A Story From the Field: Washed Three Times, Still Smelling

Khun Ton is a thirty-five-year-old freelance photographer who moved into a new single-storey house in a small estate in Nong Chok, on the far eastern side of Bangkok, less than a year ago. The walls are built from autoclaved aerated concrete blocks, the lightweight grey blocks that have become standard in new Thai housing, then rendered and painted a clean white. During the 2026 floods, water from a nearby canal spilled into the estate, rose about fifty centimeters inside the house and stayed for almost a week.

After the water receded, Khun Ton and his wife shoveled out the mud, washed the floors and wiped the walls down, not once but three times. Three weeks later the house still smelled like a basement. The lower part of every wall carried a tide line that sat noticeably higher than the level the water had actually reached. The paint had started to blister into small bubbles. In some places a white, powdery crust had formed, almost like salt, and in the corner of the bedroom tiny black dots were appearing. Even the camera gear stored in his workroom had picked up the smell.

"The water left ages ago. Why are the walls still wet?" That was the question he kept asking. The answer lies in the nature of lightweight block itself. This article explains what is happening inside the wall, why washing again and repainting straight away will not solve it, and what to do, in the right order, to make the house dry, clean and odor-free again.

Why Lightweight Block Holds Water Far Longer Than You Expect

AAC, or autoclaved aerated concrete, is popular in housing estates and new shophouses because it is light, quick to lay and good at keeping heat out. Its lightness comes from a huge number of tiny air pockets spread throughout the material. In everyday life those pockets are an advantage. When a wall stands in floodwater, however, the same network of voids can soak up and store a great deal of water.

That water does not stop at the flood line. Moisture can wick upwards through the block, above the height the flood reached. That is why Khun Ton could see a tide mark higher than the actual water level. And because the outside of the wall is sealed with render and paint, the moisture inside can only escape slowly. Depending on the wall thickness, the type of paint and how well the room is ventilated, a wall can stay damp for weeks, sometimes longer.

While the wall is still wet, minerals from the floodwater and from the cement itself are carried to the surface as the water evaporates, leaving a white crust known as efflorescence. Paint bubbles because moisture is pushing outward from behind it. And a surface that is constantly damp is exactly where mold is happiest.

Homes in Nong Chok face some particular conditions that make this more noticeable. Many houses sit close to canals and farmland, the water table is relatively high, and the air at the end of the rainy season is still humid. Walls therefore give up their moisture more slowly than they would in a drier place. Planning the drying step by step matters more than rushing to finish the repairs.

It is also worth remembering that floodwater is not clean water. On its way through the streets it picks up mud, overflow from drains and sewers, household chemicals and animal waste. The lower part of the wall has absorbed some of that mix along with the water, which is why the clean-up is about hygiene as well as drying.

What Trapped Moisture Does to Your Home and Family

Health. A wall that stays damp is a breeding ground for mold. Spores in the air can trigger sneezing, a blocked nose, itchy eyes and asthma symptoms, particularly in children, older people and anyone with a respiratory condition. The mud residue on the lower part of the wall comes from floodwater that may carry germs from drains, such as E. coli, and where water stood for days there is also a risk of leptospirosis, known in Thailand as rok chi nu. Always wear gloves and boots when cleaning. The World Health Organization notes that mold and dampness are among the lasting health problems that follow floods, and Thai health authorities advise drying and disinfecting homes before moving back in fully.

Odor. The musty smell from damp walls is stubborn. It soaks into curtains, mattresses, clothes and anything else in the room. Even if the windows are open all day, it tends to return at night when the house is closed up.

Money. Repainting before the wall is dry usually means paying twice. The new paint blisters and the mold comes back within a few months, so both the paint and the labor are wasted. Some families end up scraping and repainting the same wall three times in a year.

Belongings. Anything placed against a damp wall, such as wardrobes, bookshelves, mattresses and electronics, absorbs moisture and smell. For someone like Khun Ton, who works with expensive cameras and lenses, a humid room is also a real risk of fungus growing inside the equipment.

Stress. A house that looks repaired but still smells wrong is exhausting to live in. Families start to doubt whether the home will ever feel right again. Understanding the cause, and having a clear plan, removes a lot of that worry.

Signs Your Lightweight Block Wall Is Still Holding Water

• A tide line that is higher than the actual flood level and is creeping upward or not fading.
• Paint that is blistering, peeling or feels soft when you press it.
• A white, powdery or crystalline crust on the lower part of the wall.
• The lower wall feels noticeably cooler and clammier than the upper wall when you touch it with the back of your hand.
• The musty smell is strongest in corners, behind cabinets and in rooms that have been closed for a while.
• Black, green or fuzzy white spots along the skirting and in corners.

