Insulate and draught-proof a house and you change how it handles moisture — the vapour that used to leak out through gaps now stays in. Good installers plan for that; bad ones leave you to discover it the first winter. The proof that this is a real, routine part of insulation work: ventilation and extract works appear on 70 of our real quotes — roughly one in every eight jobs we price. This guide explains what planned ventilation is, what we check on every survey, and when your existing set-up is already enough.
Why airtight homes need planned ventilation
Every household generates water vapour constantly — cooking, kettles, showers, drying clothes, and simply breathing. In a leaky old house that moisture rides out on the draughts. The same gaps that make the house expensive to heat also, accidentally, ventilate it.
Insulation work closes those gaps — deliberately. Boards get sealed at their perimeters, membranes get taped, old vents get covered by new wall build-ups, and the air-change rate of the house drops. The heat stays in, which is what you paid for. But the moisture stays in too, and because warm air holds more moisture than cold air, indoor humidity climbs until the vapour starts condensing on whatever cold surfaces remain — window glass first, then corners and reveals. We cover that failure chain in detail in our guide to condensation, damp and mould after insulation.
The fix is not to leave the house leaky. It is the retrofit principle the industry sums up as “build tight, ventilate right”: stop the uncontrolled leakage, then provide a smaller, deliberate, controllable air path that removes moisture at source. Two components do the work in most homes: extract fans in the wet rooms, and background ventilation everywhere else.
Extract fans: remove moisture where it’s made
Most household moisture is produced in two or three rooms — the kitchen and the bathrooms. Capture it there, before it spreads through the house, and the rest of the ventilation strategy barely has to work. That is the job of extract fans.
What good looks like, from the installer's side:
- Humidity-controlled fans in bathrooms — they run when the air is actually moist rather than relying on someone flicking a switch, and continue after the shower until humidity drops. Continuously-running low-power fans (dMEV) do the same job with a steady trickle plus a boost.
- A kitchen extract that ducts outside. Recirculating cooker hoods filter grease and give the steam straight back to the room — they are not ventilation.
- Short, smooth duct runs terminating outdoors. The detail only installers seem to care about: a fan is only as good as its ducting. Long saggy flexible hose halves the airflow, and a duct that dumps into the loft instead of through an outside terminal simply moves the condensation problem into your roof timbers. We check where every duct actually ends, not just that a fan spins.
Trickle vents and background ventilation
Extract fans pull air out; that air has to come from somewhere. The supply side is background ventilation — the steady, low-level feed of fresh air that keeps the whole house slowly exchanging air even when no fan is running.
In most retrofits background ventilation means trickle vents: the slim controllable slots built into modern window heads. Each passes only a gentle airflow, which is exactly the intention — continuous and barely perceptible, not a draught. Where windows lack them, through-wall background vents do the same job. Internal door undercuts matter too: air must be able to travel from the dry rooms to the wet rooms where the extracts are, so a bathroom fan is not suffocated behind a tightly fitted door.
Two things we find over and over on surveys: trickle vents taped up or painted shut by owners chasing draughts, and old permanently-open wall vents in living rooms — often a requirement of a long-gone open fire — that now do nothing but pour cold air in. Part of a proper ventilation review is opening what should be open and closing what no longer needs to exist, not just adding hardware.
What we check on survey
Every wall-insulation survey we do includes a ventilation walk-round. It takes minutes and it is the cheapest insurance on the whole project:
- Wet rooms: is there a working extract in every kitchen and bathroom? Does it actually move air (the paper test tells you in seconds), and where does the duct terminate?
- Background provision: trickle vents present, openable and actually open; wall vents accounted for; door undercuts adequate.
- Existing moisture evidence: misting windows, mould in cold corners, musty cupboards on external walls — signs the house is already at its moisture limit before we tighten it further.
- What the insulation will change: any vents our new wall build-up will cover get re-provided, never quietly boarded over — and combustion appliances that need dedicated air keep it.
If the survey finds gaps, ventilation goes on the quote as its own itemised line — that is how it comes to appear on 70 of our quotes. You see exactly what it costs, alongside the insulation itself. That is what a fixed, itemised quote is for: get yours free here.
The honest caveat: sometimes it’s already fine
Roughly one in eight of our quotes carries ventilation works — which means about seven in eight do not. Plenty of homes, particularly those with recent windows and decent fans, already have adequate ventilation: humidity-controlled extracts in the wet rooms, trickle vents throughout, no condensation history. Where that is what we find, we say so and add nothing. Paying to duplicate working ventilation is as wasteful as skipping it where it is needed.
The point of this guide is not “buy fans”. It is that ventilation must be checked whenever a house is made more airtight — by the installer, on the survey, before the quote. An installer who never mentions ventilation on a whole-house wall job has either not thought about moisture or is hoping you will not. Compare what else should appear on a proper quote in our insulation cost guide and the 2026 London price report built from our own quote book.
Ventilation FAQs
Why does insulation mean I need more ventilation?
Insulation and the draught-proofing that comes with it make a house more airtight. The moisture your household produces — cooking, showers, laundry, breathing — used to leak out through gaps along with your heat. Seal the gaps and the moisture stays inside at higher concentration, looking for cold surfaces to condense on. Planned ventilation replaces that accidental leakage with deliberate, controlled air change — removing the moisture without throwing away the warmth the insulation just bought you.
Do I need new extract fans when I insulate?
Only if the existing ones are missing or inadequate — which, in the homes we survey, they often are. Kitchens and bathrooms are where most household moisture is produced, so a working extract fan in each wet room is the backbone of the ventilation strategy. Ventilation works appear on 70 of our real quotes — roughly one in eight — precisely because so many homes turn out to be under-ventilated once they are made airtight.
What are trickle vents and do they actually work?
Trickle vents are the small controllable slots in modern window frames that admit a steady background feed of fresh air. Individually they pass only a small airflow, but that is the point: they provide continuous low-level air change across the whole house, so extract fans in wet rooms have make-up air to pull from and moisture never accumulates. They work when they are open — a surprising number we find on survey have been taped shut or painted over.
Will ventilation make my newly insulated house cold again?
No — and this is the point people miss. An uninsulated draughty house loses heat through uncontrolled gaps everywhere, all the time. Planned ventilation moves a much smaller, controlled volume of air, and in an insulated house the fabric itself stays warm, so modest air change barely dents the room temperature. You are swapping large accidental heat loss for small deliberate air change.
Does every insulation job include ventilation works?
No. Roughly one in eight of our quotes includes a ventilation line, which means about seven in eight do not — because the survey found the existing provision adequate. Where there are working humidity-controlled fans in the wet rooms and clear background ventilation, we say so and price nothing. It goes on the quote when the house needs it, not to pad the total.