The ventilation mistake behind most condensation problems

Most UK condensation isn't a building defect — it's an airflow strategy failure. The mistake takes two forms: sealing the house tight in the name of efficiency, and airing it in short bursts that feel virtuous and change almost nothing. Fix the strategy and the window streaming, the black mould behind the wardrobe and the musty spare room mostly resolve without a builder.

How much water a normal day makes

The glass industry's condensation research puts numbers on ordinary life: two sleeping adults release roughly a litre of moisture into the air in eight hours; a gas cooker cooking adds on the order of a litre an hour while it works; showers, kettles and drying laundry add their own — an indoor clothes horse alone can put litres into a day's air (why that habit made the mistakes shelf). That vapour only stays harmless while it can leave the building. Modern homes — draught-proofed, double-glazed, chimney blocked — have removed most of the accidental escape routes older houses relied on, without adding deliberate ones.

Why short bursts don't work

Opening a window for five minutes in a cold room mostly cools the room: the surfaces get colder, and colder surfaces condense more of the moisture the next hour of living produces. Ventilation works on volume and time — the point is to exchange the moist air for dry outside air, not to briefly shock a single room. The workable version: a deliberate cross-flow of ten to fifteen minutes — windows open at opposite ends of the house — once or twice a day, while keeping the background trickle going all the time: trickle vents left open on modern windows, wall vents unblocked, extract fans actually switched on (and their run-on timers left at the setting the installer chose — the fan that stops two minutes after you leave the shower hasn't finished the job). Moist air moves through the whole house; leave internal doors ajar so the bathroom and kitchen can actually purge.

The heating half of the fix

Condensation lands on cold surfaces, so the other lever is keeping surfaces warm — which is why steady low-level background heating produces less condensation than sharp hot bursts in a cold house. A room heated for two hours at night is a cold room for the other twenty-two, and every cold surface is a trap. Cost reality bites here, and the honest version is: prioritise the rooms you sleep in, keep air moving everywhere, and treat persistent problems in unheated rooms as insulation questions rather than mould questions. While you're at it, check what's heating the room — some heaters add litres of moisture a day all by themselves.

When it isn't you, it's the building

If patches grow after rain, if a tide mark climbs the ground-floor wall, if mould returns despite genuine airflow — stop buying sprays and read the diagnosis guide: penetrating and rising damp are building defects, and in a rental they're the landlord's repair duty, with new statutory timescales arriving around the UK (the duties guide has the dates). A dehumidifier is the honest short-term bridge while the real fix happens — not a permanent tenant of the hallway.

Sources: Glass and Glazing Federation condensation guidance (moisture-production figures); UK ventilation and building-regulations background guidance. Last reviewed September 2026 · general information, not professional advice.