly specified system should not do that, but the concern is not baseless. It describes accurately what happens when the specification, the preparation or the surrounding detail is wrong.
BRE research on solid wall insulation examined the unintended consequences of solid wall insulation and identified penetrations through the system, including rainwater goods, as a recurring weak point. That matches what we find on site. The insulation is seldom the cause. It is what makes an existing problem invisible until it has grown.
Roofline detail deserves particular attention. Adding insulation increases the thickness of the wall, so eaves, verges and rainwater goods that suited the original brickwork may no longer discharge clear of the new surface. BRE guidance on avoiding risks, published as BR262, sets minimum overhangs according to wind driven rain exposure, typically in the region of 40mm for sheltered or moderate exposure and 50mm where exposure is severe. Where the roofline is not adapted to suit, water runs onto the face of the system rather than away from it.
If damp has appeared since your system was installed, the useful question is not whether EWI causes damp in general, but where water is entering and why it is not drying. That is a diagnostic job. See EWI inspection and diagnostics.
Adhesive needs a sound, clean, stable surface. Loose coatings, friable masonry, dust or organic growth all work against the bond, and preparation is the stage most often shortened.
Coverage pattern and mechanical fixings must suit the substrate and follow the system manufacturer’s requirements rather than habit. This is exactly what PAS 2030 means by installing to the system supplier’s recommendations.
System guidance requires the reinforcing mesh to be fully embedded in the basecoat at the correct depth, with the board face prepared beforehand. Mesh sitting too close to the surface is a common origin of later cracking.
Where the wall beneath is moving, cracked or bulging, the system is under continuous stress and adhesion alone will not hold it indefinitely. The substrate is stabilised first or the problem returns.
Signs worth acting on: areas that sound hollow when tapped, boards lifting at edges or corners, visible bulging, or render cracking that lines up with board movement. These are hard to assess accurately from ground level, which is the main reason a close access inspection is worth more than a photograph.
Most of what protects a system happens before the first board goes on the wall. If you are still at the quoting stage, these are the things a considered installation accounts for, and they are worth looking for in what you are offered.
Manufacturers publish their system technical documentation openly, which means the specification is verifiable rather than a matter of opinion.
Every item above takes time, materials or both. Preparing a substrate properly, embedding mesh to the correct depth, extending a roofline, adapting rainwater goods and detailing every reveal are not optional extras. They are the difference between a system that performs for decades and one that needs remedial work inside a few years.
They are also the reason quotes vary so widely for what looks like the same job. Published cost guidance from Checkatrade and the Federation of Master Builders gives useful ranges, but a range cannot tell you what a particular quote includes. The cheapest price on the table is often cheapest precisely because the detailing, preparation and system specification have been trimmed out of it.
That is the practical distinction between a competent installer and a genuinely good one. Both will put insulation on your wall. Only one is pricing the work that determines whether it is still sound in fifteen years. For context on what the numbers look like, see our guide to EWI costs and grants.
Resist the urge to fill a crack and hope. Cosmetic repairs over an unresolved cause tend to reappear, and they can make the eventual diagnosis harder and more expensive.
A sensible order is to record what you are seeing and when it changes, raise it in writing with the original installer if a guarantee is in place, and get an independent assessment of the cause before agreeing to any repair scope. An independent consultation or a documented inspection gives you something concrete to work from. For background on how these systems are built up, see our plain English guide to external wall insulation.
If something has appeared on your walls and you want a straight answer about what is causing it, we will tell you honestly whether it needs work now, monitoring, or nothing at all.