Every one of these myths gets said to us on surveys — usually in the first ten minutes, usually as the reason a house has stayed cold for a decade. After 500+ installations and a 9.87/10 Checkatrade rating from 122 verified reviews, we have heard them all and seen what the truth looks like on real walls. Here are the seven biggest, each tested against building physics and against what we actually find on jobs.
Myth 1: "EWI stops walls breathing"
The reality: whether an insulated wall can pass moisture vapour depends on the system chosen, not on EWI as a category. Vapour-open build-ups — mineral wool or wood-fibre boards under a breathable render — let the wall keep managing moisture the way a solid wall always has. Vapour-closed systems (EPS with certain renders) exist and are fine on the right substrate. Saying "EWI seals walls" is like saying "coats make you sweat" — some do, and you pick the right one. The physics is laid out in our breathable wall insulation guide.
What we see on real jobs: breathable build-ups appear on 103 of our real quotes — it is routine specification for period solid walls, not an exotic upgrade. The failures that feed this myth are mismatches: vapour-closed systems slapped onto damp, solid, pre-1919 walls with no assessment. The system was wrong; the category gets the blame.
Myth 2: "Thicker is always better"
The reality: insulation obeys diminishing returns. Going from a bare wall to a modest thickness cuts heat loss enormously; each centimetre after that saves less than the one before, because you are halving an ever-smaller remainder. Meanwhile the costs of thickness — money, lost room space on IWI, deeper reveals, sill and eaves complications on EWI — keep stacking up linearly. Past the design point, you are paying full price for marginal gains.
What we see on real jobs: the best-performing houses we hand back are the ones where the thickness was chosen per wall — a sensible EWI build-up outside, a slimmer board where a small room could not spare the space — and the detailing got the attention instead. A wall with 20 per cent less board and properly insulated reveals beats a thicker wall with cold bridges every time. Where the heat actually goes is covered in where homes lose heat — and it is rarely solved by another 50mm on one surface.
Myth 3: "Cavity wall insulation always causes damp"
The reality: failed installs cause damp; the measure does not. Millions of filled cavities perform quietly and well. The genuine failures cluster around specific conditions — severely exposed elevations, walls in poor repair, unsuitable narrow or rubble-filled cavities — pumped anyway by installers paid per house. Water tracking across slumped or saturated fill is a real mechanism; it is also an avoidable one, caught by a competent pre-assessment.
What we see on real jobs: both halves of the truth. We see sound filled cavities in most post-war homes we survey — and we see the failures too, which is why we wrote a whole guide on cavity wall insulation problems. The pattern in the failures is nearly always missing assessment, not bad luck. The lesson is "check the wall first", not "never insulate".
Myth 4: "Replace the windows before the walls"
The reality: walls usually lose more. Windows feel like the weak point — you can feel cold radiating off glass — but in a typical solid-walled house the walls are several times the area of the glazing, and an uninsulated solid wall loses heat prolifically per square metre. Multiply area by loss rate and the walls dominate the heat bill in most pre-war homes. Swapping working double glazing for slightly better double glazing moves that total very little; insulating the walls moves it a lot.
What we see on real jobs: houses that had £10,000 of new windows fitted two years before we arrived — still cold, because the walls around those lovely windows were untouched. Order matters for detailing too: insulate walls first (or together with glazing) and the reveals and sills can be detailed as one warm assembly. If the windows genuinely are failing — blown units, rot — replace them; just do not let a windows salesman sequence your retrofit.
Myth 5: "You have to move out while it's done"
The reality: no. EWI happens entirely outside on scaffold — from indoors you notice netting on the light and some noisy fixing days. IWI is done one sealed room at a time while you live in the rest of the house. Loft and floor jobs are measured in days. The full room-by-room playbook is in insulating while you live there.
