Humidity, Mold, and Your AC: Why "Cool" Isn't the Same as "Comfortable"

If you’ve ever walked into your house in July, seen the thermostat reading 74°F, and still felt sticky, you’re not imagining it. In Jacksonville, temperature is only half the comfort equation. The other half is humidity, and it’s the half most homeowners (and plenty of AC installers) overlook.

High indoor humidity doesn’t just feel bad. It leads to musty smells, mold on vents and ceilings, warped wood floors and trim, and dust mites that aggravate allergies. The good news: most humidity problems have a clear cause and a practical fix.

Your AC Is Also a Dehumidifier

Air conditioners remove heat and moisture. As warm, humid air passes over the cold indoor coil, water condenses out of it, just like on a glass of iced tea, and drains away through your condensate line.

There are two jobs happening at once:

• Sensible cooling lowers the temperature you see on the thermostat.
• Latent cooling removes moisture from the air.

The catch is that dehumidification takes time. The coil has to get cold, stay cold, and keep air moving across it long enough for water to condense and drain. Anything that shortens that process leaves moisture behind in your home.

For most homes, indoor relative humidity between 40% and 50% is the comfort sweet spot. Consistently above 60% is where mold, mildew, and dust mites start to thrive.

Problem #1: An Oversized AC That Short-Cycles

It seems logical that a bigger AC would do a better job. In Florida, the opposite is often true.

An oversized system cools the house down to the thermostat setting very quickly, then shuts off. These short run cycles, often just a few minutes, aren’t long enough to pull much moisture out of the air. The result is a house that hits the right temperature but feels damp and clammy.

Signs your system may be oversized:

• The AC turns on and off frequently, especially on mild days
• The house feels cold but sticky
• You keep lowering the thermostat to feel comfortable
• Indoor humidity readings regularly sit above 55–60%

Oversizing is common. Many systems are sized using a rough “square feet per ton” rule of thumb, or replaced with a bigger unit “just to be safe.” The right way is a Manual J load calculation, which accounts for your home’s insulation, windows, orientation, ductwork, and Jacksonville’s climate.

How it’s fixed:

• When it’s time to replace, size the new system properly instead of matching (or upsizing) the old one.
• Consider a two-stage or variable-speed system. These run at lower capacity for longer stretches, which is exactly what dehumidification needs.
• If replacement isn’t in the cards yet, a whole-home dehumidifier can make up the difference.

Problem #2: The Thermostat Fan Is Set to “On”

This one surprises a lot of homeowners. Most thermostats have a fan setting with two options:

• Auto: The blower runs only when the AC is actively cooling.
• On: The blower runs all the time, even when the AC compressor is off.

“On” sounds like it would keep air fresh and circulating. But in a humid climate, it works against you. When the compressor shuts off, the indoor coil is still wet. With the fan still running, that moisture gets blown right back into your house and re-evaporates. You’ve essentially paid to remove water from the air, then put it back.

Our recommendation for Jacksonville homes: set the fan to “Auto.” It’s one of the fastest, free fixes for a clammy house.

Some modern thermostats and systems also offer a dehumidify mode or “overcool to dehumidify” setting, and variable-speed blowers can slow airflow to wring out more moisture. If your system has these features, it’s worth having them configured properly.

Problem #3: Moisture Sneaking In

Sometimes the AC is fine, but humidity is getting into the house faster than the system can remove it. Common culprits in Northeast Florida homes:

• Leaky return ducts in the attic or crawlspace. A leak on the return side pulls in hot, humid attic air directly into your system. Attics here routinely exceed 130°F in summer.
• Air leaks around doors, windows, recessed lights, and attic access hatches.
• Bathroom and kitchen exhaust fans that don’t work, aren’t used, or vent into the attic instead of outside.
• Clothes dryers with disconnected or crushed vents.
• Crawlspaces with standing water or no vapor barrier, especially in older homes in neighborhoods like Riverside, Avondale, and Murray Hill.

The Fix That Works When Nothing Else Does: Whole-Home Dehumidifiers

A whole-home dehumidifier is installed alongside your HVAC system and connects to your ductwork. It removes moisture from the air independently of your AC, controlled by its own humidity setting.

It’s a strong option if:

• You’ve corrected thermostat settings and the house is still humid
• Your AC is properly sized but humidity spikes during mild, rainy weather (spring and fall in Jacksonville are notorious for this, when it’s not hot enough for the AC to run much)
• You have a well-sealed, energy-efficient home that holds in moisture
• Someone in the home has allergies or asthma
• You’ve had recurring mold or mildew issues

Unlike portable dehumidifiers, a whole-home unit treats every room, drains automatically, and doesn’t need to be emptied. It can also let you set the thermostat a couple of degrees higher while feeling just as comfortable, since dry air feels cooler. That can help offset rising energy costs.

The Musty Smell Problem

A musty or “dirty sock” smell when the AC kicks on almost always means something is growing in or around your system. Common sources:

• Mold or bacteria on the evaporator coil. The coil is cold and wet for much of the year, a perfect environment for growth.
• A clogged or slow condensate drain. Algae and sludge in the drain line cause water to back up into the drain pan, where it sits and grows. Left alone, it can overflow and cause water damage to ceilings and floors.
• Dirty or damp ductwork, especially where ducts are leaking or sweating in the attic.
• Dirty air filters that trap moisture and debris.
• Mold growing elsewhere in the house that the AC is circulating.

What you can do:

• Change your air filter on schedule (every 1–3 months, depending on the filter and household).
• If your system has an accessible condensate drain line, flush it periodically per your technician’s recommendation.
• Check for water in or around the indoor unit and auxiliary drain pan.
• Watch for mold on supply vent covers, which is often a sign of high humidity or condensation at the vent.

When to call a professional: If the smell persists after a filter change, if you see visible mold, if there’s water around the air handler, or if anyone in the home is experiencing worsening allergy or respiratory symptoms. A technician can clean the coil, clear the drain, inspect the ductwork, and identify the underlying humidity cause, so the problem doesn’t just come back.

A Simple First Step: Measure Your Humidity

You can’t fix what you don’t measure. An inexpensive digital hygrometer from any hardware store will tell you your indoor humidity. Many smart thermostats display it too.

Check readings at different times of day over a week or two. If you’re regularly above 55%, especially with the AC running, it’s time to dig into the cause.

Quick Checklist for a Drier, More Comfortable Home

• Set your thermostat fan to Auto, not On
• Measure indoor humidity and aim for 40–50%
• Change air filters on schedule
• Keep condensate drains clear
• Use bathroom and kitchen exhaust fans, and make sure they vent outdoors
• Have ductwork inspected for leaks, especially on the return side
• If you’re replacing your AC, insist on a proper load calculation
• Consider a whole-home dehumidifier if humidity stays high

How Copper & Co. Can Help

Copper & Co. helps Jacksonville homeowners solve humidity and air quality problems at the source, with:

• Humidity and comfort assessments
• Properly sized AC and heat pump replacements, including two-stage and variable-speed systems
• Whole-home dehumidifier installation
• Coil cleaning and condensate drain service
• Duct inspection, sealing, and repair
• Thermostat setup and optimization

Related reading: How to Cut Your A/C Costs in Jacksonville as JEA Rates Go Up and What’s Wrong With My AC? Common Air Conditioning Problems and How They’re Fixed