Home / Duct Cleaning / Guide
Duct cleaning - humidity
A commercial kitchen makes a great deal of moisture - from boiling, steaming, dishwashing and the general heat of cooking - and if that moisture is not removed, the kitchen becomes humid, with all the discomfort, condensation and mould that brings. The tool for removing it is extraction, so a humid kitchen is usually a kitchen whose extraction is not keeping up. Here is how better extraction reduces kitchen humidity, and why.
The short answer
A commercial kitchen generates a lot of moisture: boiling and steaming release water vapour directly, dishwashing adds more, and the general heat drives evaporation - so a working kitchen constantly loads its air with humidity. If that moisture is not removed, the humidity builds, making the kitchen hot and clammy, fogging surfaces, and, over time, causing condensation and mould on cool surfaces. The way moisture is removed is extraction: the canopy and ductwork draw the humid, hot air out of the kitchen and vent it outside. So the humidity in a kitchen is essentially a balance between how fast the cooking makes moisture and how fast the extraction removes it - and a humid kitchen is one where the extraction is losing that race. Reducing the humidity, then, means better extraction: enough extract rate to remove the moisture the kitchen makes, balanced by enough make-up air to let the extract work, and kept clean and maintained so it delivers its rate. Better extraction pulls the moisture out as fast as the kitchen makes it, so the humidity stays down; poor or degraded extraction lets the moisture build, so the kitchen goes humid. So if a kitchen is persistently humid, the answer is usually in its extraction - its rate, its balance, or its condition.
Why a kitchen is humid
The reason a commercial kitchen tends to be humid is simple: cooking makes moisture, and a lot of it. Boiling water and steaming food release water vapour straight into the air. Dishwashing - especially a hot commercial dishwasher - adds a burst of steam every cycle. And the general heat of a busy kitchen drives evaporation from wet surfaces, food and cleaning. So a working kitchen is continuously adding water vapour to its air, from several sources at once, all day. This is the moisture load, and it is inherent to what a kitchen does - you cannot cook without making some. A kitchen is, by nature, a moist environment.
If that moisture is not removed as fast as it is made, it accumulates, and the humidity rises. Humid kitchen air is unpleasant to work in - hot and clammy - and it causes practical problems: condensation forming on cooler surfaces (windows, walls, ceilings, cold equipment), and, where that damp persists, mould and the hygiene and building problems it brings. So the moisture a kitchen makes, if it is allowed to build up, turns into humidity, condensation and eventually mould. The humidity is not a mystery; it is the visible result of the kitchen making moisture faster than it is being removed. Which points straight to the remedy: remove the moisture faster, which means extraction.
Extraction is the remedy
Extraction is the tool that removes a kitchen's moisture. The extraction system - the canopy over the cooking line and the ductwork behind it - draws the hot, humid, greasy air up and out of the kitchen and vents it outside. In doing so, it removes the water vapour along with the heat and grease, taking the moisture out of the kitchen air before it can build up. So extraction is what keeps the humidity down: it is continuously removing the moisture the cooking is continuously making, and as long as it removes it as fast as the kitchen makes it, the humidity stays manageable. The canopy over the cooking is not only for grease and smoke; it is also the kitchen's main route for getting rid of moisture.
This means the humidity in a kitchen is a balance: moisture in (from the cooking) versus moisture out (through the extraction). When the extraction removes moisture as fast as the cooking makes it, the balance holds and the kitchen stays reasonably dry. When the extraction cannot keep up - because it is undersized, unbalanced or degraded - the moisture builds faster than it leaves, and the humidity climbs. So a persistently humid kitchen is almost always one where the extraction is losing this balance: not removing the moisture as fast as it is being made. Which is why reducing the humidity is, in practice, a matter of improving the extraction - restoring its ability to remove the moisture at the rate the kitchen produces it. Better extraction, less humidity; that is the core of it.
What better extraction means
Better extraction, in the sense that reduces humidity, comes down to three things. The first is enough extract rate: the system has to be able to remove air, and with it moisture, fast enough to match the kitchen's output at its busiest. An extraction system sized for the cooking load will keep up; one that is undersized for the kitchen it serves will always be behind, letting the humidity build. So adequate extract capacity for the kitchen's actual load is the foundation. The second is balance: an extraction system can only pull air out as fast as replacement air comes in, so it needs enough make-up air (replacement air supplied to the kitchen) to let the extract work. Without balancing make-up air, the extract is throttled - it cannot achieve its rate - so the moisture removal falls short even if the fan is sized correctly. Balanced extraction removes moisture at its full rate; unbalanced extraction cannot.
The third is condition: an extraction system only delivers its rate if it is clean and maintained. Ductwork clogged with grease has a restricted airflow, so the extract rate drops and moisture removal with it. A grease-laden, degraded system that once handled the moisture may no longer, letting the humidity creep up as the system silts up. So keeping the extraction clean and maintained keeps it delivering the rate that keeps the humidity down. Put the three together - adequate rate, balancing make-up air, and a clean, maintained system - and the extraction removes the moisture as fast as the kitchen makes it, so the humidity stays low. Fall short on any one - undersized, unbalanced, or clogged - and the moisture removal falls behind the moisture production, and the kitchen goes humid. So reducing kitchen humidity through better extraction means attending to all three: the rate, the balance, and the condition.
