Nine Months After the Flood the Mould Keeps Coming Back, and Their Daughter Wheezes Every Night Two Coats of Anti-Mould Paint Did Nothing
Last updated: 6 Sep 2026 | 8 Views |
Ozone disinfection service 2026 for flood-damaged and mould-affected homes
Nine Months After the Flood the Mould Keeps Coming Back, and Their Daughter Wheezes Every Night — Two Coats of Anti-Mould Paint Did Nothing
Why mould returns after every repaint, and why what you cannot see matters more than what you can.
After the water goes down, what you see is the tide mark on the wall and the ruined furniture. What stays for years is the mould spores that spread through the whole house on day one and wait only for the right humidity to grow again. This is the true story of one family that took nine months to understand that.
Almost every homeowner after a flood does the right things. They pump the water out, air the house, strip out damaged flooring, wipe the tide marks, scrub off the visible mould, and repaint with paint advertised as mould-resistant.
Then three to six months later, black and green spots reappear in the same corner. Sometimes in a new corner that never had them. And the damp musty smell they thought had gone comes back on rainy days.
This article explains why scrubbing and painting is not enough, why the mould spores you cannot see are the real problem, and what ozone treatment — a gas that reaches wherever air reaches — actually does about it.
The story
Sixty centimetres of water, three days
Supatra's house is a two-storey home in a central province. The flood that year stood for three days at about sixty centimetres above the ground floor.
After the water went down the family did everything they should. They pumped it out, opened the house up for two weeks, stripped out all the swollen laminate flooring downstairs, threw away the downstairs sofa and mattresses, and wiped the walls with disinfectant.
In the second month Supatra hired a contractor to scrub off the mould on the downstairs walls and repaint the whole floor with paint specifying a mould inhibitor. With the new flooring, that came to about 120,000 baht.
The house looked far better and everyone thought it was over.
Month five: the black spots returned in the same corner
Five months after painting, small black spots appeared in the corner of the downstairs bedroom — the same corner as before — and within two weeks had spread to about the size of a palm.
Supatra had it scrubbed and repainted a second time, this time with higher-grade paint in three coats. Another 45,000 baht.
Four months later the spots were back. This time in two corners rather than one, with a new patch behind the wardrobe that had never had any.
What actually worried the family was not the walls. It was their seven-year-old daughter, who had begun wheezing at night from the third month, worse in the rainy season, needing her inhaler three or four times a week.
The question the doctor asked that they could not answer
When they took their daughter to the hospital, the doctor's first question was whether the house had been flooded, and whether there was still mould growing now.
Supatra said yes it had flooded, and yes there was still some mould in certain corners, but they had repainted twice.
The doctor explained that the visible patches are only the part that has grown large enough to see. What affects the respiratory system is the spores suspended in the air, which are invisible and present throughout the house — not just at the black patches.
The question Supatra could not answer was the doctor's next one: had they done anything about the air in the house, or only about the walls they could see? The answer was the latter.
What the survey team found, and what they did
When the team surveyed the house, the first thing they did was measure moisture in the walls and floor with a meter, rather than looking with their eyes.
The result: the downstairs walls below one metre still held above-normal moisture in several places, even though the surface was completely dry and painted over — because the water that soaked into the brick and plaster on the day of the flood had not fully evaporated, and the paint over the top had slowed that evaporation further.
They also found mould on the coil and in the ducts of the downstairs air conditioner — which explained why every time it was switched on, spores were blown throughout the house and down to breathing height.
So the team addressed two things together: advising on the structural moisture first, then ozone-treating the whole house to deal with the spores suspended in the air, in the furniture, in the air conditioning and in the places nobody can see.
What it actually costs
What you can see is a small fraction of the problem
The black patches on a wall are the part that has grown visible. The spores suspended in the air and settled in furniture, curtains, mattresses and the air conditioning are far more numerous — and they are the part that affects breathing.
Children and the elderly show symptoms first, always
In a house with accumulated mould spores, small children, elderly people, and anyone with allergies or asthma show symptoms first — particularly at night and in the rainy season when humidity rises.
Painting over slows the moisture down
Paint applied over a wall that is still damp inside acts like a sealing film, slowing the evaporation of water held in the brick and plaster. That is why mould returns to the same corner within months.
Repeated spending that never touches the cause
Supatra spent 120,000 baht on new flooring and the first repaint, then 45,000 baht on the second scrub and repaint — 165,000 baht in total — and the problem still came back.
Why the usual fixes fail
Scrubbing visible mould and painting over it
Scrubbing addresses only the growth on the wall surface, while the mould's roots penetrate the plaster and spores remain distributed throughout the house. Painting over is covering the problem while the cause remains.
