Upstate South Carolina summers are humid — that's just the reality. But if your home feels muggy even with the AC running, something is off. Comfortable indoor humidity is typically 40–55%. Above 60%, you'll feel it — and above 70%, you're creating conditions for mold growth.
Your AC Is Oversized
This is the most common cause of high indoor humidity that we see. An oversized AC system cools the air quickly and shuts off — before it has time to remove adequate moisture. Air conditioning removes humidity as a byproduct of cooling: warm, humid air passes over the cold evaporator coil, moisture condenses on the coil, and drains away.
A system that short-cycles (runs for 5–10 minutes, shuts off, runs again) doesn't run long enough to dehumidify effectively. The result: a home that reaches the set temperature but still feels clammy.
If your system was recently replaced and humidity got worse, oversizing is a likely cause. Proper sizing requires a Manual J load calculation — not just matching the old system's tonnage.
Duct Leaks
Leaky ducts in unconditioned spaces (attics, crawlspaces) pull in hot, humid outside air and distribute it through your home. This is particularly common in older homes with flex duct that has separated at connections.
Duct leaks also reduce system efficiency — you're paying to condition air that escapes before it reaches the living space. A duct pressure test can quantify the leakage, and sealing the ducts addresses both the humidity and efficiency problems.
Inadequate Ventilation Control
Homes need some fresh air exchange, but uncontrolled infiltration — air leaking in through gaps in the building envelope — brings in outdoor humidity. In Upstate SC summers, outdoor humidity is often 70–90%.
Common infiltration points include attic hatches, recessed lights, gaps around plumbing penetrations, and crawlspace vents. Air sealing these areas reduces the humidity load on your AC system.
The AC System Isn't Removing Enough Moisture
Even a properly sized system can struggle with humidity if the evaporator coil is dirty, the refrigerant charge is off, or the blower speed is too high. A higher blower speed moves air across the coil faster, which reduces the contact time and moisture removal.
Some thermostats and systems allow you to adjust the blower speed. Running the blower at a slightly lower speed during cooling increases moisture removal at the cost of slightly reduced airflow.
Whole-Home Dehumidifiers
If your home has persistent humidity problems that can't be solved by fixing the AC system, a whole-home dehumidifier is worth considering. These units install in the duct system and run independently of the AC, removing moisture even when the AC isn't running.
They're particularly useful in homes with high occupancy, lots of cooking or showering, or in very humid climates where the AC alone can't keep up.
Frequently Asked Questions
What indoor humidity level is comfortable?
Most people find 40–55% relative humidity comfortable. Below 30% feels dry (common in winter with forced-air heating). Above 60% feels muggy and promotes mold and dust mite growth.
Can I use a portable dehumidifier instead of fixing the AC?
A portable dehumidifier can help in specific rooms, but it doesn't address the root cause. It also adds heat to the space (dehumidifiers generate heat as a byproduct) and requires regular emptying. Fixing the underlying cause is usually the better long-term solution.
My new AC system makes the house more humid than the old one. Why?
The new system is likely oversized. A larger system cools faster but dehumidifies less. This is a common result of replacing a system without doing a proper load calculation.