There is no shortage of mold prevention advice online and most of it is the same eight bullet points, written by people who have never opened a wall.
This is the version from the job side: the places we actually find it, in the order we actually find it, and what would have stopped it.
The single highest-value thing on this page is the 48-hour rule, further down. Nearly every large remediation bill we write started as a small amount of water that sat over a weekend.
The 48-hour rule
When water gets into a house — a supply line, an overflowing washer, a roof leak in a storm, water pushed under a door — you have roughly 24 to 48 hours before mold begins to establish on wet materials in warm weather.
That window is the difference between a drying job and a remediation job. It is also the difference between a few hundred dollars and several thousand.
What to do in that window
- Stop the water. Obvious, and people skip it while they deal with the mess.
- Get standing water out immediately — wet vac, mop, whatever moves it fastest.
- Pull up what traps water. Rugs, and pad under carpet especially. Pad holds water against the floor for days and looks dry on top.
- Get air moving across wet surfaces — fans on the wet materials, not pointed at the room in general.
- Run the air conditioning, or a dehumidifier, so the water you evaporate leaves the house instead of moving into the walls.
- Open what you can. Cabinet toe-kicks, the space under a vanity, closet doors. Trapped pockets dry last and grow first.
- Photograph everything before you move it, in case an insurance claim follows.
Where the 48-hour rule quietly fails
Wet insulation inside a wall does not dry from the outside. Neither does the underside of a subfloor, or the back of a base cabinet. A room can feel completely dry while the cavity behind the baseboard is still wet a week later.
If water went into something rather than sitting on top of it, drying the surface is not the same as drying the material. That is the situation where a fan and a hopeful weekend turns into a wall coming out later.
Room by room
In the order these show up on our schedule.
Bathroom
- Run the exhaust fan during the shower and 20 minutes after. A timer switch removes the human factor and costs very little.
- Check the fan actually vents outside, not into the attic. Venting a bathroom into an attic is common in older work here and it creates an attic problem instead of a bathroom one.
- Leave the door open afterwards if there is no fan. Closed door plus wet room is the reliable recipe for ceiling growth.
- Re-caulk where tile meets tub or floor when it cracks. That gap sends water behind the surface, where you will not see it until the wall is soft.
- Hang the bath mat to dry rather than leaving it flat on the floor.
Under sinks and behind appliances
- Look under every sink twice a year. Two minutes with a flashlight. A supply line weeping a drop an hour will not reach the floor — it soaks the cabinet bottom, and the first sign is a smell or a soft base months later.
- Replace rubber washing machine hoses with braided steel. Rubber hoses fail, and they fail while you are out.
- Check the dishwasher and refrigerator lines. Ice maker lines are a frequent quiet leak.
- Do not store things tightly under a sink. You want to be able to see the deck.
Air conditioning and vents
- Fan on AUTO, not ON. The most common mistake in a Louisiana house. Why that one setting matters so much here.
- Change the filter on schedule — restricted airflow means less water removed.
- Use a MERV 4 or MERV 5 filter. This is the opposite of what most people have been told, and it matters here. A high-MERV filter restricts airflow, and an air conditioner that cannot move air cannot pull humidity out of the house. In this climate airflow beats filtration — you want the system drying the house, not straining through a dense filter.
- Make sure the condensate line is draining. A blocked line puts water back into the house and into the unit.
- If you see dark speckling around supply vents, that is a wet system rather than dirty ducts — see mold in air vents.
- Have the ducts checked for leaks if part of the house is always humid. The mechanics of all of this are in how to lower the humidity in your house.
Attic
- Look at the underside of the roof deck twice a year, in summer. Dark staining on the sheathing is the early sign.
- Confirm every bathroom fan and the dryer vent terminate outside, not into the attic space.
- Keep insulation clear of the soffit vents so the attic can actually breathe.
- Look for daylight and for staining around the chimney, the vent boots and the valleys after heavy weather.
- What grows up there does not stay up there — see attic mold.
Crawl space
- Ground moisture rises into the floor system year round. A vapor barrier over the dirt is the single biggest improvement in most raised houses.
- Keep the grade sloping away from the house, and keep gutters discharging away from the foundation rather than beside it.
- Look for standing water after heavy rain, and at whether it drains within a day.
- Crawl space work is often what stops it coming back when floors smell musty and nothing else explains it. It is worth being clear that sealing a crawl space is the prevention side — if growth is already established, the subfloor above it frequently has to be replaced, and no amount of moisture control changes that.
Closets, exterior walls and unused rooms
- Leave a gap between furniture and exterior walls so air can move behind it.
- Do not pack closets on exterior walls solid — that is a cool surface with no airflow.
- Keep vents open in rooms you do not use. Closing them creates warm unconditioned pockets.
- Put clothes away dry. Damp laundry in a closed closet is a small greenhouse.
Laundry and garage
- Vent the dryer outside, and check the duct is still connected. A disconnected dryer vent puts warm wet air into the house every load.
- Leave the washer door open between loads on a front loader.