The knock-on benefits
Reducing the humidity through better extraction is worth doing for more than comfort, because the humidity was causing knock-on problems that go with it. Lower humidity means less condensation - the moisture is being removed before it can settle on cool surfaces - so the fogging of windows, the damp on walls and ceilings, and the dripping from cold equipment all ease. And less condensation means less of the persistent damp that leads to mould, so the hygiene and building problems mould brings recede too. So better extraction does not only make the kitchen more comfortable; it removes the moisture that was driving condensation, damp and mould, addressing the cause of a whole cluster of problems at once.
It also helps the kitchen fabric and equipment last, because persistent humidity and condensation corrode, rot and degrade over time - so a drier kitchen is easier on the building and the kit. And it makes the kitchen a better place to work, which matters for the staff who spend their shifts in it. So the case for reducing humidity through better extraction is broad: comfort, hygiene, the building, the equipment and the staff all benefit from a kitchen where the moisture is removed as fast as it is made. And because the extraction that removes the moisture is the same system that removes the grease and heat, improving it serves all of those at once - a single system, kept doing its job, that keeps the kitchen dry, clean-aired and safe. Which is why, when a kitchen is humid, looking at the extraction is looking at the root of it.
The takeaway
A commercial kitchen makes a lot of moisture - from boiling, steaming, dishwashing and heat - and if that moisture is not removed as fast as it is made, the humidity builds, bringing discomfort, condensation and eventually mould. The tool that removes the moisture is extraction: the canopy and ductwork draw the humid air out and vent it outside. So the humidity in a kitchen is a balance between the moisture the cooking makes and the moisture the extraction removes, and a persistently humid kitchen is one where the extraction is losing that balance - not removing the moisture fast enough.
So reducing kitchen humidity is, in practice, a matter of better extraction, which means three things: enough extract rate for the kitchen's load, enough make-up air to let the extract achieve that rate, and a clean, maintained system that delivers it. Get all three right and the extraction removes the moisture as fast as the kitchen makes it, so the humidity stays down - and with it the condensation, damp and mould that high humidity was causing. Fall short on any one and the moisture builds and the kitchen goes humid. So when a kitchen is humid, the question to ask is about its extraction - its rate, its balance, and its condition - because that is where the humidity is decided, and where reducing it begins.
Questions
Usually because the extraction, though running, is not removing moisture as fast as the kitchen makes it - which points to one of three things. It may be undersized for the kitchen's load; it may be starved of make-up air, so the fan cannot achieve its rated extract rate; or it may be clogged with grease, so the airflow is restricted and the effective rate has dropped. Any of these leaves the moisture building faster than it is removed, so the kitchen stays humid despite the fan running. The fix is to find which one applies - rate, balance or condition - and address it.
Yes, indirectly but importantly. An extraction system can only remove air, and the moisture in it, as fast as replacement air comes in. Without enough make-up air, the extract is throttled - it cannot achieve its rated rate - so it removes less moisture than it should, and the humidity stays higher than it would with the system balanced. So a kitchen short of make-up air can be humid even with a properly sized extract fan, because the fan cannot do its job. Getting the make-up air right lets the extraction reach its full rate and remove the moisture as designed.
Yes. Ductwork clogged with grease has a restricted airflow, which reduces the extract rate the system can achieve. A lower extract rate means less moisture removed per hour, so the humidity climbs. A system that once kept the kitchen dry can, as grease builds up in the duct, lose enough airflow that it no longer keeps up with the moisture, and the kitchen becomes humid. So keeping the ductwork clean is part of controlling humidity: a clean duct lets the extraction deliver its full rate, and its full moisture removal, keeping the humidity down.
It is both. High humidity is uncomfortable to work in - hot and clammy - which matters for the staff. But it also causes real problems: condensation on cool surfaces, which leads to persistent damp, which leads to mould, a hygiene and building issue; corrosion and degradation of the building fabric and equipment over time; and a generally unpleasant, poorly performing kitchen. So humidity is not just a comfort question. Reducing it protects the building, the equipment, hygiene and the staff, as well as making the kitchen more pleasant - which is why it is worth taking seriously.
Extraction over the cooking line is the main tool, because that is where most of the moisture is made and where the canopy captures it at source. But general ventilation of the kitchen space helps too, clearing the moisture that escapes the canopy, and make-up air is essential to let the extraction work. So it is not extraction alone but a balanced system - extraction capturing moisture at source, make-up air enabling it, and general ventilation clearing the rest. That said, the extraction is the heart of it: if the canopy and duct are removing the moisture as they should, the humidity is mostly controlled at source.
A clean system that still cannot keep the humidity down under a normal load is likely undersized or unbalanced; a system that used to cope but has gradually got worse is more likely clogged or degraded. A cleaning contractor can tell a lot by inspecting the ductwork (how much grease is restricting it) and checking the airflow against what the system should deliver. If cleaning restores the extract rate and the humidity improves, condition was the issue; if a clean, balanced system still cannot keep up, the rate is genuinely too low for the kitchen, which is a design question rather than a maintenance one.
If your kitchen is humid, the extraction is usually behind - undersized, unbalanced or clogged. We clean and assess extraction systems so they remove the moisture as fast as the kitchen makes it. Ask us to look at yours.