Using paint advertised as mould-resistant
Mould-resistant paint genuinely slows growth on the paint surface in normal conditions. But on a wall holding moisture inside permanently, that moisture pushes out to the surface and mould will grow even on that paint.
Airing the house with the windows open for weeks
Airing genuinely removes humidity from the air and from material surfaces. It cannot draw out water that has soaked into brick and plaster in a matter of weeks — which is the source of the moisture still present months later.
Running an air purifier in the bedroom
An air purifier does filter spores that pass through it and can ease symptoms somewhat. But it does not address the source releasing the spores — particularly the air conditioner, which blows them out every time it runs.
Ordinary air conditioning servicing
Standard servicing addresses the coil and the filter to a degree, but mould in the ducts, the casing and the insulation cannot be stripped out — so spores keep being blown out every time the unit runs.
Two things have to be addressed together: moisture and spores
Mould needs two things to grow: moisture and organic matter for food. In a flooded house both are present in abundance and distributed throughout — not just at the visible patches.
So a lasting fix has to do two things together. First, bring the structural moisture back to normal levels, which may mean removing some surface materials and takes time. Second, eliminate the spores distributed through the air, the furniture, the fabrics and the air conditioning system.
It is the second of these that ozone treatment does best, because ozone is a gas and reaches wherever air reaches. The Master Ozone Generator used by this service is the only brand in Thailand certified by the Department of Medical Sciences, Ministry of Public Health, as removing deep-seated mould in air conditioning and furniture to 99.99 per cent, and tested by Intertek in the UK.
10 reasons the AIROFOG U260 solves it
1. It addresses the spores spread through the whole house, not just visible patches
Black patches on a wall are a small part. The real spore load is suspended in the air and settled on curtains, in mattresses, inside wardrobes and behind furniture. Ozone gas reaches all of it in one treatment.
2. It reaches the air conditioning, which is what spreads the spores
An air conditioner with mould on the coil blows spores out every time it runs. Ozone treatment with the unit on recirculation carries ozone to the coil and the ducts — where ordinary servicing cannot reach.
3. Certified at 99.99 per cent effectiveness against mould
The Master Ozone Generator is tested by the Department of Medical Sciences, Ministry of Public Health, as removing deep-seated mould in air conditioning and furniture to 99.99 per cent, and by Intertek in the UK.
4. It adds no moisture to a house that already has a moisture problem
This is the most important point for a flooded house. Steam treatment or liquid spraying adds moisture to a house that is trying to dry out. Ozone is a gas and adds none at all.
5. No chemical residue in a house with children
Ozone reverts to oxygen after reacting, so there is no chemical residue on floors, toys, mattresses or anything a child touches — a necessary condition in a home with small children.
6. It removes the damp musty smell at the same time
The musty smell that returns whenever it rains comes from compounds released by mould and bacteria. Ozone reacts with those molecules directly, so the smell goes along with the spores rather than being masked.
7. Surface moisture is verified before results are confirmed
The standard procedure for a mould-affected house is to check surface moisture levels after treatment, to confirm no mould remains and to tell the owner which areas still need structural moisture work.
8. Furniture, mattresses and fabrics can be saved rather than thrown away
Many items owners assume must be discarded because of musty smell or suspected mould can be ozone-treated and kept — far cheaper than replacing them, particularly mattresses and sofas.
9. About 30 minutes per cycle, with no need to move out
The treatment runs about thirty minutes per cycle on a timer, then the house is ventilated until the ozone smell has gone. The family leaves briefly and moves back in the same day.
10. It costs less than one repaint
Supatra spent 45,000 baht on the second scrub and repaint and the problem still returned, while ozone treatment of the whole house costs less than that and addresses what painting never touched.
Before and after
Before
- Flood water standing 3 days at 60 centimetres
- Laminate flooring stripped, downstairs sofa and mattresses discarded, walls wiped with disinfectant, house aired for 2 weeks
- Mould scrubbed and mould-resistant paint applied — 120,000 baht including new flooring
- Month 5: black spots back in the same corner; second scrub and repaint, another 45,000 baht
- Month 9: mould in two corners plus a new patch behind the wardrobe
- A seven-year-old wheezing at night, needing her inhaler 3-4 times a week
After
- Moisture in walls and floors measured with a meter rather than judged by eye
- Downstairs walls below 1 metre found to still hold high moisture despite dry, painted surfaces
- Mould found on the coil and in the ducts of the downstairs air conditioner — the spreader for the whole house
- Structural moisture addressed alongside a whole-house ozone treatment
- Ozone run with the air conditioning on recirculation, treating air, furniture and the AC system
- Surface moisture re-checked after treatment to confirm no mould remained
How we get you started, in 3 steps
1. Survey and measure moisture, rather than judging by eye
For a flooded or mould-affected house we check the level of contamination and measure moisture in walls, floors and materials before planning the work — because a house with dry surfaces but wet material underneath will grow mould again however many times it is treated.