If you want certainty rather than guesswork, many builders and painters carry a moisture meter that can tell you whether the wall is dry enough to repaint. It is a small step that can prevent an expensive mistake.

Why the Usual Fixes Never Quite Finish the Job

Washing the walls again and again. Washing removes mud and stains, but every time you hose down a lightweight block wall, it drinks more water back in. Use a damp, well-wrung cloth to clean and disinfect instead of spraying the wall.

Painting over it straight away. Fresh paint seals the surface, trapping moisture inside. The result is blistering paint and mold growing beneath the new coat, often within weeks.

Relying on bleach alone. Bleach lightens stains but does nothing about the moisture in the body of the wall. Never mix bleach with ammonia or with certain toilet and bathroom cleaners, because dangerous gases can form.

Closing the house and running the air conditioner. Air conditioning removes some humidity from the air, but a sealed room with no fresh air exchange keeps the musty smell and spores circulating.

Perfume, candles and air fresheners. Fragrance hides the smell for a while but does nothing at all to reduce mold.

The Right Order: Dry, Disinfect, Then Paint

Step 1: Let the wall breathe. Scrape off blistered paint, move furniture away from the walls, open doors and windows on dry days, and run fans in a way that gives humid air somewhere to go. A dehumidifier helps a great deal if you can get one.

Step 2: Have the wall assessed. If you see cracks, loose render, or suspect damage to the structure or reinforcement, ask a builder or engineer to inspect it. Sockets and switches that were under water must be checked by a qualified electrician before use.

Step 3: Disinfection fogging and ozone treatment. Once the mud, waste and damaged materials are out, this is the moment for the disinfection service and the ozone treatment service from World Health Disinfection (WHD), which deal with germs, surface mold on the walls and the musty odor soaked into the rooms.

Step 4: Wait until the wall is genuinely dry before painting. Talk to your painter about measuring moisture and choosing a primer suited to a wall that has been flooded. Do not repaint just because the surface looks dry.

The fogging uses CHEMGENE HLD4H disinfectant applied with an ultra-low-volume (ULV) fogger. The very fine droplets drift onto wall surfaces, skirting boards, corners and crevices that a cloth cannot reach, without drenching the wall the way hosing does. Ozone treatment then deals with spores in the air and on surfaces and breaks down the odor molecules trapped in the room. Neither method dries the wall in place of ventilation; what they do is reduce germs and smell while the wall dries slowly, and before it is sealed under new paint.

How the WHD Team Works

1. Site survey. The team looks at the tide lines, blistering paint, mold spots and the rooms where the smell is strongest.
2. Preparation advice. Clear out mud, move belongings away from the walls and remove damaged materials, so that the disinfectant and ozone can reach the wall surfaces.
3. Disinfection fogging with CHEMGENE HLD4H. Focus on the lower walls, skirting boards, corners and floors.
4. Closed-room ozone shock treatment. No people, pets or plants may be in the area. The team controls the dose and duration according to the size of each room. Sensitive electronics, cameras and lenses, artworks and valuables should be removed or covered first.
5. Ventilation. Doors and windows are opened and the house is aired thoroughly before the family goes back in.
6. Re-check and advice. The team checks the smell again and explains how to keep drying the walls until they are ready to paint.

WHD provides disinfection fogging and ozone treatment only. Wall repair, rendering, painting and electrical work should be handled by the appropriate qualified trades.

A Practical Drying Plan, Week by Week

Week one after the water leaves. Get the mud out, remove soaked items such as carpets, cardboard boxes and mattresses that cannot be saved, and pull every piece of furniture away from the walls. Scrape off any paint that is clearly blistering so the wall has somewhere to release moisture. Open the house whenever it is not raining. This is also the week to have sockets and the electrical board checked by an electrician, and to call a builder if you see cracks.

Week two. Keep air moving. Fans work best when they push humid air out through an open window rather than simply stirring it around a closed room. If you have a dehumidifier, run it in the room that smells worst and empty it often; the amount of water it collects will surprise you. Once the mud and damaged materials are gone, this is a good moment to book disinfection fogging and ozone treatment, so that germs and odor are dealt with while the wall continues to dry.

Weeks three and four. Watch the tide line and the white crust. If the crust keeps returning after you brush it off, the wall is still pushing out moisture. Brush it off dry with a stiff brush rather than washing it, which only adds water back. Check the corners and the area behind where cabinets stood for any new mold spots, and wipe and disinfect them if they appear.

Before painting. Ask your painter to measure the moisture in several places, low and high, on each wall. Only when the readings are consistently acceptable should primer and paint go on. If one wall stays damp much longer than the others, there may be another water source, such as a leaking pipe or rising damp from the slab, which a builder or plumber should investigate.

