In short. Indonesian Health Ministry regulation No. 1077/2011 sets for housing a humidity of 40-60 percent, a temperature of 18-30 degrees, ventilation of at least 10 percent of the floor area and not a single mould spore in the air. In Bali the humidity is 81-85 percent all year round, so mould here is a feature of the climate, and it has to be fought by drying: air conditioning in dry mode, an extractor fan venting outside, light and a draught, fix roof and pipe leaks at once, and dry anything flooded within 24-48 hours. Black spots on walls are washed off with soap and water or a bleach solution no stronger than a cup per four litres, in gloves and with the windows open; do not mix bleach with other products. Soaked mattresses and rugs that cannot be dried at once are thrown away. Water standing in saucers, barrels and gutters means mosquitoes: empty it once a week.
Where the damp comes from. By the BMKG table for 2023, air humidity at the Ngurah Rai station stayed between 81 and 85 percent in all twelve months, more in the guide to weather by month. Health Ministry regulation No. 1077/2011 names a different norm for living rooms: humidity of 40-60 percent, a temperature of 18-30 degrees, lighting of at least 60 lux, an air exchange speed of 0.15-0.25 metres per second and ventilation openings of at least 10 percent of the floor area. By the same regulation there must be no mould in the home's air at all: the norm is 0 colonies per cubic metre. The risk factors are listed directly in the regulation: a leaking roof, a floor and walls without waterproofing, too little light. A Bali villa collects them all: an open plan without glass, a bathroom under the sky, concrete walls that draw water from the ground, and an air conditioner switched on at night while the rooms stand closed by day. The smell of damp appears before the spots: if the wardrobe smells like a cellar, mould is already growing.
How to dry the house. For humidity above 60 percent, the Health Ministry regulation names two means: an air dehumidifier and light, even glass roof tiles. The CDC adds a figure and an order: keep the humidity in the home no higher than 50 percent all day, with air conditioning and a dehumidifier helping; the extractor fans in the kitchen and bathroom must vent air outside; fix leaks in the roof, walls and pipes at once; after a flood the house is washed and dried completely within 24-48 hours. In practice it looks like this. Buy a hygrometer, it costs about a lunch in a warung, and hang it where mould has already been: without a number you argue with the owner blind. Switch the air conditioner to dry mode (the droplet on the remote): it dries the air without strong cooling, and for an hour or two a day in a closed room that is enough; count the electricity use on the meter using the guide to PLN tokens. In the morning open everything wide for an hour, when the sun is already up and the rain is not yet. Keep wardrobe doors ajar, do not hang clothes tight together, dry laundry outside under a roof, the bedroom is not suitable for it. In the bathroom after a shower switch on the extractor or leave the door open: a closed wet bathroom is an incubator. If the house has a room without windows, put a dehumidifier or at least a fan there: still air is what mould likes most. Before the rainy season check the roof, gutters and drains using the guide to checking a villa: water pouring down the outside of a wall will be inside in a month.
What to clean mould with. The CDC rules are simple. Mould is removed as soon as it is seen or smelled, you do not need to identify the kind, and the CDC does not recommend a mould test: there are many kinds, there are no norms, and you do the same thing anyway. The main condition: along with cleaning, remove the source of moisture, otherwise the spots return within a week. Hard surfaces, that is tiles, painted concrete, plastic and glass, are washed with household products, soap and water or a bleach solution no stronger than one cup per four litres of water. In a shop look for bleach under the word pemutih. Bleach must not be mixed with ammonia and other cleaning products: a poisonous gas is released. Open windows and doors, work in rubber gloves, goggles and closed shoes, and follow the instructions on the bottle. Porous things in which mould lives inside are listed by the CDC: paper, cardboard, wood, plasterboard, fabric, rugs, upholstery. Mattresses, rugs and upholstery that got soaked and did not dry at once are thrown out: they cannot be cleaned from the inside. Ceiling panels and plasterboard suspended ceilings after a leak are usually replaced too, which is a conversation with the owner.
Clothes, shoes, electronics. This is already our advice, not a norm. Keep clothes you do not wear every week in closed plastic boxes with silica gel or charcoal bags, and change the bags once a month. Wipe leather shoes and bags and keep them in the light: in a dark wardrobe a white bloom appears on leather after two weeks. Keep books and documents off the floor and away from an outer wall. Put your passport and immigration papers in a zip bag: damp spoils the pages, and a spoiled passport at the border is a separate story, covered in the guide to replacing a passport. Keep electronics in a room with air conditioning, and cameras and lenses in a dry box: fungus on a lens cannot be rubbed off. Do not leave a phone and laptop overnight in an open gazebo, as the morning mist settles inside.
Health. By the CDC page, mould causes nothing in some people, while in others it gives a stuffy nose, a sore throat, a cough or wheezing, burning eyes and a rash. It is harder on people with asthma and mould allergy, chronic lung disease and weakened immunity: lung infections are possible in them. In some children, early contact with mould is linked to the development of asthma. The CDC cites a 2004 Institute of Medicine review: a link has been proven between damp indoor spaces and respiratory symptoms, asthma flare-ups and hypersensitivity pneumonitis in susceptible people. If the house smells of damp and a cough and runny nose do not go away, first dry the house, then see a doctor: the list of hospitals with round-the-clock reception is in the guide to hospitals.
Mosquitoes, rats and other neighbours. The rainy season is the dengue season, and the most dangerous thing in a damp house is standing water, mould is minor next to it. The Health Ministry's "3M plus" rule from the guide to dengue: once a week drain and clean everything that holds water, cover containers, remove the unneeded ones. At a villa these are saucers under pots, barrels of rainwater, clogged gutters, a bucket in the bathroom, the tray under the air conditioner and even a bottle cap in the garden. Water after a flood and puddles where rats run mean leptospirosis: how not to catch it is in the guide to leptospirosis. Termites, rayap in Indonesian, give themselves away by clay tunnels on walls and doors that sound hollow: this is the owner's problem, report it at once before the beams are eaten. Ants and cockroaches come to food and drains: keep food in closed jars and plug drains at night. Gecko lizards (cicak) are harmless and eat mosquitoes, so leave them alone.
What to tell the owner. A leaking roof, blocked drains, damp walls and a broken extractor fan are the condition of the house, and under an ordinary rental contract the owner fixes them: what should be in the contract is in the guide to monthly rental. Write with photos and the figure from the hygrometer, in Indonesian: atap bocor (the roof leaks), dinding lembap (the wall is damp), ada jamur di kamar (there is mould in the room), talang tersumbat (the gutter is blocked), exhaust rusak (the extractor is broken), tolong diperbaiki (please fix it). Painting over spots after a fixed leak without treatment is useless, which is also worth mentioning: the CDC advises adding an anti-mould product to paint and washing bathrooms with products that kill it.
Our advice. A hygrometer in the bedroom and in the wardrobe, air conditioning in dry mode for an hour a day with the doors closed, windows wide open for an hour in the morning, laundry dried outside, boxes with silica gel for things that lie unused, and a weekly round of saucers and gutters with a bucket. That is enough for the rainy season to pass without black walls and without a hospital.
What this means for you. Mould in Bali is a feature of the climate, the house is not to blame: humidity of 81-85 percent outside against 40-60 under the Health Ministry norm is closed only by drying. Dry the air, fix leaks at once, throw out what got soaked and wash spots off with soap or a weak bleach solution in gloves. And do not forget the water in saucers: dengue in the rainy season is scarier than any mould.