Why is my kitchen humid even with extraction on?

Because the extraction isn't removing enough of the steam - usually a greasy, underperforming system pulling too little air, or a make-up air problem starving it; the steam is produced faster than the system takes it away. Cooking produces a lot of moisture (steam), and one of the extraction's jobs is to remove it - drawing the steam-laden air up and away. So if your kitchen is humid even with the extraction on, it means the extraction isn't removing enough of the moisture: the steam is being produced faster than the system is taking it away, so it builds up. The commonest reason is that the extraction is underperforming because it's greasy - grease build-up chokes the airflow, so the system moves less air and removes less steam. Another common reason is a make-up air problem - the extraction needs enough replacement air coming in to pull properly, so if the make-up air is inadequate, it's starved and removes less moisture even when clean. Heavy steaming or humid weather add to the load. So clean the extraction (the common fix), and check the make-up air if it stays humid. So the extraction isn't pulling enough air - usually grease or make-up air. So it's not removing enough steam. This is general guidance.

Yes - grease chokes the airflow, so a greasy system moves less air and removes less steam, leaving the kitchen humid despite running; it's the same choked-airflow problem behind weak smoke capture and a hotter kitchen. Yes - a greasy extraction can make the kitchen humid. The extraction removes moisture by drawing the steam-laden air away, and how much it removes depends on how much air it moves. A greasy system moves less air, because grease build-up (in the filters, ductwork and on the fan) chokes the airflow - narrowing the path and loading the fan. So a greasy extraction removes less steam, leaving the kitchen humid even with it running. This is the same choked-airflow problem that makes a greasy system capture smoke poorly and remove less heat - here it shows up as humidity (the steam not being carried away). So a greasy, grease-choked system is the commonest cause of a humid kitchen despite the extraction. And it's the readily fixable cause: cleaning the system restores the airflow, so it removes more steam and the humidity improves. So yes - a greasy system removes less steam. So yes; suspect grease first. This is general guidance.

Yes - the extraction needs enough replacement (make-up) air coming in to pull properly, so if the make-up air is inadequate, the extraction is starved and removes less moisture even when clean - leaving the kitchen humid; that's a ventilation matter for a specialist. Yes - inadequate make-up air can cause a humid kitchen. The extraction needs enough replacement (make-up) air coming into the kitchen to pull properly - you can only extract as much air as you can replace. So if the make-up air is inadequate, the extraction is starved and pulls less air (removing less moisture), even if the system itself is clean and in good order (the kitchen goes into negative pressure, choking the airflow from the supply side). So a clean extraction can still leave the kitchen humid if it's starved of make-up air - the problem is the ventilation (the make-up air), not the cleaning. This is a common, under-appreciated cause of an extraction underperforming. So if the system is clean but the kitchen is still humid, the make-up air or ventilation design may be the issue - a matter for a ventilation specialist to assess and improve. So yes - a make-up air problem can leave even a clean system unable to remove the steam. So yes; it starves the extraction. This is general guidance.

If the system is greasy and underperforming, yes - cleaning restores the airflow so it removes more steam, reducing the humidity; but if it's clean and still humid, the cause is likely make-up air or ventilation, needing a specialist. If the humidity is due to a greasy, underperforming extraction (the commonest cause), then yes - cleaning it will reduce the humidity. A proper extraction clean removes the grease choking the airflow (in the filters, ductwork and fan), so the system moves more air and removes more steam - carrying the moisture away and reducing the humidity. So cleaning is the readily fixable cause and the first thing to address, especially if the system is greasy or overdue a clean. However, if the extraction is already clean and well-performing but the kitchen is still humid, then cleaning won't be the answer - the cause is likely a make-up air problem (the extraction starved of replacement air) or a ventilation design issue, which needs a ventilation specialist to assess and improve. So cleaning reduces the humidity if grease was the cause; if the system's clean and still humid, look at the make-up air/ventilation. So yes if it's greasy; otherwise check the make-up air. So cleaning helps if grease is the cause. This is general guidance.

Then the cause is likely the make-up air or ventilation design (an inadequate supply of replacement air starving the extraction, or a design issue) - a matter for a ventilation specialist to assess; also manage the steam load and note humid weather adds to it. If the kitchen is still humid after the extraction is cleaned and performing well, the cause is probably not grease but the make-up air or ventilation. A clean, well-performing extraction that still can't keep up with the steam suggests it's being starved of make-up air (not enough replacement air coming in to let it pull properly) or that the ventilation design is inadequate for the kitchen's steam load. These are ventilation matters - a job for a ventilation specialist to assess and improve (better make-up air, a ventilation redesign). Also consider managing the steam load where possible (lids on pots, avoiding over-boiling, spreading out heavy steaming) and note that humid weather adds moisture to the ambient air, making it harder. So after cleaning, if it's still humid, get a ventilation specialist to look at the make-up air and ventilation design, and manage the steam load. So investigate the make-up air/ventilation with a specialist. So look at the make-up air and ventilation. This is general guidance.

Yes - persistent humidity causes condensation (on cold surfaces and under the canopy), damp, discomfort, and can affect hygiene and the fabric of the kitchen; so removing the steam (by a well-performing extraction) matters beyond comfort. Yes - persistent humidity in a kitchen causes several problems beyond discomfort. It leads to condensation - moisture condensing on cold surfaces, windows, walls and under the canopy (as covered in the when-condensation-forms-under-your-canopy page) - which can cause damp, dripping, and staining. It creates a damp, uncomfortable working environment (unpleasant and tiring for staff). It can affect hygiene (damp conditions and condensation can encourage mould and make surfaces wet). And over time, persistent moisture can affect the fabric of the kitchen (surfaces, fittings, the building). So removing the steam effectively - keeping the humidity down - matters for more than comfort: it prevents condensation, damp and the associated hygiene and fabric problems. This is why an underperforming extraction (leaving the kitchen humid) is worth addressing: a well-performing extraction that removes the steam keeps the kitchen drier and avoids these knock-on problems. So yes - humidity causes condensation, damp and hygiene issues, so removing the steam matters. So yes; condensation, damp and discomfort. This is general guidance.