Quick answer
- Indoor relative humidity should stay between 40-55% — Charleston summers regularly push outdoor RH past 80%.
- An AC dehumidifies as a byproduct of cooling — but only when it runs long enough.
- Oversized units short-cycle (cool the air without removing enough moisture), leaving the house cold and clammy.
- Variable-speed compressors and ECM blowers handle humidity better than single-stage equipment.
- Standalone whole-home dehumidifiers (Ultra-Aire, Aprilaire) solve persistent humidity that the AC alone can't.
Why Lowcountry humidity is its own design problem
Charleston, Summerville, Mount Pleasant — anywhere east of I-95 — averages outdoor relative humidity in the upper 70s to mid 80s through summer. That's roughly double the humidity load that systems designed for Atlanta or Birmingham face. Standard HVAC sizing methods that work fine inland produce uncomfortable houses here.
The comfort target inside is 40-55% RH. Above 60%, mold and dust mites thrive and the air feels heavy even at lower temperatures. Below 35%, woodwork shrinks and skin gets dry — but that's rare in our climate even in winter.
Why an AC is also a dehumidifier (when it runs long enough)
When warm humid air passes over a cold evaporator coil, water vapor condenses out and drains away. That moisture removal is happening continuously while the AC is running — but only while it's running. The longer the cycle, the more moisture removed.
This is the single most common comfort complaint we hear in Charleston. The fix is rarely 'add more cooling.' It's almost always 'remove more moisture.'
Equipment choices that handle humidity
Variable-speed compressor + ECM blower
A variable-speed compressor can run at 30% output for 45 minutes — pulling much more moisture out of the air than a single-stage unit at 100% output for 10 minutes. Paired with an ECM blower (variable airflow), this combination is the modern standard for humid climates.
Two-stage as a middle ground
Two-stage compressors (low and high) cost less than variable-speed and deliver most of the humidity benefit. Often the right choice for replacement budgets that can't reach full variable-speed.
Whole-home dehumidifier (when AC alone isn't enough)
Brands like Ultra-Aire, Aprilaire, and Santa Fe make dedicated whole-home dehumidifiers that tie into your ductwork. They run independently of the AC, removing moisture even when cooling demand is low (cool, rainy days, shoulder seasons). For coastal homes or homes with persistent humidity problems, this is often the only real fix.
Common mistakes
- Sizing the AC by square footage instead of a Manual J load calc — produces oversizing in every Charleston home we've measured.
- Setting the thermostat fan to 'on' continuously — re-evaporates moisture from the coil back into the house during off cycles.
- Closing supply registers in unused rooms to 'redirect' airflow — increases duct static pressure and reduces moisture removal.
- Ignoring crawlspace or attic humidity — humid spaces around your ductwork dump moisture into the conditioned air.
What to do next
If your house feels sticky even when the thermostat is satisfied, the issue is almost always either equipment sizing or a separate humidity source. We can measure RH at multiple points in your home, identify the source, and propose a fix that doesn't require oversizing your AC.
Have a question about your system?
Talk to a real Charleston Lowcountry technician — a family-owned team with 30+ years of local experience that installs and services Carrier, Trane, Rheem, and more.