Kitchen ventilation mistakes that cause long-term damp and mould

Cooking is the biggest moisture factory in the house — a working gas hob alone can put roughly a litre of water vapour into the air in an hour of use, before the kettle and the pasta pot weigh in. Where that vapour goes decides whether the kitchen stays fresh or slowly grows its own ecosystem. The most common answer, in millions of kitchens, is "nowhere in particular" — because the appliance that looks like ventilation isn't one.

Mistake one: trusting a recirculating hood

A recirculating (ductless) hood filters grease and odours through carbon, then blows the air — moisture and all — straight back into the room. UK Building Regulations say the quiet part in numbers: under Approved Document F, a cooker hood ducted to outside counts as extract ventilation at 30 litres per second (mounted over the hob); a recirculating hood doesn't count as extract ventilation at all — which is why a kitchen relying on one needs a 60 l/s extract fan elsewhere in the room to reach the same standard. The taste-and-smell improvement is real; the moisture removal is zero. Steam that stays in the kitchen condenses on the coldest surfaces — the window, the external wall — and that's the daily breadcrumb trail to black mould and the damp-diagnosis guide.

Mistake two: the fan that exists but doesn't work

Ducted extraction fails quietly too. Grease filters clog until the hood hums without moving air — clean them monthly (most pop in a dishwasher). Ducting that's crushed, longer than designed, or — the classic — venting into the loft instead of outside moves the problem upstairs and out of mind. And the fan switched on after cooking starts the extraction ten minutes too late: the discipline is on before the pan, off ten minutes after — the kitchen cousin of the whole-house ventilation rules.

Mistake three: no make-up air

Extraction needs replacement air; a sealed kitchen can't give the hood anything to extract. Trickle vents closed, vents blocked, door sealed — and the 30 l/s fan becomes a 5 l/s disappointment. Leave the air route open (that's deliberate ventilation — never seal it), and crack a window when the hob is working hard.

Mistake four: ignoring what combustion adds

A gas hob adds combustion by-products to the steam — water vapour plus nitrogen dioxide and, when a flame is misbehaving, the quiet risk this desk keeps flagging (unflued combustion belongs outside the moisture story too). Ducted extraction is the only setup that removes combustion products from the room, which is why it's the specification the regulations prefer — and why an audible, in-date CO alarm near a gas kitchen isn't paranoia. Electrical note for remodels: new kitchen circuits and extracts are notifiable work in the UK — the same rule as the induction guide.

The five-minute audit

Turn the hood on and hold a sheet of toilet paper at the filter: it should cling. Check the duct actually exits (go outside and look for the vent in operation). Look at the window and external-wall corners after a week of cooking: dry, or beading? And match what you find to the standard — 30 l/s ducted over the hob, or 60 l/s extracted from the room — because the regulation number is genuinely a good proxy for "this kitchen will not grow mould".

Sources: UK Approved Document F (2021): intermittent extract — kitchen 30 l/s (ducted hood over hob) / 60 l/s (elsewhere or recirculating); recirculating hoods not classed as extract ventilation; GGF moisture-production figures. Reviewed September 2026 · general information, not professional advice.