Most of the mold we are called out to look at did not start with a leak. It started with air.
That surprises people. They expect a burst pipe or a roof problem, and they go looking for one. There isn't one. The house is simply holding more moisture than the materials in it can tolerate, month after month, and eventually something grows.
This is the single most Louisiana problem there is, and it is worth understanding before you spend money on it.
The number that matters
| Indoor relative humidity | What happens |
|---|---|
| 30–50% | The target range. Comfortable, and materials stay dry |
| 50–60% | Tolerable, but you have no margin. A cool surface or a closed-off room will find trouble |
| Above 60% | Sustained, this grows mold on its own — no leak required |
| Above 70% | Expect visible growth on cool surfaces within weeks in warm weather |
Buy a hygrometer. They cost about fifteen dollars, they are sold in every hardware store, and until you own one every conversation about humidity in your house is guesswork — including ours.
Put one in the room that feels worst, not in the living room. Whole-house averages hide the problem. The closet on the north wall, the back bedroom nobody uses, the room over the garage — those are where the number is actually high, and where growth starts.
Why it is worse here than the number suggests
Two things stack up in this climate, and neither of them is about rain.
The air outside is loaded
For a good part of the year the outdoor dew point on the Gulf Coast sits in the seventies. That is the temperature at which the air has to give up its water. Any surface in your house that is cooler than that dew point — a supply vent, an exterior wall behind furniture, a slab floor under vinyl, the underside of a roof deck — is a place where water forms out of the air, without a drop of rain being involved.
So every gap that lets outside air in is not just an energy loss. It is a water delivery.
An oversized air conditioner makes humidity worse, not better
This is the one that costs people the most money, because the instinct is exactly backwards.
An air conditioner dehumidifies by running long enough for water to condense on the coil and drain away. Cooling is fast; drying is slow. A unit that is too large for the house hits the thermostat setting quickly, shuts off, and never runs long enough to pull water out. The house gets cold and clammy.
Then someone turns the thermostat down further to fight the clamminess, which makes the cycles shorter still, and the surfaces colder — which is precisely the condition for condensation.
Cold and damp is not an air conditioner that is too small. It is usually one that is too big, or one that is short-cycling. A house at 74 degrees and 45 percent humidity is more comfortable than one at 70 degrees and 65 percent, and it costs less to run.
What actually lowers it
In rough order of how much difference they make for the money.
1. Run the air conditioner so it dehumidifies
- Set the fan to AUTO, never ON. This one is free and it is the most common mistake in the house. With the fan running constantly, the water sitting on the coil gets blown back into the house between cooling cycles instead of draining outside. You are re-humidifying your own home on purpose.
- Raise the thermostat a degree or two so cycles run longer. Longer, gentler runs remove far more water than short cold blasts.
- Change the filter. A restricted filter reduces airflow across the coil and cuts how much water it removes.
- Check that the condensate line is actually draining. If the pan is holding water or the line is partly blocked, everything the coil pulls out of the air is sitting inside the unit evaporating back into the house. This also causes mold in the air vents.
2. Stop the air you are not accounting for
- Duct leaks. Return duct leaking in a hot attic pulls attic air into the system continuously. That is the single biggest hidden humidity load we see in a house with no plumbing problem.
- The attic hatch, recessed lights, the gap around plumbing penetrations, the top of interior walls. Individually small, collectively a chimney.
- A crawl space with an open vent and bare ground under it. Ground moisture rises into the floor system year round. Crawl space work is often what solves the moisture side of it in an older raised house.
3. Exhaust the moisture you make
- A shower puts a surprising amount of water into the air. Run the bathroom fan during the shower and for twenty minutes afterwards — a timer switch costs very little and removes the human factor. That is most of what causes mold on a bathroom ceiling.
- Vent the clothes dryer outside, and check that the duct is still connected. A disconnected dryer vent dumps warm, wet air into the house every load.
- Use the kitchen hood when you cook, and make sure it goes outside rather than recirculating.
4. Add a dehumidifier in the main living area
If the house cannot hold 50 percent or under on its own, put a dehumidifier in the main living area. One unit, central, working on the whole house. That is what we recommend to almost everybody, and it is what our founder runs in his own home and put in both of his children's houses.
Size it well past the square footage. A 1,500 square foot house wants a unit rated for around 5,000 square feet. The rating on the box assumes conditions that do not exist in South Louisiana, so a unit sized to the house runs constantly and never catches up. Plumb it to a drain line rather than emptying a bucket, and run it hard for about two weeks to pull the house down — after that it only works when it needs to.
A sealed or unreached space — a closed-off room, an enclosed crawl space, a garage that is part of the house — is the exception, and needs its own unit. That situation is far less common than people expect.
