A Story From the Field: A Small House With a Big Smell

Auntie Sunantha is sixty-one. After retiring from an accounting job, she bought a single-storey, two-bedroom knock-down house and had it installed on her family's land beside a canal in Lam Luk Ka district, Pathum Thani, two years ago. The house has a steel frame, cream faux-wood siding outside, and inside walls made from fiber cement board, the material many Thais simply call "Smartboard", painted a clean white. She loves the house. It went up quickly, it stays cool, and it is easy to look after.

In the October 2026 floods, the canal overflowed onto the land and water stood about twenty centimeters deep inside the house for a full week. Auntie Sunantha stayed with her niece in town. When she came back, the walls inside and out looked normal. Nothing had swollen or bubbled. There was just a straight tide line marking how high the water had come. She wiped the line away. Two weeks later, though, a musty smell was getting stronger every day, especially in the air-conditioned bedroom. Small black dots appeared on the paint low down along the board joints, and brown rust stains seeped out around some of the screw heads. She started waking up every morning with a blocked nose.

"The walls aren't wet at all. Where is the smell coming from?" she asked us. The answer is that a knock-down house has double-skinned walls with a hollow space in the middle, and the floodwater went into that space.

Her situation is very common among owners of lightweight homes along the canals of Lam Luk Ka. These houses are well suited to the area in many ways, but they hide flood damage better than brick homes do, which means owners often discover the problem late.

How Knock-Down House Walls Are Built, and Where the Water Goes

Most knock-down and prefab homes use a lightweight wall system that is quite different from the brick-and-render walls of a conventional Thai house.

• Wall frame: usually steel box section or light steel profiles. Some are galvanized; others are painted with anti-rust primer.
• Outer skin: faux-wood siding planks or fiber cement sheets, screwed to the frame.
• Inner skin: fiber cement board or gypsum board, painted or wallpapered.
• The cavity in between: sometimes empty, sometimes filled with thermal insulation such as glass wool or foam boards. It is also where the electrical cables and sockets run.
• Floor: either a concrete slab on the ground, or a raised floor of steel joists covered with fiber cement floorboards or faux-wood decking.

Fiber cement and faux-wood products cope with water fairly well. They do not rot like real timber, which is why they look fine after a flood. But floodwater flows in through the joints at the bottom of the panels and settles inside the cavity and in the bottom track of the frame. Glass wool insulation soaks up water and slumps. Gypsum board on the inside wicks water well above the flood line. A dark, sealed cavity like this dries extremely slowly, and it is an excellent place for mold to grow.

There is also a design detail that works against owners after a flood. Lightweight walls are built to keep rain out from the outside, so the outer skin, joints and flashings are designed to shed water downward and away. When floodwater rises from below instead, it enters exactly where the wall is least protected, at the bottom track, and then has no natural way to drain back out. The bottom track of a steel frame can hold a shallow channel of dirty water for weeks, quietly rusting the screws and feeding mold on the back of the boards.

That is why a knock-down house can smell worse two or three weeks after a flood than it did on the first day home. The smell is not coming from what you cleaned. It is coming from what you could not reach.

Signs the Wall Cavity Is Still Wet and Moldy

1. A strong musty smell in closed rooms, especially near the lower walls and power sockets.
2. Black or green spots along the board joints and the skirting line.
3. Rust stains bleeding from screw heads, a sign that the steel frame or screws inside are staying damp.
4. Paint bubbling or flaking in thin sheets low on the wall.
5. Gypsum board on the inside feeling soft, denting when pressed, or showing water stains higher than the flood line.
6. For raised-floor houses: standing water or mud under the floor and a musty smell from the floorboards.

Any one of these on its own is worth investigating. Two or three together almost always mean the cavity needs to be opened.

The Risks That Follow

• Health. Mold spores from the cavity leak out through joints and socket holes, causing a blocked nose, sneezing, a scratchy throat, and flare-ups of allergies and asthma. Older people who spend the whole day at home are affected most. Floodwater trapped in the cavity may also carry germs from sewage and rat urine, which is linked to leptospirosis.
• Electrical safety. Cables and sockets low in the wall may have been underwater, creating a risk of current leakage and electric shock. A qualified electrician must check them before they are used.
• Structure. Steel frames and screws that stay damp for a long time rust, shortening the life of the house.
• Resale and rental value. A house with a musty smell and mold marks is hard to sell or let.

This article is not medical advice. If anyone develops a fever, calf pain, red eyes or other unusual symptoms after contact with floodwater, see a doctor and mention the flood exposure.

Do the Walls Need Opening? A Straightforward Way to Decide

Open the lower wall when water rose above the skirting and stood for several days, when the wall has insulation inside, or when there is a musty smell and mold along the joints. Builders usually remove one strip of inner boards along the bottom, or cut gypsum board a little above the flood line, so they can pull out the wet insulation, let the cavity dry and inspect the steel frame.

