The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. That is the number every article gives you, and it is worth aiming for. However, it is not the only number that decides whether mold grows in your house.
Mold grows based on the moisture at a surface, not the average of the air in the room. A hygrometer in your hallway can read a comfortable 48 percent while the wall behind a closet on the shaded side of the house sits well above 80.
Elite Mold Services has been testing Central Florida homes since 2006, and that gap is where most of what we find lives.
The Numbers Worth Knowing
Three organizations publish indoor humidity guidance, and they do not perfectly agree. The differences are small but worth understanding.
| Source | Guidance |
|---|---|
| EPA | Below 60%, ideally 30% to 50% |
| CDC | No higher than 50%, all day |
| ASHRAE 62.1 | Not above 65%, and dew point not above 60°F |
The dew point ceiling in that last row is the one Florida homeowners should pay attention to. Relative humidity changes with temperature, so the same air reads differently depending on where you measure it.
Dew point does not move. When the outdoor dew point sits in the mid-70s for months, which it does here from June through September, every cool surface in your house is a candidate for condensation.
A $10 hygrometer is still worth owning. Just understand what it is telling you, which is the condition of the air in one room, at one height, at one moment. If you self-check and get an elevated reading, then it’s time to involve professionals ASAP.
Why a Good Reading Can Still Mean Trouble
This is the part almost no article covers, and it is the reason humidity problems get missed.
Relative humidity is relative to temperature. Warm air holds more moisture, so the same amount of water vapor produces a lower reading in a warm room and a higher one at a cold surface. Building science research puts real numbers on it: air sitting at 70°F and 40 percent relative humidity in the middle of a room reaches 70 percent relative humidity at any surface cooler than 54°F.
That is a thirty-point swing from a sixteen-degree temperature difference. Nothing about the air changed. Only the surface did.
In a Central Florida home, the cold surfaces are predictable:
- Exterior wall corners in closets, where air does not circulate
- Wall cavities behind furniture pushed flat against an outside wall
- Supply registers and duct boots, especially where a diffuser blows directly onto a wall
- Ductwork running through a 95-degree attic
- Slab edges and the base of exterior walls
- Toilet tanks and cold water lines
ASHRAE’s moisture standard sets the mold criterion at a 30-day running average surface relative humidity below 80 percent. Note the word surface. The standard that actually governs mold risk is measured at the material, not in the air.
That is why a whole house can pass a hygrometer check and still be growing something behind a baseboard.
Where the Water Is Coming From
Before you buy equipment, it helps to know how much moisture is entering and from where. Ordinary living generates a surprising amount.
Extension research puts everyday sources in perspective:
- Each occupant releases about three pints of water a day just by breathing
- A ten-minute shower adds roughly one pint
- Building materials in a house release six to sixteen pints a day as they dry
Those are real, and exhaust fans handle them. Then there is the category that dwarfs all of it. Moisture moving up through a concrete slab or a damp crawl space can introduce as much as 100 pints a day, which is more than every occupant activity in the house combined.
This is why the order of operations matters. A dehumidifier running against an unsealed moisture source is treating a symptom, and it can hide the problem long enough for damage to develop out of sight.
Why the AC Is Not Fixing It
Plenty of Central Florida homeowners set the thermostat to 74, feel clammy anyway, and assume the unit is failing. Usually it is working exactly as designed, which is the problem.
An air conditioner removes humidity as a side effect of cooling. Warm air passes over a cold coil, water condenses on it, and that water drains away. The key word is drains. It takes runtime.
The Department of Energy explains the failure directly. An oversized system “will quickly meet the sensible cooling requirements (temperature) and will not run for a significant enough time to allow the condensate to drain off of the evaporator coil.” Worse, “in many cases, the condensate on the evaporator coil will evaporate back into the airstream as the fan continues for a few minutes at the end of the cooling cycle.”
Translated: the unit wins the temperature race, shuts off early, and the water it just pulled out of your air goes right back into the house.
Two related tells are easy to check yourself. If your thermostat fan is set to ON rather than AUTO, the blower keeps running between cycles and re-evaporates coil water continuously.