What a dehumidifier is not is a substitute for fixing the three things above. Running one against a leaking return duct is paying for electricity to lose an argument.
And a countertop HEPA purifier is not doing this job or any part of it. A real air scrubber is a different machine at a different scale.
Do dehumidifiers stop mold?
They stop the conditions. Kept below 50 percent, a space will not grow new mold. That is genuinely useful and it is why we recommend them.
They do not remove mold that is already there. Growth already established in drywall or wood does not die back and disappear because the room dried out — it goes dormant and waits. Drying is prevention, not remediation.
Things that do not work
- Closing the vents in unused rooms. Cuts airflow, unbalances the system, and creates a closed warm room with no conditioning — which is where growth starts.
- Buckets of calcium chloride crystals. They pull a small amount of water out of a very small space. Fine for a gun safe. They will not change a room.
- Houseplant advice, either direction. Plants add a little moisture. It is not your problem.
- Turning the thermostat way down. Covered above. It makes condensation more likely, not less.
- An air purifier. Useful for particles. It does nothing to humidity, and it does not address a mold source — it filters some of what the source releases.
When humidity has already become a mold problem
There is a point where lowering the humidity stops being the whole answer, because something is already growing and will keep releasing spores into the house until it is removed.
The signs that you are past that point:
- A musty smell with nothing visible. Smell is the most reliable early indicator there is, and it usually arrives well before anything shows.
- Growth in more than one room, which points at a whole-house humidity problem rather than a local one.
- Staining that comes back after cleaning. Bleach in particular makes this worse, because it is mostly water.
- Dark speckling on ceilings near supply vents, or on the exterior-wall side of a closet.
- Anything soft, cupping or discolored in the attic, where our hottest, wettest air ends up.
Fixing the humidity without removing what grew leaves you with a clean-smelling problem. Removing what grew without fixing the humidity means paying for the same job twice. Both halves, or neither is worth much.
What we would tell you at your kitchen table
Buy the hygrometer first. Put it in the worst room. Set the fan to AUTO. Look at whether the condensate line is draining.
That costs about fifteen dollars and an afternoon, and in a fair number of houses it is all the prevention the house needs. If the number comes down and stays down, you never needed us.
If it stays above 60 percent with the air conditioner running normally, something is bringing outside air in, and that is worth finding before it becomes a remediation bill. Our evaluations are free and a fair number of them end with us telling someone what to have their HVAC contractor look at.
Common questions
What should indoor humidity be in a South Louisiana house?
Between 30 and 50 percent. Above roughly 60 percent sustained, mold can grow on ordinary surfaces with no leak involved. In this climate the practical target through summer is closer to 45 to 50 percent, because outdoor dew points in the seventies mean any air that gets in arrives loaded with water.
Why is my house humid even with the AC running?
Usually one of four things: the fan is set to ON instead of AUTO, so water on the coil is blown back into the house between cycles; the unit is oversized and short-cycles, cooling quickly without running long enough to dehumidify; the condensate line or pan is not draining; or a return duct is leaking and pulling humid attic air into the system continuously.
Does a dehumidifier help with mold?
It prevents new growth by holding the house below about 50 percent humidity, which is why we recommend one in the main living area of almost any house that cannot hold that on its own. It does not remove mold that is already established in drywall or wood. Drying a space causes existing growth to go dormant rather than disappear, so a dehumidifier is prevention, not remediation.
Does turning the air conditioner colder lower humidity?
No, and it commonly makes things worse. A lower setting produces shorter cooling cycles, and short cycles remove less water because the coil never runs long enough to drain properly. It also makes surfaces colder, which is what condensation needs. A house at 74 degrees and 45 percent humidity is more comfortable than one at 70 degrees and 65 percent.
What humidity level causes mold?
Sustained relative humidity above roughly 60 percent will support mold growth on ordinary building materials, and above 70 percent visible growth can appear within weeks in warm weather. The relevant number is at the surface rather than in the middle of the room, which is why cool exterior walls, closets and rooms with poor airflow grow mold while the rest of the house seems fine.
Can high humidity alone cause mold, with no leak?
Yes, and in South Louisiana it is the most common cause we see. Outdoor dew points in the seventies mean any surface cooler than that collects water out of the air. Leaking ducts, an open crawl space, gaps at the attic hatch and unvented bathrooms all deliver that air into the house continuously, with no plumbing failure anywhere.
Should I run a dehumidifier and the AC at the same time?
Yes. If the house is having trouble holding 50 percent or under, put a dehumidifier in the main living area and run it alongside the AC. They are not working against each other. A sealed room or an enclosed crawl space is the exception and needs its own unit. What is not worth doing is running a dehumidifier to compensate for a leaking return duct or a fan left on ON, because the moisture will keep arriving faster than the machine removes it.