Understanding damp · 15 Oct 2026 · 4 min read
Condensation or leak? How to tell the difference
Wet walls and black spots can come from condensation inside the room or from water getting in from outside. Simple checks to tell which one you have.
A wet wall does not always mean water is getting in. In humid Indian cities, especially during the monsoon, the moisture can come from the air inside the room. It settles on the coldest surfaces and stays there. That is condensation, and the fix is completely different from the fix for a leak.
Getting this wrong is expensive in both directions. Waterproofing a wall that only has condensation does nothing. Treating a leak as condensation lets it run for another season.
How condensation happens
Air holds water vapour. The warmer the air, the more it can hold. When humid air touches a surface that is cold enough, some of that vapour turns back into water on the surface. Cooking, bathing, drying clothes indoors, sleeping in a closed room and running an AC that cools one wall more than others all play a part. The condensation source page covers the common patterns.
Signs that point to condensation
- Spots spread across a wall or ceiling, often worst in corners and behind furniture where air does not move.
- Black mould spots on the surface, wiped off easily, returning in the same places.
- Water droplets on windows, mirrors and the cold side of the room in the morning.
- It is worse in bathrooms without a working exhaust, in bedrooms kept closed, and in rooms where clothes dry indoors.
- No clear edge or tide mark, and no salt deposit.
Signs that point to a leak
- A defined patch with an edge, often with a brown ring or a tide mark.
- Efflorescence or salt crust, which needs water moving through the wall, not just sitting on it.
- The patch lines up with something: a bathroom above, a pipe route, a window, a terrace edge, a shaft.
- It follows the rain, or the use of a bathroom, rather than the time of day.
- Paint bubbling or plaster softening from inside.
Simple checks you can do
| Check | Condensation | Leak |
|---|---|---|
| Tape a square of clear plastic to the wall for two days | Droplets form on the room side | Droplets form on the wall side, or the wall stays wet under it |
| Open windows and run a fan for a week | Spots reduce | Patch stays the same |
| Compare with rain | Little link to rain | Worse after heavy rain |
| Look at the other side of the wall | Nothing obvious | Bathroom, pipe, external face or shaft |
| Look at the surface | Even film, spots, no salt | Edge, ring, salt, bubbling |
The plastic sheet test is old and rough, but it is useful. If moisture collects between the plastic and the wall, it is coming from the wall. If it collects on the outer face, it is coming from the room.
When it is both
A wall that is slightly damp from a leak is also colder, because of evaporation. A colder wall attracts more condensation. So a small leak can look like a condensation problem with black spots spreading around it. This is why a moisture meter reading against a dry reference wall helps: condensation wets the surface, while a leak usually raises readings into the plaster as well. A thermal camera can show whether the cool area has the shape of a pipe, a joint or a wet patch.
What helps with condensation
- Run the bathroom and kitchen exhaust while bathing and cooking, and for a while after.
- Keep a gap between wardrobes and external walls so air can move.
- Dry clothes on the balcony or in a ventilated utility where you can.
- Open windows on drier days, even during the monsoon.
- Check that the AC drain is not dripping onto the wall. See AC drain lines.
WHO's 2009 guidelines on dampness and mould link damp homes with respiratory symptoms; speak to a doctor about health concerns.
If you still cannot tell which one you have, a damp inspection compares every wall against a dry reference and states the likely source.
Find the damp before you pay for it.
Book a moisture and thermal inspection. Report on WhatsApp within 24 hours.