05/09/2026 - Webrich Software
The Ugly Truth Is a Physics Problem, Not a Product Problem
External wall insulation is one of the few realistic options for the millions of UK homes with 9-inch solid brick walls. Once the loft is done, the suspended floor is insulated, the glazing is double and the doors are draught-proofed, those solid walls are the last big leak. Wrapping them in a bonded and mechanically fixed insulation board with a render finish can make a dramatic difference to comfort and bills.
Roger Bisby’s whiteboard talk on EWI is worth every retrofit installer’s attention because it describes what happens after a competent crew leaves. The system works. The house is warmer. And a few months later the householder is pointing at black mould and blaming the insulation.
Did you know? A typical family home produces around 20 litres of airborne moisture a day from breathing, cooking, washing and drying clothes. Insulation does not reduce that figure by a single drop.
The mechanism is simple once you see it, and it sits squarely inside the building physics that PAS 2035 and PAS 2030 expect every retrofit professional to understand.
Why 95% Coverage Creates a New Problem
EWI installers drill, screw, glue and render. They are not carpenters, plumbers or gas engineers. So when the board reaches an obstruction, it gets cut around and boxed in with proprietary trims. Common obstructions on a Victorian or Edwardian terrace include:
- A 100-year-old cast iron soil stack on the rear elevation
- A boiler flue terminal
- Gas and electricity meter boxes that nobody is going to relocate
- A neighbour’s fence post fixed to the wall beside the side gate
- Shallow eaves overhangs where the board cannot reach the top of the wall
- Window and door reveals where the frame sits flush with the outer face
The crew will cover 95% of the wall, sometimes more. That remaining 5% is the part that matters for moisture.
Before EWI
Every external wall was a cold surface. The 20 litres of daily moisture condensed thinly across the whole wall area. On a cold morning you might feel a slight dampness on the plaster, but the moisture was spread over so much surface that it soaked into the lime plaster and migrated outwards with the heat flow. Nobody noticed.
After EWI
The bulk of the wall is now warm on the inside face. Moisture no longer condenses there and no longer migrates outwards through the polystyrene. The house is also more airtight, and warmer air holds more water vapour. All that moisture is still looking for a cold surface, and the only cold surfaces left are the residual thermal bridges.
Tip: Condensation is not spread evenly. It concentrates on the coldest surface available. Reduce the number of cold surfaces without reducing the moisture, and each remaining one gets hit harder.
The result is a predictable pattern: a damp patch around the soil stack, a strip under the eaves, mould at the corners of window reveals and a stain behind the meter box. The installation did nothing wrong. The house simply has fewer places for the same moisture to go.
Residual Thermal Bridge Risk Table
Use this as a site-survey prompt before and after installation.
| Location | Typical cause | Risk after EWI | Practical response |
|---|---|---|---|
| Soil stack | Board cut around pipe | High | Insulate behind stack if possible, note as bridge |
| Meter box | Cannot be removed | High | Insulated box surround, householder warning |
| Fence post or gate | Fixed to wall by others | High | Agree removal and refixing before start |
| Eaves gap | Insufficient overhang | Medium | Extend verge or eaves, angled capping |
| Window reveal | Frame flush with brick | Medium | Reveal boards, check frame position |
| Boiler flue | Board cut around terminal | Low | Flue warmth usually offsets |
The point of the table is not to say these details make EWI a bad idea. Each one is a known, documented risk that should be flagged, priced and explained rather than discovered by the tenant in February.
Moisture Management Is the Other Half of the Job
The video’s central message is blunt: if you have EWI installed, you must manage the moisture in the house. That means:
- Extract at source with properly commissioned fans in kitchens and bathrooms.
- Consider a dehumidifier or mechanical ventilation with heat recovery for homes with high occupancy or drying loads.
- Make it automatic. Humidity-controlled fans and continuous running systems work because they do not rely on anyone remembering.
- Tell the householder plainly which surfaces will stay cold and why.