What we see on real jobs: across 500+ installations, customers staying put is the norm, not the exception — including families with young children and people working from home through the job. The one honest exception: a whole-house back-to-brick renovation with rewiring and plumbing running at once. That is a renovation decision, not an insulation one.
Myth 6: "Insulation causes mould"
The reality: mould needs two things — moist air and a cold surface for it to condense on. Insulation warms surfaces, which is why it is one of the best anti-mould measures there is. What causes post-insulation mould is bad detailing: cold bridges left at reveals, corners and junctions become the new coldest spots and concentrate all the condensation the wall used to spread out; and tightening a house without ventilation traps the moisture indoors. Blame the execution, not the physics.
What we see on real jobs: this is a decent slice of our remedial work — damp and mould remediation appears on 32 of our real quotes, and ventilation or extract works on 70. The pattern in the failures we are called to fix is always the same: partial insulation with skipped details, no thought given to where the moisture goes. Done completely — walls, reveals, junctions, plus ventilation — insulation removes mould conditions rather than creating them.
Myth 7: "Spray foam is the quick modern fix"
The reality: spray foam is the one insulation measure that can make your home harder to sell. Foam bonded to the roof hides the timbers from surveyors, can trap moisture against them, and is effectively irreversible without expensive removal — which is why lenders and surveyors flag it and why an industry now exists to strip it out. Speed of installation is a poor trade for decades of consequences.
What we see on real jobs: only 3 jobs in our entire quote book involve spray foam — we are the installer who declines it, not one that sells it. What we do see is the aftermath: homeowners asking us to quote for putting right a foamed roof, or for conventional loft and rafter insulation after a surveyor's report killed a sale. The full story is in why we don't install spray foam. Mineral wool between and over joists does the same thermal job, removably, for less.
One honest closing caveat: scepticism itself is not a myth. Insulation done badly has caused real problems in real homes — that is precisely why half these myths exist. The answer is not to leave the house cold; it is to insist on assessment, complete detailing and ventilation as part of the job. If you want claims you can check rather than promises, get a free, fixed, itemised quote and compare it line by line against anyone else's.
Myth-busting FAQs
Does external wall insulation stop walls breathing?
It depends entirely on the system, not on EWI as a category. Vapour-open systems — mineral wool or wood fibre with a breathable render — let moisture vapour pass through the whole build-up and are routinely used on solid-walled period homes. Vapour-closed systems exist too, and belong on walls designed for them. The myth comes from treating all EWI as if it were one material.
Is thicker insulation always better?
No — returns diminish with every added centimetre. The first increments of insulation cut heat loss dramatically; each further increment saves less than the one before, while costs, lost space and detailing problems keep growing linearly. There is a sensible design thickness for each wall and measure, and going far beyond it buys very little.
Does cavity wall insulation cause damp?
Properly assessed and installed cavity fill performs fine in the vast majority of homes — millions of filled cavities are quietly doing their job. The horror stories are real but specific: exposed walls pumped without assessment, unsuitable properties, and poor workmanship. The problem is failed installs, not the measure itself.
Should I replace my windows before insulating my walls?
Usually the other way round. In a typical solid-walled house, the walls are a far larger area than the glazing and lose more heat in total, so wall insulation moves the bills and comfort further than swapping already-functional double glazing. If the windows are failing, replace them — but do it in the right order and detail the reveals with the wall insulation.
Do you have to move out during insulation work?
No. External insulation happens outside on scaffold; internal insulation is done one sealed room at a time; loft and floor jobs take days. Across 500+ installations our customers have stayed in their homes as the norm — the only scenario where decamping makes sense is a whole-house back-to-brick renovation with every trade running at once.
Can insulation cause mould?
Bad insulation detailing can — good insulation prevents it. Mould needs a cold surface and moist air. Insulation warms surfaces, which fights mould; but a sloppy job that leaves cold bridges at reveals and corners, or seals a house without ventilation, concentrates condensation on the few cold spots left. The fix is complete detailing plus ventilation, not avoiding insulation.