Water at the Kitchen Exhaust Duct When It Hasn't Rained: Condensation or a Leak?

A stain spreads on a ceiling tile under the kitchen exhaust duct. It hasn’t rained. No one has touched a joint or a seal. The usual first call is to whoever fabricated the ductwork, asking them to reseal it, because a wet duct usually means a leak. Sometimes that is the answer. Sometimes the water is condensation — and the useful question is which duct it is forming on, and on which side of the metal, because that decides the fix.

Here is how to tell the possibilities apart, why condensation happens around kitchen ductwork in a Singapore building, and what actually addresses each case.

Rule out rain and a failed joint first

Water near a kitchen exhaust duct has a handful of realistic causes. Two of them are not condensation at all, so check them first.

• Rain. Where the duct or fan is exposed outdoors — through a roof, a facade or a riser — a storm can put water into the system from the open end. If the wet patch appears the morning after heavy rain and dries out over the following days, that is weather, not condensation. What to check on a kitchen exhaust after heavy rain covers that case, including what to do about a fan that has tripped.
• A joint that has opened up. Grease duct is joined at intervals, and a joint under strain — from age, from a light-gauge panel flexing, from a seal that was never right — weeps grease-laden moisture at that one point, every day the system runs, rain or shine. That water is usually faintly oily or discoloured, and it tracks back to a single seam you can find by following the stain uphill. Why grease duct joints fail covers what that looks like and why gauge and joint type decide it.

Condensation happens in two different places

Water condenses wherever moist air meets a surface colder than that air’s dew point. Around a commercial kitchen that can happen in two quite different ways, and they look different on the ceiling.

Inside the exhaust duct. Cooking pushes a lot of moisture into the exhaust air — steam from woks, stockpots, steamers and dishwashing all goes up the hood. That air is hot, so it holds the moisture while it is near the kitchen. As it travels along the duct it cools, and it cools faster where the duct runs through a colder space: an air-conditioned ceiling void over a dining area, a cooled corridor, a long run before it reaches the riser. When the inside surface of the duct drops below the dew point of the exhaust air, water condenses inside the duct. It picks up grease on the way, runs to the lowest point or the nearest joint, and weeps out. So this water is usually oily or discoloured — which is exactly why it gets mistaken for a failed joint.

On the outside of a cold duct. The other case is a duct that is colder than the air around it. In a kitchen that is usually not the exhaust duct — it is the air-conditioning supply duct, or a fresh-air duct carrying cooled air, running through the hot, humid kitchen or through a ceiling void that is not air-conditioned. The humid air around it condenses on its outer surface and drips. This water is clean and runs along the length of the duct rather than appearing at one seam.

A hot exhaust duct passing through a cool space does not sweat on the outside — its surface is warmer than the room. If clean water is dripping from what looks like the exhaust run, check whether a cold duct runs alongside or above it before anyone touches the exhaust.

What addresses each case

• Clean water from a cold duct: the fix is on that duct — insulation with a continuous vapour barrier, with no gaps at hangers, flanges or joints, because humid air that reaches the cold metal through a gap condenses there anyway. A drip at one bracket every few metres is the typical sign of lagging that stops short.
• Oily water from condensate inside the exhaust duct: resealing the joint the water comes out of helps, but the moisture is still forming inside. The questions are how the duct is laid (whether liquid can pool at a low point or run back toward the hood), whether the joints are built to hold liquid, how long the run is through cold space, and whether the duct can be reached for cleaning so grease-laden condensate does not build up. Any insulation or enclosure on a kitchen exhaust duct is a fire-safety detail as well as a thermal one, and where the duct passes through protected spaces it is a job for fire-rated ductwork, not an off-the-shelf wrap.
• Oily water at one seam that does not change with the cooking load: that is most likely the joint itself — see the joints link above.

These are decisions that belong with the duct route at design stage — the same stage at which BC Air’s design & build page describes commissioning that “tests and balances airflow so the system performs from day one.” Correcting them after the ceiling is finished means opening it along the affected run, which is a bigger job than a small stain suggests. Whether that duct can even be opened without cutting the ceiling apart depends on decisions made at the original fit-out.

How to check it yourself before calling anyone

• Note whether the stain grows on dry days, not just after rain. Rain-driven water needs weather; condensation does not.
• Look at the water, carefully, away from anything electrical. Clean and odourless points to a cold duct sweating on the outside. Oily or grease-tinted points to the exhaust duct — either a joint, or condensate from inside finding its way out.
• Note when it happens. Condensate from inside the exhaust duct tends to follow heavy steam cooking and service times; a sweating cold duct tends to follow when the air-conditioning is running.
• Trace which duct the water is actually on. Air-conditioning and fresh-air ducts often run right beside the exhaust in the same ceiling void.
• If water is anywhere near a light fitting, a control panel or other electrical equipment, switch off that circuit at the board before doing anything else.

On its own this is a maintenance problem rather than an emergency — unless the water is tracking toward electrics, in which case treat it as urgently as a leak. Grease-laden water collecting inside an exhaust duct is also a cleaning matter, so it is worth raising at the next exhaust clean either way.

Seeing a stain and not sure which of these it is? WhatsApp us on 8989 2833 with where it is and what the water looks like, before anyone opens a ceiling. Scope and pricing follow a look at the actual run.

Related: if the drip only ever follows a storm, start with what to check after heavy rain instead.

Also worth reading: why grease duct joints fail · the duct you cannot open is the duct that never gets cleaned