Fiber cement boards that are not cracked can often be removed, cleaned and refitted, depending on how they were installed and the house manufacturer's advice.

Gypsum board on the inner skin that was flooded should be cut out and replaced, because the gypsum core and its paper facing absorb water and feed mold.

Glass wool insulation in the cavity that got wet in the flood should be removed and replaced once the cavity is completely dry.

Water only touched the outside and never reached the indoor floor: cleaning the exterior and keeping an eye on things may be enough.

The best first step is to contact the company that made or installed your knock-down house. They know the wall system for that model and may have a way of taking panels off without damaging them.

Being clear about our role: World Health Disinfection (WHD) does not remove or install walls, insulation or house frames. That work should be done by the house manufacturer, a contractor, or a builder experienced with lightweight wall systems, and the electrical system must be checked by an electrician. WHD takes care of the stage when the walls are open: disinfection fogging inside the cavity, on the steel frame and the floor, followed by ozone treatment to reduce mold and musty odor before the builder closes the walls again.

Why Shortcuts Do Not Work

• Painting over the mold spots hides them only briefly, because the source is inside the cavity and the mold will push the paint up again.
• Running silicone along the board joints traps even more moisture inside the cavity.
• Running the air conditioner to dry the room helps the room air, but the sealed cavity stays just as damp.
• Spraying bleach into the socket holes is dangerous for the electrical system and does not reach the whole cavity.
• Charcoal bags or deodorizing sprays reduce the smell for a while without dealing with the source.

The Recommended Order of Work

1. Switch off the power and have an electrician inspect before using any sockets or switches low on the walls.
2. Take photos and contact the house manufacturer about how to remove panels and what the warranty covers.
3. The builder opens the lower walls, pulls out wet insulation, removes damaged gypsum and checks the frame and screws for rust.
4. Clear the mud and dry everything, opening the house for airflow and using fans and a dehumidifier.
5. WHD fogs and ozone-treats while the cavities are open.
6. The builder closes the walls, fitting new insulation, boards and paint once everything is confirmed dry.
7. Check the smell again, with a further ozone treatment if repair work has left an odor.

Questions to Ask the House Manufacturer or Builder

• "Which panels can be removed and refitted, and which must be cut?" Some systems use concealed fixings or interlocking siding that can be taken off cleanly; others cannot.
• "Is the frame galvanized or painted?" Galvanized steel tolerates short wetting better. Painted steel with scratched coating may need rust treatment before closing up.
• "What insulation is in the walls, and what should replace it?" If you live in a flood-prone spot, ask whether a less absorbent insulation is suitable for the lower part of the wall.
• "Can the bottom track be drained or raised next time?" Small design changes, such as a raised sill or drainage gaps, can reduce how much water sits in the cavity in a future flood.
• "Does the warranty cover any of this?" Flood damage is often excluded, but it is worth asking and keeping the answer in writing.

Knock-Down Homes in Lam Luk Ka

Lam Luk Ka has a lot of canal-side land, from Khlong 4 out to Khlong 8, and many families choose knock-down homes as retirement houses, a second house on the family plot, rental rooms or site offices, because they go up quickly and the budget is easy to control. Many of these homes, however, sit on a concrete slab at ground level close to a canal. When the water rises, every lower wall cavity floods at the same time, and water standing around the house keeps the moisture in for a long time. Restoration should therefore cover the walls, the floor and the space under the house, not just the surfaces you can see.

Raised-floor knock-down homes have an extra space to consider. The gap under the floor often holds mud and standing water for weeks after the flood, and the floor structure above it stays damp. It is worth having that space cleared and dried, and including it in the disinfection plan, before you judge whether the smell inside the rooms has really gone.

Living Arrangements While the Walls Are Open

Knock-down homes are often small, with only one or two bedrooms, so there is rarely a dry upstairs to retreat to. For older owners in particular, it is usually better to stay with relatives for the few days when the walls are open and the cavities are drying. Exposed insulation fibers, dust from cutting boards and mold spores released during removal are all things you do not want to breathe while sleeping.

Before work starts, move clothes, bedding and soft furnishings out of the rooms being opened, or seal them in large plastic boxes. Keep important documents and medicines somewhere dry and easy to reach. Agree with the builder on how the house will be secured each evening, since some wall sections may be partly open overnight. And plan your return around the ozone treatment: the house must be empty while the generator runs, and the team will tell you when ventilation is complete.

Getting Ready for the Next High-Water Season

Living beside a canal in Lam Luk Ka means planning for water, not just recovering from it. A few practical changes can make the next flood much less damaging for a lightweight house.