- Watch cardboard boxes on a garage or slab floor — cardboard wicks slab moisture and grows on the underside.
Before and after a storm
Storm prevention is a different job from ordinary prevention, and the mistakes are predictable.
Before
- Clear gutters and check downspouts discharge away from the house.
- Move anything valuable off the floor, particularly in a garage or a room that has taken water before.
- Know where the water shutoff is.
After
- The 48-hour clock starts when the water arrives, not when the power comes back. This is the reason post-storm mold is so widespread here — houses sit wet and closed up, in heat, for days.
- If it is safe and outdoor humidity is lower than indoor, open up and move air through. If it is a hot wet day, closing up and running the air conditioning does more.
- If there is that much water, call a water mitigation company now. Not a mold company, and not next week. They carry the drying equipment, they usually show up within hours, and the 48-hour clock is the whole game. We are not the right call at this stage and we will tell you so.
- Do not start pulling up pad or cutting walls open yourself. Wet material has to come out, but doing it without containment or the equipment to dry what is left behind generally makes the job bigger, not smaller.
- Photograph everything for the claim before anything is removed.
A word about "mold-resistant" products
Mold-resistant drywall, mold-inhibiting paint additives and treated lumber all do what they say: they resist growth on the product itself. They are worth using in a bathroom rebuild.
They are not a moisture strategy. Mold does not need the drywall — it will grow on the dust, the paper, the paint film and whatever else is on the surface. A wet mold-resistant wall still grows mold. Anyone selling one as a substitute for controlling water is selling you the wrong thing.
The twice-a-year walk
Fifteen minutes, spring and late summer, with a flashlight:
- Under every sink, including the far corners of the cabinet deck
- Around the base of every toilet — a rocking toilet leaks slowly into the subfloor
- Behind and under the washing machine and dishwasher
- The water heater pan and the line from it
- The attic, at the roof deck and around every penetration
- The crawl space, for standing water and for a vapor barrier that has been pulled aside
- Window sills and the wall below them, for staining
- The hygrometer reading in the worst room of the house
Nearly everything we get called out for would have been visible on one of those walks, months earlier, as something small.
What prevention cannot do
If a house already smells musty, or growth keeps returning in the same spot after cleaning, prevention is no longer the right tool. Something is already established and it will keep releasing spores until it is removed.
- A musty smell with nothing visible is the most reliable early signal there is.
- Growth returning after cleaning means the water source was never found.
- Bleach makes this worse, because it is mostly water and it only removes the color.
At that point the question is what it takes to remove it — what mold removal costs covers the real ranges — and, more importantly, where the water is coming from. Removing mold without fixing that means paying for the same job twice.
Common questions
How do you prevent mold in a house?
Control moisture. Keep indoor humidity between 30 and 50 percent, exhaust bathrooms and the dryer outside, check under sinks and behind appliances twice a year, keep the attic ventilated and the crawl space covered, and dry any water intrusion completely within 24 to 48 hours. No spray, paint or additive substitutes for those, because mold only grows where a material stays wet.
How long does it take for mold to grow after water damage?
Generally 24 to 48 hours on wet materials in warm conditions, which is why the response window matters so much. Materials that hold water internally — carpet pad, insulation inside a wall, the underside of a subfloor, the back of a cabinet — take far longer to dry than the surface suggests, and are where growth commonly establishes while the room appears dry.
What humidity level prevents mold?
Below 50 percent relative humidity, measured in the worst room rather than the middle of the house. Sustained readings above roughly 60 percent will support growth on ordinary building materials with no leak involved, which in South Louisiana is the most common cause we see.
Does mold-resistant drywall stop mold?
It resists growth on the product itself and is worth using in a bathroom rebuild, but it is not a moisture strategy. Mold will grow on dust, paper facing, the paint film and other surface material regardless, so a wet mold-resistant wall still grows mold. It reduces one risk; it does not replace controlling water.
Should I run a dehumidifier to prevent mold?
Yes. If the house cannot hold 50 percent or under on its own, put one in the main living area, sized well above the square footage and plumbed to a drain. A sealed room or an enclosed crawl space is the exception and needs its own unit. Held below about 50 percent, a space will not grow new mold. It is not a substitute for fixing a leaking duct, a blocked condensate line or a fan left in the ON position, because moisture will arrive faster than the machine removes it.
How do I prevent mold after a storm or flood?
Treat the clock as starting when the water arrived, not when the power returned. If there is standing water or soaked material, call a water mitigation company — they have the drying equipment and usually respond within hours, and that speed is what decides whether this becomes a mold job. Get air moving and run air conditioning or a dehumidifier so evaporated water leaves the house. Photograph everything before anything is removed, for the claim.
Can you prevent mold in a Louisiana house at all?
Yes, but not by keeping spores out — they are in every building and always will be. Prevention here means denying them sustained moisture: humidity under 50 percent, water intrusion dried within 48 hours, and the handful of quiet leak points checked twice a year. The climate makes it harder than in a dry state, not impossible.