Damp and mould 2026: the 48-hour rule and when to call a professional

Most people tackle mould in the wrong order: they look for a strong cleaning product, wipe it off, and a month later it is back in the same place.

This article follows the order the US EPA sets out, which gives three measurable thresholds — how long you have, how big an area you can handle, and what humidity to hold — and one of them directly contradicts the advice you will hear most often.

Rule one: 24–48 hours

This is the most valuable number here, and hardly anyone mentions it.

EPA: if wet or damp materials or areas are dried 24–48 hours after a leak or spill happens, in most cases mould will not grow.

Which means this is not about which product you use. It is about speed. A leak on Friday left until Monday turns a mopping job into a mould remediation job.

What to do inside the first 48 hours:

1. Stop the source first — close the valve, fix the leak, or move things out from under the drip.
2. Extract or soak up as much water as possible. Do not wait for it to evaporate.
3. Run fans and open windows so air moves across the wet surfaces. Airflow matters more than heat.
4. Lift porous items off the floor. Carpet, mattresses and cardboard hold water and dry slowest.
5. Check under and behind things. Water runs downhill and hides under cabinets, behind stud walls and beneath timber floors.

⚠️ Things that look dry on the outside may still be wet inside, particularly plywood, plasterboard and stud walls. This is why a pin-type moisture meter is genuinely useful in a house that has had flooding or a pipe leak.

Photo: Asurnipal — CC BY-SA 4.0 (Wikimedia Commons)

Rule two: find the moisture source first

Once mould has appeared, wiping it off alone buys a few weeks. Mould needs moisture to grow; as long as the moisture remains, it returns.

Moisture sources in Thai homes, in rough order of frequency:

An easy way to tell condensation from a leak: see whether the patch changes with the weather. Condensation gets worse in humid weather and better in dry. A leak persists and slowly spreads regardless.

Rule three: hold humidity below 60%

The EPA recommends keeping indoor relative humidity below 60%, and within 30–50% where possible.

Above 60% is where condensation starts forming inside the building, which is the condition mould needs. Thai rainy-season humidity often sits at 75–90%, so the climate naturally favours it.

Ways to reduce humidity, cheapest and most effective first:

1. Extractor fans in the bathroom and kitchen — 500–2,000 baht, about 28 baht a month to run, and they address the biggest steam source directly.
2. Opening windows when it is sunny — free, but useless on a rainy day when outside air is more humid than inside.
3. Silica gel sachets for wardrobes and closed cabinets.
4. A dehumidifier for rooms that genuinely are damp and can be closed — 300–800 baht a month to run. See how to size one.
5. Air conditioning — if it is already running, it removes humidity as it works.

⚠️ Buy a 200–500 baht hygrometer first. It tells you whether the room really has a humidity problem or just poor ventilation, which need different fixes.

Rule four: the line for calling a professional is about 1 sqm

The EPA gives a clear, measurable threshold:

If the mouldy area is less than about 10 square feet (roughly a 3 ft by 3 ft patch, about 1 sqm), in most cases you can handle the job yourself.

Larger than that, call a professional — and call one regardless of size in these cases:

• It came from flooding or sewage, which brings pathogens beyond mould.
• Mould in the air conditioning or ductwork, because it will spread spores through the house every time it runs.
• You suspect a leak inside a wall, because the wall has to be opened and the plumbing fixed first.
• Someone in the house has asthma, severe allergies or a compromised immune system.

Photo: Infrogmation — CC BY 2.5 (Wikimedia Commons)

Cleaning it safely, and the bleach question

The protection EPA recommends

• An N-95 respirator, because scrubbing puts spores into the air. A cloth mask does not filter them.
• Long gloves reaching the middle of the forearm.
• Goggles without ventilation holes, since vented ones let spores reach your eyes.
• Ventilate throughout, and close the door to keep spores out of other rooms.

⚠️ About bleach — this contradicts the usual advice

The common recommendation is to wipe mould with diluted bleach. But the EPA does not recommend bleach as routine practice for hard surfaces. What it actually recommends is:

Scrub mould off hard surfaces with detergent and water, and dry completely.

The reason is that what stops mould returning is dryness, not chemistry. Wiping bleach on and leaving the surface damp adds water to a place that already has a water problem. EPA notes bleach is considered only in particular cases, such as when someone immune-compromised is present.

⚠️ Never mix bleach with anything containing ammonia — it produces toxic gas, and plenty of cleaning products contain ammonia without saying so prominently on the front label.