This aligns directly with the PAS 2035 whole-house approach, which requires a ventilation assessment before fabric measures are installed. Making a house warmer and tighter without dealing with ventilation is exactly the unintended consequence the standard exists to prevent. For a broader look at how these risks play out in real homes, see our guide to damp, mould and condensation for retrofit professionals and the practical lessons in eight real damp and mould fixes.
Remember: Social landlords insulating whole streets face this at scale. A retrofit that saves fuel but produces mould complaints six months later has not delivered its outcome.
What This Means for the PAS 2030 Installer
PAS 2030 places the installer inside a system that starts with assessment and ends with monitoring. The installer is not responsible for designing the ventilation strategy, but is responsible for installing the measure in accordance with the design and for raising anything on site that departs from it.
For EWI that means:
- Checking the design shows how each obstruction will be treated, not just the flat wall areas.
- Confirming the base tray, rodent barrier and top capping are specified and fitted.
- Using the correct insulation grade for the system. Expanded polystyrene is the norm outside; PIR and PUR are generally unsuitable in damp external conditions.
- Recording where coverage could not be achieved and handing that information back, because those locations are where the Retrofit Coordinator and monitoring stage need to look.
- Knowing the planning position. A render finish or brick-effect system changes the facade, and in a conservation area that is a live issue.
Our earlier post on what retrofit installers must get right with EWI covers the fixing, detailing and system-approval side in more depth. This post is about the physics that follows the fixing.
Did you know? Exam questions on thermal bridging frequently describe a scenario rather than asking for a definition. Being able to explain why mould appears at a reveal after EWI is worth more than reciting a U-value.
Is EWI a Good Thing or a Bad Thing?
Both, and that is the honest answer. Insulating solid walls is one of the highest-impact fabric measures available for older UK housing. It also concentrates moisture risk onto whatever it cannot cover. Installers who understand that second half deliver better outcomes, fewer callbacks and clearer conversations with householders.
If you want to go deeper, hands-on manufacturer academy courses and specialist EWI suppliers offer product training that covers exactly these details. The people to talk to are the ones you consult before the job, not after the mould appears.
How RetrofitReady Fits Into This
Thermal bridging, moisture movement, ventilation and the interaction between measures are core examination territory for installers working under PAS 2030. The RetrofitReady: Installer app provides PAS 2030 Retrofit Installer revision with 480 questions across the two core units and six fabric measures, including multi-answer and diagram-based practice. External wall insulation sits within those fabric measures, and scenario questions on cold bridges and condensation risk are exactly the kind covered.
The wider RetrofitReady suite offers 1900+ questions across 4 apps, so if you also work as an assessor or coordinator the same topic appears from their perspective in the Retrofit Assessor and RetrofitReady: Coordinator apps. Practise the physics by topic first, then move to mixed sessions closer to your assessment date.
Continue with the free quizzes or browse the full RetrofitReady app suite.
FAQ
Why does mould appear after external wall insulation is fitted?
EWI warms most of the wall, so the same household moisture now condenses on the few surfaces left cold: soil stacks, meter boxes, fence posts, eaves gaps and window reveals. Concentrated condensation on small areas produces damp patches and black mould where the house previously showed none.
Which parts of a solid wall are hardest to insulate externally?
Anything fixed to or passing through the wall: cast iron soil stacks, boiler flues, gas and electricity meter boxes, fence posts, gates, shallow eaves overhangs and window reveals. These are the usual locations for residual thermal bridges after EWI.
Which insulation is used for external wall insulation?
Most EWI systems use a specified grade of expanded polystyrene, mechanically fixed and adhesively bonded to the wall, then finished with a render or brick-effect system. PIR and PUR boards are generally avoided in damp external conditions.
Does PAS 2035 require ventilation to be considered when installing EWI?
Yes. PAS 2035 treats ventilation as part of the whole-house assessment and requires the ventilation strategy to be considered before fabric measures are installed, precisely because making a house more airtight and warmer without adequate extract creates condensation risk.
Do I need planning permission for external wall insulation?
Often not, but changing the external appearance of a house may need permission, especially in conservation areas or on listed buildings. Installers should confirm the planning position before work begins rather than after the render is on.
Related Apps
Continue Practising
Use a free quiz, then continue in the app with the full question bank.