Mould in Sri Lankan homes: why it keeps coming back, and how to stop it for good
24 July 2026 · 7 min read · DRYAir Dehumidifiers
The short answer
Mould grows whenever relative humidity stays above roughly 60% and it cannot survive below about 50%. In Sri Lanka indoor humidity commonly sits between 70% and 90%, which is why cleaning alone never works — the spores simply regrow on the damp surface. The permanent fix is to lower the moisture in the air itself and hold the room between 45% and 55% relative humidity, which a dehumidifier does automatically.
Almost every Sri Lankan household has fought the same battle. A dark patch appears in the corner of a bedroom, behind the wardrobe, or along the bathroom ceiling. You scrub it with bleach, it disappears, and within a few weeks it is back in exactly the same place. Most people conclude the wall is defective. It usually is not.
Mould is not a stain. It is a living organism, and like every living thing it needs water. In our climate the water is not coming from a leak — it is dissolved in the air of the room itself.
Why mould grows in Sri Lanka almost everywhere
Sri Lanka's relative humidity typically ranges from 70% to 90% year-round, and rises higher during the monsoon months and along the coast. Mould spores are always present in the air — they are harmless and unavoidable. What decides whether they germinate is moisture.
| Relative humidity | What happens indoors |
|---|---|
| Below 45% | Air feels dry; dust and static become noticeable |
| 45–55% | The healthy target: mould dormant, dust mites collapse |
| 55–60% | Borderline; mould begins to establish on cool surfaces |
| 60–70% | Active mould growth, musty smell develops |
| Above 70% (typical Sri Lanka) | Rapid mould colonisation, dust mites thrive, metals corrode |
This is why mould appears first in the coolest, least ventilated places: behind wardrobes and sofas pushed against outside walls, inside built-in cupboards, in bathrooms, and in rooms that are shut all day. Those surfaces are cooler than the room air, so moisture condenses on them and the surface stays damp long enough for spores to take hold.
What mould actually does to your health
Mould does not just look unpleasant. It releases spores and microscopic fragments into the air you breathe all night. Health authorities including the World Health Organization link damp and mouldy indoor environments to a measurable increase in respiratory problems.
- Worsened asthma — more frequent attacks and higher medication use, particularly in children
- Allergic rhinitis: persistent sneezing, blocked or runny nose, itchy eyes that never fully clear
- Coughing, wheezing and throat irritation, often worse in the morning after a night in the room
- Skin irritation and eczema flare-ups in sensitive individuals
- Higher risk for infants, the elderly, and anyone who is immunosuppressed
Why bleach and cleaning never fix it
Bleach kills the mould you can see on a hard surface and removes the black pigment, which is why the wall looks cured. But two things remain unchanged: the porous material underneath often still holds living hyphae, and — far more importantly — the air in the room is still carrying the moisture that let mould grow in the first place.
Nothing about the conditions has changed, so the outcome does not change either. Repainting has the same problem: anti-fungal paint slows regrowth on that one surface while the humidity keeps working on every other surface in the room.
The fix: control the air, not the wall
Remove the moisture and mould simply stops. It cannot metabolise, cannot spread and cannot regrow, regardless of how many spores are floating around. The target is 45–55% relative humidity — comfortable for people, hostile to mould and dust mites.
- 1Clean the existing mould properly (mask and gloves on, ventilate, and discard badly affected soft materials).
- 2Fix any genuine water ingress — a leaking pipe or roof needs a plumber, not a dehumidifier.
- 3Run a dehumidifier sized for the room and set it to around 50%. It will cycle on and off automatically to hold that level.
- 4Pull furniture a few centimetres off external walls so air can move behind it.
- 5Check the affected spots after two weeks — dry surfaces stay clean.
An air conditioner removes some moisture as a by-product of cooling, but it only does so while it is cooling, and it stops the moment the room reaches temperature. A dehumidifier targets humidity directly, runs on a fraction of the power, and keeps working overnight and while you are out.
Which size do you need?
Sizing is about room volume and how damp the space is. As a rough guide: a bedroom or study up to about 34 m² is served by a 12L/day unit; a living area up to 50 m² by a 20L/day unit; larger open-plan spaces and offices up to 80 m² by a 35L/day unit; and whole homes, basements or commercial floors up to 110 m² by a 50L/day unit. Coastal properties and ground floors usually need one step up.
If you are unsure, send us your room dimensions on WhatsApp — we have been sizing dehumidifiers for Sri Lankan homes since 2010 and will tell you honestly what you need, including when a smaller unit is enough.
Fix it at the source
Hold your rooms at a healthy 45–55%
DRYAir dehumidifiers set a target humidity and hold it automatically — day, night and right through the monsoon. Islandwide delivery, local service since 2010.
Frequently asked questions
What humidity level stops mould growing?+
Mould cannot grow when relative humidity is held below about 50%. The practical indoor target is 45–55%, which is comfortable for people and prevents both mould and dust mites.
Will a dehumidifier remove mould that is already there?+
No — you still need to clean existing mould off the surface. What the dehumidifier does is stop it returning, because the spores can no longer find the moisture they need to regrow.
Is an air conditioner enough to prevent mould in Sri Lanka?+
Usually not. An air conditioner only dehumidifies while it is actively cooling and stops once the room reaches its set temperature, so humidity climbs back overnight and while you are out. A dehumidifier controls humidity directly and continuously.
How long does it take to dry out a mouldy room?+
A dehumidifier typically brings a normal room from 75–80% down to around 55% within the first day, and to a stable 50% within two to three days. Walls and soft furnishings that have absorbed moisture take a little longer to fully release it.
Why does mould keep coming back on the same wall?+
That wall is almost always the coolest surface in the room — usually an external or north-facing wall, or one blocked by furniture. Moisture in the air condenses there first, so it is the first place to become damp enough for spores to grow.
Keep reading
Written by the DRYAir team — dehumidifier specialists in Sri Lanka since 2010. Questions about your own space? Call +94 75 666 00 00 any time, or message us on WhatsApp. This guide is general information, not medical advice — for persistent symptoms, please speak to your doctor.