2. Disinfect surfaces first, then run the ozone treatment
Where mould or virus is present, the standard procedure is to disinfect those surfaces first and then run the ozone treatment immediately afterwards, with the space sealed and everyone out of the area. The timer completes the process in around 30 minutes.
3. Ventilate and verify surface moisture to confirm the result
When the process finishes, doors and windows are opened until the ozone smell has gone. We then check surface moisture levels to confirm no mould remains in that area, and identify which areas still need structural moisture work.
What a customer says
“I used mould-resistant paint twice — over 165,000 baht — and it came back in the same corner. When I took my daughter to the doctor about the night-time wheezing, he asked me one question I could not answer: had we done anything about the air in the house, or only the walls we could see? It was only when the team came and measured the moisture that I learned the walls were still wet inside and the downstairs air conditioner had mould spreading it through the whole house.”
Supatra — Homeowner after flooding, central Thailand
Why mould always returns to the same corner
A room corner is where air moves least. Airflow from an air conditioner or fan circulates through the middle of a room and weakens by the time it reaches the corners, so moisture at the wall surface there evaporates more slowly than anywhere else.
A corner is also where two walls meet, giving a greater mass of masonry and a slightly lower surface temperature than the rest of the wall — so water vapour in the air condenses there first.
In a flooded house, both of those factors plus the moisture still held in the brickwork make the corner the place mould returns first, however many times it has been painted.
So seeing black spots return in the same corner does not mean the painter did a poor job. It means the moisture in the structure has not been resolved.
Mould spores and children's airways
Mould spores are only a few microns across, so they stay airborne for a long time and can be drawn deep into the lower airways — unlike larger dust, which is trapped in the nose.
In small children the airways are far narrower than an adult's, so a small amount of swelling or inflammation affects breathing much more. Children also breathe more often relative to body weight, so they take in proportionally more spores.
Symptoms are usually worse at night, because children sleep in a closed room with the air conditioning on for hours — and if there is mould in the air conditioning box, that bedroom becomes the highest-concentration spore exposure in the house.
This is why managing the air and the air conditioning matters as much as managing the walls, and why doctors ask about flooding and household mould when a child has persistent wheezing.
Frequently asked questions
After ozone treatment, will the mould stay away for good?
Ozone treatment removes the spores and mould present, but if the structure still holds moisture, mould will eventually grow again. That is why we measure moisture before and after and tell you which areas need moisture work alongside.
Do we need to strip the walls or repaint before treatment?
Not necessarily. We assess from the moisture readings. If wall moisture is within an acceptable range, treatment can proceed. If it is too high, we will advise on reducing moisture first so you do not pay to treat twice.
Do mattresses and sofas suspected of mould have to be thrown out?
Usually not. Ozone reaches foam and fabric because it is a gas, which is far cheaper than replacing them — except where the material is physically damaged from direct immersion.
Is it safe for a child with asthma?
Yes, when the procedure is followed. The child and everyone else must be out of the area during treatment, and you wait for ventilation to clear the ozone smell before returning. Afterwards there is no chemical residue, because ozone reverts to oxygen.
How many cycles are needed?
It depends on the contamination found at survey. A house with visible mould in several places usually needs more than one cycle, and should be treated again once the moisture work is complete. We set out the plan at assessment.
Can it be used on the air conditioning too?
Yes, and it should be, because an air conditioner with mould is what spreads spores through the whole house. We run the unit on recirculation during treatment so ozone reaches the coil and the ducts.
Does the damp musty smell go too?
Yes, because that smell comes from compounds released by mould and bacteria. When ozone reacts with those molecules the smell goes along with the spores — it is not masked.
How long does it take, and how long must we leave the house?
The treatment runs about 30 minutes per cycle, then the house is ventilated until the ozone smell has gone. The family leaves briefly and moves back in the same day.
What you cannot see is the part affecting your child's health
Supatra's family spent 165,000 baht on flooring and paint over nine months, dealing only with what they could see — while the spores in the air and in the air conditioning, the part directly affecting their daughter, were never touched.
Ozone treatment addresses exactly that unseen part: the air, the furniture, the mattresses and the air conditioning system. It adds no moisture to a house that is trying to dry out, and leaves no chemical residue in a home with children.
If your home has been flooded, has mould returning, or has someone whose allergies or asthma worsen in the rainy season, send us the details and photos of the affected areas and we will come and measure the moisture and assess it.
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