Room by Room: Where the Smell Usually Hides

Bedrooms. Wardrobes and beds pushed against outside walls trap moisture behind them. Mattresses that touched the floodwater should usually be discarded; those that stayed dry still benefit from airing and a dust mite treatment once the room is clean.

Kitchen. Cabinets under the sink and along the wall hide damp block and often a little leftover mud. Empty them, leave the doors open, and include this area in the fogging.

Bathroom. Bathrooms are humid even in normal times. After a flood, check the wall behind the toilet and basin, and keep the exhaust fan running if it has been checked as safe to use.

Workroom or study. Paper, books and equipment absorb odor easily. Move valuable items, especially cameras and electronics, to the driest part of the house, and do not return them until the room smells clean.

10 Reasons to Disinfect Walls Before You Repaint

1. Surface mold is reduced before it is sealed under fresh paint, using a professional disinfection service.
2. The musty odor in the rooms is broken down by ozone treatment rather than masked.
3. ULV droplets do not soak lightweight block the way hosing does.
4. The team uses CHEMGENE HLD4H, designed for surface disinfection work.
5. Fogging reaches skirting, corners and the space behind cabinets that people often forget.
6. Family members with allergies sleep more comfortably while the walls are drying.
7. It lowers the chance of mold spots appearing through your new paint a few months later.
8. Curtains, bedding and belongings stop absorbing the smell from the walls.
9. You get advice on drying that suits your specific house.
10. It extends easily to residential ozone disinfection for other rooms of the home.

In the Customer's Words

"I was about to get the whole house repainted. The team suggested scraping off the blistered paint and waiting for the walls to dry first, and they did the fogging and ozone treatment in the meantime. The house smells much better and we sleep more easily. Now I am waiting for a painter to measure the moisture before we paint, so we will not have to pay twice."

Khun Ton, single-storey house, Nong Chok (a scenario from the field)

Looking After the House While the Walls Dry

• Open windows on opposite sides of the house on dry days so air can flow straight through.
• Keep furniture at least a hand's span away from the walls, and do not lean boxes or mattresses against the lower walls.
• Wipe small mold spots with a damp cloth and follow with a spray disinfectant such as the CHEMGENE HLD4H spray.
• Store electronics and camera equipment in the driest room, or in sealed boxes with desiccant packs.
• Watch the tide line. If it keeps rising, ask a builder or plumber to check for another water source, such as a leaking pipe or water under the floor slab.
• Do not hang wallpaper, panels or wall decorations on the lower wall until it has been confirmed dry.
• Note the date of the flood and keep simple photos of the walls each week. It helps you and your painter see whether drying is progressing.

Frequently Asked Questions

Q: Does a flooded lightweight block wall have to be demolished?
A: In most cases, no, provided the structure is not damaged. It does need time to dry and proper mold control. If there are cracks or the render is coming loose, have a builder assess it.

Q: How long before the wall can be repainted?
A: It depends on wall thickness, paint type and ventilation. Ask a painter to measure the moisture rather than picking a date by guesswork.

Q: Does ozone help the wall dry faster?
A: That is not what ozone does. Ozone deals with mold spores and musty odor; drying depends on ventilation, fans or a dehumidifier.

Q: Is the white crust on the wall mold?
A: A white powder or crystalline crust is usually salt brought out by moisture. Mold tends to appear as black, green or white fuzzy spots with a thread-like texture. Both are signs the wall is still damp.

Q: Do we need to leave the house during ozone treatment?
A: Yes. No people, pets or plants may be in the area during treatment, and the space must be ventilated adequately before you return. The team will give you clear timings.

Q: Can I speed things up with heaters or by lighting charcoal indoors?
A: Please do not burn charcoal or use fuel-burning heaters inside a closed house, because of the serious risk of carbon monoxide poisoning and fire. Steady ventilation, fans and a dehumidifier are safer and work well over time.

Q: Will the smell come back after ozone treatment?
A: If the source of moisture is still there, some odor can return as mold regrows. That is why drying, removing damaged materials and treating the room go together. If the smell comes back strongly, check for remaining wet materials or another water source before repeating treatment.

Q: Does WHD repair walls or paint?
A: No. WHD provides disinfection fogging and ozone treatment only. Wall repairs and painting should be done by a builder or painter.

Clear the Musty Smell From Your Nong Chok Home Before Repainting Day

Talk to the World Health Disinfection team about post-flood disinfection fogging and ozone treatment any day of the week.

Book Ozone Treatment - Click Here

See Disinfection Service - Click Here

Call 065-556-6294 · LINE @whd268 · worldhealthdisinfection@gmail.com

Related Links

• Residential ozone disinfection
• Ozone odor and chemical residue removal
• Master Ozone Generator
• All articles
• External source: WHO: Floods and health and Department of Health, Thailand