Why Your South Carolina Home Feels Humid Even When the AC is Running
Sticking point.
Is your home built to withstand the Grand Strand?
Control Moisture and Manage Humidity in Your Home
If you live in Myrtle Beach, you know the feeling: you walk into your home on a humid summer afternoon, the thermostat is set to 72∘F, the AC is humming away, but the air still feels heavy, sticky, and damp. You might even notice a faint, musty smell or see condensation on your windows.
You’ve probably done the first logical thing: lowered the thermostat even further. The temperature drops, but the stickiness remains. Why? Because you’re fighting the wrong enemy. The problem isn’t your air conditioner's ability to cool; the problem is your home's ability to manage moisture.
In the challenging, hot, and relentlessly humid coastal climate of the Grand Strand, your home is constantly battling what we call latent heat. Understanding this concept is the key to solving your perpetual humidity problem—and saving a fortune on your electric bill.
The Root Cause: Latent Heat vs. Sensible Heat
Your typical air conditioning unit is designed to handle two things: sensible heat and latent heat.
Sensible Heat (Temperature): This is the heat you feel and measure with a thermometer. When your AC is successful at dropping the indoor temperature from 80∘F to 72∘F, it is removing sensible heat.
Latent Heat (Moisture): This is the energy locked inside the water vapor in the air. This moisture is what makes the air feel sticky. When the AC runs, the cooling coil must get cold enough to condense this water vapor into liquid water (which drains out as condensate) before it can fully tackle the temperature.
In Myrtle Beach, our outdoor air regularly holds 70%, 80%, or even 90% relative humidity. Your AC unit simply cannot shed all that moisture load while simultaneously handling the high temperatures, especially if the house is constantly refilling the air with new moisture.
The humidity problem is a symptom of one or more major structural failures that are allowing the "swamp air" to infiltrate your home’s thermal envelope.
Heat Index is the “feels like” temperature, combining temperature and relative humidity. Image by the National Weather Service and NOAA.
Four Coastal Culprits Hiding in Your Home
For residents in this area, three common (and expensive) structural vulnerabilities act as direct conduits for humid outdoor air, forcing your AC unit to work endlessly and inefficiently.
Culprit #1: The Vented Crawlspace Sponge
This is arguably the single largest cause of high indoor humidity in older, traditional South Carolina homes built over a crawlspace.
The Myth: For decades, building codes mandated open vents in crawlspaces, believing this air circulation would dry the space out.
The Reality in Myrtle Beach: In the summer, those vents do the opposite. They suck in humid, 95∘F air from the outside. When that hot, moist air hits the cooler surfaces of the crawlspace (like the foundation and the AC ducts), it condenses, creating an environment that is 90% relative humidity.
The Damage: This perpetually damp air migrates upwards through cracks, utility penetrations, and floor joists, directly into your living space, making your carpet feel damp, causing your wood floors to warp, and feeding mold spores growth that can trigger allergies. It also attacks and corrodes the metal components of your HVAC system.
Culprit #2: Leaky Ductwork in Unconditioned Spaces
Your air ducts are designed to be a closed highway for cooled air. When they run through hot attics, garages, or damp crawlspaces, any break in that highway becomes a huge problem.
The Problem: Over time, the flexible ductwork commonly used becomes brittle, joints separate, and the aluminum tape used to seal them deteriorates.
The Consequence: When your AC kicks on, the return ducts (the ones sucking air in) are under a slight negative pressure, and the supply ducts (the ones blowing air out) are under a positive pressure. Any leak in the return ductwork in a humid attic or crawlspace acts like a giant, pressurized vacuum, sucking in hot, damp, often moldy air and delivering it directly into your cooled rooms. You are literally paying to cool the air from the outdoors, which is why your AC never seems to catch up. Duct leakage alone can account for up to 30% of your cooling loss.
Ensure all joints are securely connected and sealed to minimize loss of conditioned air and prevent infiltration.
Culprit #3: Uncontrolled Air Infiltration (Blowing Money Out the Wall)
Beyond the big issues, thousands of tiny holes in your home's exterior are letting the outside air sneak in constantly.
The Hidden Gaps: The typical Myrtle Beach home has massive air leaks where materials meet: the rim joist in the attic, the plumbing and electrical penetrations under sinks and in utility rooms, the space behind wall outlets, and the tiny gaps around light fixtures.
The Result: When the wind blows off the ocean or the temperature differential between inside and out is large, these gaps act as miniature portals for unconditioned, humid air. This uncontrolled airflow overwhelms the dehumidifying capacity of your AC and forces it to cycle constantly, driving up your power bill with little improvement in comfort.
Culprit #4. Building and Occupant-Added Moisture
Your house can bring in moisture through a wet crawlspace or basement. New construction materials and furnishings can release several gallons of moisture per day. Even if your AC is perfectly sized and your house is completely sealed, the people living inside are constantly generating humidity. Everyday activities—from showering and cooking to simply breathing—add significant amounts of water vapor directly into your home's air, forcing your AC to work overtime to manage it.
The Problem: The moisture created by the building and occupants rapidly raises the interior humidity level. This damp air leads to that sticky feeling or even potential mold issues if not properly addressed.
The Result: The extra humidity strains your existing AC system, which may not be powerful enough to handle both the external SC moisture and the internal occupant load.
The total moisture load requiring control within a structure originates from a combination of everyday human and household activities and the water vapor introduced by the building and its construction materials themselves.
Why You Need a Scientific Home Performance Consultation
Attempting to solve these problems by simply buying a larger AC unit or running a portable dehumidifier is like taking aspirin for a broken leg—it addresses the symptom but ignores the injury. You will continue to run up electric bills and risk costly moisture damage.
A professional Home Performance Consultation moves beyond guesswork and uses science to pinpoint the exact source of your home’s moisture and air leaks.
The Tools of the Trade
The Blower Door Test: This machine is installed in an exterior doorway to safely depressurize your home. It pulls air out, which draws exterior air in through every single hole and gap. We can use a smoke pencil or thermal imaging to see precisely where the conditioned air is escaping and where the humid air is entering.
Duct Blaster Test: This dedicated test measures the exact percentage of conditioned air your duct system is leaking into unconditioned spaces (like the attic or crawlspace). This number tells us exactly how much you are losing.
Infrared Thermal Imaging: Used during the Blower Door test, the camera highlights temperature differences, clearly showing where cool, conditioned air is escaping and where hot, humid air is infiltrating, particularly in walls, ceilings, and around windows.
By using these three scientific tools, we create a precise roadmap to fix the structure of your home, permanently isolating your conditioned air from the humid coastal climate.
From Swampy to Serene: The Path Forward
Addressing your home’s performance is a one-time investment that stops the humidity cycle, reduces your cooling costs, and protects your home from long-term moisture damage.
The solution usually involves a combination of:
Crawlspace Encapsulation: Sealing the crawlspace entirely, isolating it from the ground and the outside air, turning it into a semi-conditioned space.
Air Sealing: Methodically sealing the hundreds of penetrations and gaps found during the blower door test.
Duct Sealing and Insulation: Ensuring your ductwork is fully intact, sealed, and properly insulated.
Creating New Habits: Small changes can make a big difference for indoor humidity levels. Try running the bathroom exhaust fan while showering, using a lid when boiling water, drying firewood outside, and more tips to reduce moisture.
Stop paying huge electric bills just to feel sticky! We can scientifically diagnose the problem and provide a clear, prioritized list of solutions with a projected return on investment.
Ready to find out exactly where the Myrtle Beach humidity is getting into your home? Call us now to schedule your scientific Home Performance Consultation with Rappid Energy.