• Keep power sockets and the electrical board as high as practical, and ask an electrician whether low sockets can be moved up when the walls are open.
• Store spare boards, insulation offcuts and paint from this repair, so future patches match and can be done quickly.
• Keep a clear gap around the house so water drains away rather than pooling against the walls after the flood recedes.
• Raise the bed, wardrobe and appliances on sturdy stands if water entered the house this year.
• Note the date and level of this flood on the inside of a cupboard door. It is a simple record that helps you and any future buyer understand the house's history.

None of this changes the need to dry and disinfect properly after a flood, but it can shorten the work and lower the cost next time.

The Solution: Disinfection Fogging in the Wall Cavities and Whole-House Ozone Treatment With WHD

Once the builder has opened the lower walls, the WHD team can deal with areas that are normally unreachable. Our disinfection fogging service uses CHEMGENE HLD4H disinfectant delivered through an ULV fogger so the fine mist travels into the wall cavities, the bottom tracks of the steel frame and the space under raised floors. This is followed by whole-house ozone treatment to reduce mold spores and the musty odor absorbed by furniture, curtains and mattresses.

How the Service Works

1. Survey. We look at the water level, the wall system, insulation, mold and rust marks, and the condition under the floor.
2. Preparation advice. We coordinate with the builder or manufacturer on which wall lines to open and when.
3. Disinfection fogging. We fog CHEMGENE HLD4H into the open cavities, over the steel frame, the floor and high-touch surfaces.
4. Ozone shock treatment. We close the house and run the ozone generator for a time suited to its size. No people, pets or plants may be inside.
5. Ventilation. We open the house to air it and tell you when it is safe to return.
6. Re-check. We assess the odor after the walls are closed and give advice on ongoing moisture control.

Ozone safety. Treatment is only carried out when the house is empty. The team controls the dose, removes or covers sensitive electronics, artworks and natural rubber items, and the house is always ventilated before anyone goes back in.

10 Reasons Knock-Down Home Owners in Lam Luk Ka Choose WHD

1. We understand lightweight wall systems and know the odor source is inside the cavity.
2. We are upfront that opening and closing walls is work for the builder or manufacturer.
3. We work while the walls are open, when disinfectant can reach the source.
4. We use CHEMGENE HLD4H for surface disinfection.
5. ULV mist reaches steel frame tracks and the space under raised floors.
6. Whole-house ozone treatment reduces musty odor in bedrooms and furniture.
7. Controlled ozone dose, with care for electronics.
8. Well suited to older owners who spend all day at home.
9. Can be combined with our mosquito spraying service for canal-side land that still has standing water.
10. Practical advice on moisture control and raising belongings before the next high-water season.

Before and After

Customer Voice (Example Scenario)

"I thought walls like these didn't mind water. The team explained that the water was inside the wall and suggested I call the company that installed the house first. When the builder opened the wall, the insulation was a soggy lump. The team fogged and ozone-treated while the walls were still open. Once they were closed up again, the bedroom stopped smelling musty and my blocked nose cleared up."

— Auntie Sunantha, canal-side knock-down house, Lam Luk Ka (example scenario from the field)

Frequently Asked Questions

Q: Fiber cement board is water-resistant, so why is there a musty smell?
A: The board itself copes well with water, but floodwater gets trapped in the cavity, insulation and bottom frame track, which dry slowly and become a source of mold.

Q: Does the whole house need to be stripped?
A: Usually not. Builders normally open only the lower part of the walls that the water reached. Consult the house manufacturer or a builder experienced with lightweight wall systems.

Q: Does WHD remove or repair walls?
A: No. WHD provides disinfection fogging and ozone treatment. Wall, insulation and electrical work should be done by the relevant trades.

Q: Can you ozone-treat without opening the walls?
A: It can reduce the smell in the rooms, but if the insulation inside the cavity is still wet, the smell will return. We recommend opening and drying the walls first.

Q: Do we need to leave the house during ozone treatment?
A: Yes, along with pets and plants. The team will tell you when it is safe to return after ventilation.

Q: Do you serve Lam Luk Ka and nearby areas?
A: Yes. We serve Lam Luk Ka, Thanyaburi, Khlong Luang and other parts of Pathum Thani, Bangkok and the surrounding provinces. Contact us on LINE @whd268.

Walls Open? Let WHD Disinfect Before They Close

Book a survey of your knock-down home in Lam Luk Ka or nearby areas.

See Disinfection Service - Click Here

Book Ozone Treatment - Click Here

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

Related Links

• Residential ozone disinfection
• Ozone odor and chemical residue removal
• Mosquito spraying service
• All articles

Health reference: World Health Organization (WHO): Floods · Department of Health, Thailand