Sealing a house without ventilating it is how damp starts

A house is a moisture source before it is anything else

People breathe, and breathing puts water into the air. So does cooking, washing, showering, mopping a floor, drying clothes indoors and keeping houseplants. A household of several people generates a surprising quantity of water vapour every day, and none of it disappears on its own. It either leaves the building or it condenses somewhere inside it.

For most of the history of housing, that removal happened by accident. Buildings leaked air through gaps around windows, through suspended floors, up open chimneys and through unsealed lofts, and the moisture went with it. The cost was heat, and heat was cheap enough that nobody minded.

Efficiency work removes that accidental ventilation, which is the point of it. What it doesn’t do, unless somebody thinks about it, is replace the function the leaks were performing.

Condensation is a surface temperature problem

Air holds water vapour, and warm air holds a great deal more of it than cold air. When moist air touches a surface below its dew point, some of the vapour condenses onto that surface. This is the entire mechanism, and it explains why condensation appears on windows, on the corner of an external wall and behind a wardrobe pushed against a cold wall.

Two levers control it. Reduce the moisture in the air, or raise the temperature of the cold surfaces. Insulation works on the second lever, which is why a properly insulated wall is drier than an uninsulated one at the same humidity, and why insulation is part of the solution rather than the cause of the problem.

The failure mode arrives when airtightness improves faster than ventilation does. Humidity climbs, and the coldest remaining surfaces start to collect water. Those surfaces are usually the ones that were not insulated: a thermal bridge at a lintel, a window reveal, the back of a north-facing bedroom wall. Mould follows, and mould is a health matter as much as a building one.

Ventilate at the source, and ventilate the background

The most efficient ventilation removes moisture where it is produced, before it spreads through the house. That means an extract fan in the bathroom that actually runs long enough to clear the room, and one over the hob that vents outside rather than recirculating through a filter. Both are cheap and both are frequently the single most effective damp measure in a house.

Background ventilation handles the rest: the slow, continuous exchange that keeps humidity from creeping upward over a winter. Trickle vents in window frames are the common approach, and they are widely taped over by occupants who feel the draught and don’t connect it with the mould in the corner.

Continuous low-rate mechanical extract is the step up, running quietly all the time rather than in bursts, and it suits a house that has been tightened up substantially. Full heat recovery ventilation goes further, warming incoming air with outgoing air, and it only earns its considerable cost in a genuinely airtight building. Fitting it to a leaky house is an expensive way to move air around while the leaks carry on doing what they always did.

The loft is where this goes wrong most often

A conventional cold roof relies on air moving through the loft space to carry away moisture that has migrated up from the house. That airflow enters at the eaves, and the single commonest error in retrofit insulation is stuffing the insulation right into the eaves so that the path is blocked.

The result is a loft that stays humid, timbers that stay damp, and condensation forming on the underside of the roof covering in cold weather. It looks exactly like a leak and it isn’t one, and the fix is to reopen the ventilation path rather than to chase a hole in the roof.

The same principle applies to suspended timber floors, where air bricks provide the cross-ventilation that keeps joists dry. Blocking them because a floor feels cold is an understandable instinct and a bad idea, and the correct remedy is to insulate between the joists while leaving the airflow beneath them alone.

A sensible order of operations

Deal with the moisture sources first, because they are cheap. Vent the tumble dryer outside or use a condensing one. Run the extract fans. Dry laundry outdoors when the weather allows, since drying a full load indoors releases a large quantity of water into the house in a few hours.

Then insulate, and while doing it, keep the ventilation paths intact. Then, once the building is measurably tighter, reassess whether the background ventilation is still adequate, ideally with a humidity meter rather than a hunch. A cheap hygrometer in the worst room of the house is a more useful diagnostic tool than any amount of speculation.

And accept that this is a trade rather than a free win. Ventilation costs heat, always, and the alternative costs the building. Anyone offering a retrofit that promises to seal a house without addressing how it will breathe is offering a problem dressed as a solution, and it is worth being blunt about that before the work starts rather than after the mould appears.