What can be cleaned, and what has to go

This is where people get sentimental and decide wrongly. A ceiling tile scrubbed until it looks clean usually still has mould growing inside the material, and it will be back within months. Replacing a panel for a few hundred baht is cheaper than doing the job three times.

Can mould in the house actually make you ill?

Short answer: yes, but not equally for everyone.

Mould continuously releases spores into the air, and breathing them can trigger symptoms, particularly in people already sensitive.

• People with allergies or asthma usually react most clearly — sneezing, congestion, a persistent cough, wheezing.
• Small children and older adults are more sensitive than the general adult population.
• Anyone immune-compromised is the group to be most careful with, and the case where EPA says a disinfectant may be warranted.
• Healthy adults may notice nothing obvious, but living with a heavy spore load is not a good idea regardless.

Signs your home may have a problem even before you can see any staining:

• A musty smell that disappears when you leave and returns when you come back.
• Allergy symptoms that improve when you sleep elsewhere and return at home.
• Symptoms that worsen only in the rainy season, or only in one particular room.

⚠️ If someone at home has unexplained chronic respiratory symptoms, damp and mould are worth checking. An air purifier filters spores out of the air but does not address the cause — see how to choose an air purifier if you want to reduce the spore load while you fix the moisture.

Mould in the air conditioner — the most common Thai case

An air conditioner contains both moisture and darkness, which makes it the easiest place in the house for mould to establish — and the worst, because every time you switch it on you blow spores around the room.

Signs of mould in an AC:

• A musty smell when you first turn it on, fading after a while
• Dark spots on the blower wheel or vanes when you shine a light in
• Allergy symptoms that flare only in the air-conditioned room
• Water dripping from the unit, meaning the drain line is blocked and water is standing inside

You can rinse the filter yourself once or twice a month. The coil and blower need a full service by a technician, which MEA already recommends every six months for efficiency — one job, two benefits. See the electricity side in our article on cutting your bill, and if you are replacing the unit, the air conditioner buying guide.

⚠️ Mould in an AC system is a case where EPA advises calling a professional regardless of how small the visible area is, because the air handling spreads spores through the whole house.

Photo: Dinkun Chen — CC BY-SA 4.0 (Wikimedia Commons)

Long-term prevention

• Bathroom extractor fan — run it during every shower and for 15–20 minutes afterwards, which is when the most steam is still hanging around.
• Do not dry laundry in the bedroom. One load of wet washing releases several litres into the room.
• Leave 5–10 cm behind wardrobes and beds so air can move. The dead corner behind a wardrobe is the most common mould spot in a Thai bedroom.
• Service the AC on schedule and check the drain line is clear.
• Check bathroom and basin sealant once a year. Cracked sealant is a route for water into the wall.
• Inspect after every heavy rain — ceilings, wall corners, under windows. A fresh stain is the one that is still easy to fix.

Common mistakes

Wiping the mould and painting over it. Paint does not kill mould and does not stop moisture. Within months it blisters and peels away with more mould than before. Remove and dry first, always.

Reaching for a stronger chemical instead of fixing the moisture. See the bleach section above.

Shutting the door on a mouldy room. A closed damp room is exactly what mould likes best. Shutting it makes things worse, not better.

Running a dehumidifier without fixing the leak. That is paying hundreds of baht a month indefinitely to mask a symptom.

Not wearing a mask while scrubbing. Scrubbing is the single point in the process that releases the most spores.

Assuming what you see is all there is. Visible surface mould is usually a fraction of the total. If a patch spreads quickly, or the musty smell persists after cleaning, something is behind the wall.

Summary: do it in this order

1. Within 48 hours of a leak — get it dry, and the story ends there.
2. If mould has appeared, find the moisture source first. Everything else is buying time.
3. Measure the humidity, aim below 60%, and try ventilation before buying a machine.
4. Check the area. Under about 1 sqm you can do it; larger means calling someone.
5. Wear an N-95, long gloves and sealed goggles, then scrub with detergent and water and dry completely.
6. Throw away porous materials that have gone mouldy. Do not try to save them.

Sources

• US EPA — A Brief Guide to Mold, Moisture and Your Home
• US EPA — Mold Course Chapter 2: moisture and mould growth
• US EPA — Mold Cleanup in Your Home
• MEA — current Ft announcement (used for the appliance cost figures)

The technical guidance here — the area threshold, protective equipment, cleaning method, humidity range and the 24–48 hour window — follows published US EPA material, which is the international reference. This is general information, not an on-site assessment. Where mould covers a large area, follows flooding, or where someone in the household has a respiratory condition, consult a professional directly.