And if the humidity feels worst in October, November, March, and April rather than in August, that is the signature of low cooling demand with high moisture load, when the AC barely runs at all.
What Actually Lowers Humidity
Roughly in order of how much difference each one makes.
- Find and fix bulk water first. Roof leaks, plumbing leaks, grading that slopes toward the house, sprinklers hitting the wall. Liquid water beats vapor by orders of magnitude, and something as ordinary as clogged gutters dumping water at the foundation can feed the problem year-round.
- Get AC runtime right. A correctly sized system that runs longer at lower capacity removes far more moisture than an oversized one. Your HVAC contractor can evaluate sizing and staging. Set the fan to AUTO.
- Consider a whole-house dehumidifier. It is the only equipment that removes moisture independent of whether the house needs cooling, which is exactly the shoulder season problem above. Again, an HVAC question.
- Seal the ducts. A leaking return in a hot attic pulls humid attic air straight into the system and distributes it through the house.
- Vent exhaust outdoors. Bath fans, range hood, and dryer, all terminating outside rather than into the attic. Run bath fans during the shower and for twenty minutes after.
- Address the attic and any crawl space. Encapsulation, insulation, and duct placement all change how much moisture the living space has to fight.
Notice that only two of those six involve buying a machine. Equipment is the last step, not the first, and the reason is simple: a dehumidifier lowers the number on your hygrometer whether or not you found the source.
Related Questions to Explore
These come up on most calls.
What humidity level causes mold? There is no single switch. Sustained surface relative humidity above 80 percent is the threshold in ASHRAE’s standard, and room readings above 60 percent make that far more likely.
Is 65 percent too high in a house? For Florida, yes. That is at the outer edge of the ASHRAE limit with no margin, and it means cold surfaces are already well past it.
Does a dehumidifier prevent mold? It helps, but only if the moisture source is also addressed. A dehumidifier holding the room at 45 percent does nothing for a wall cavity being fed by a slow leak.
Can humidifiers cause mold? Yes, when overused or poorly maintained. We have written more about using them safely.
Why is my house humid in the fall? Cooling demand drops before moisture does. The AC runs less, so it dehumidifies less, while outdoor dew points stay high. This is Central Florida’s real mold season.
Do I need testing if I do not see or smell anything? Not always. Testing earns its keep when there is a reason to suspect hidden moisture, such as a past leak, a musty smell with no visible source, or a persistent humidity problem you cannot explain.
When to Call a Professional
A hygrometer tells you about the air. Finding out what is happening at a surface takes different tools.
An assessment measures moisture in materials with a meter, uses thermal imaging to find cold spots and wet areas behind finishes, takes surface temperatures to calculate the actual surface relative humidity, and samples air or surfaces when there is reason to. That combination answers the question a room reading cannot.
Worth being clear about what we do and do not do. Elite Mold Services performs inspection, testing, and assessment. We do not perform remediation, and that is deliberate. When the company telling you what is wrong is not the company selling you the fix, the finding is easier to trust. For equipment questions, your HVAC contractor is the right call.
Call when a humidity problem persists after the obvious fixes, when you have had a leak or storm intrusion, when there is a musty odor without a visible source, or when someone in the house has symptoms that ease when they leave. Our indoor air quality testing and mold inspections cover Orange, Seminole, Brevard, Lake, Osceola, Volusia, and the surrounding counties.
For the underlying science, the EPA’s guidance on why and where mold grows is the clearest free resource available, and the Department of Energy’s explanation of right-sizing covers the AC side.
Conclusion
Humidity control in Florida is less about hitting a number on a display and more about finding the places where that number is wrong.
- Aim for under 60 percent in the room, but remember mold responds to surface humidity, not room humidity.
- Fix water sources before buying equipment. A dehumidifier can mask a problem it is not solving.
- If the house feels worst in spring and fall, suspect AC runtime rather than AC capacity.
If your humidity will not come down, or it has come down and you still suspect something behind a wall, schedule an inspection and get an actual measurement.