Why Is My Myrtle Beach Crawl Space So Humid Even When the AC Runs Constantly?
In almost every Myrtle Beach home, the air conditioner never touches the crawl space. A vented crawl space is outside the conditioned envelope, so it stays at whatever the outdoor air is doing. From June through September, Grand Strand dew points sit in the low-to-mid 70s for weeks at a time, and every foundation vent pulls that air across framing and ductwork that the AC has chilled into the 60s. The surfaces are colder than the dew point, so moisture condenses on them. Running the system harder often makes the condensation worse, not better.
Your Air Conditioner Does Not Condition the Space Under Your Floor
This is the part that surprises most homeowners. A traditional vented crawl space is deliberately open to the outdoors — the foundation vents exist to let outside air move through. That means the space under your floor is not part of the area your HVAC system serves. It is closer, thermally, to your yard than to your living room. What the AC does affect is the temperature of everything that passes through that space: the supply ducts, the insulated floor above, the plumbing lines, and the joists in contact with them. Those surfaces get cold. The air surrounding them does not. That temperature split is the whole mechanism, and no thermostat setting resolves it, because the thermostat has no sensor and no supply register anywhere in the crawl space.
What Grand Strand Dew Points Do to an Open Crawl Space
Humidity percentage is a distraction here — dew point is the number that matters, because it tells you the temperature at which air has to give up its moisture. Along the Horry County coast, summer dew points routinely hold in the low to mid 70s Fahrenheit for stretches of days, and they stay elevated overnight rather than dropping the way they do inland. When outdoor air at a 74°F dew point moves through a vent and contacts a duct line or a subfloor surface sitting at 65°F, condensation forms on that surface immediately. It is the same physics as a glass of iced tea on a porch in July. Homes closest to the ocean, the Intracoastal Waterway, and the marsh systems at Cherry Grove, Little River, and Murrells Inlet tend to see the effect run longest, because those areas hold elevated dew points later into the evening.
Why Running the System Harder Can Make It Worse
Homeowners often respond to a musty, clammy house by dropping the thermostat, which is a reasonable instinct and usually the wrong lever. A lower setpoint chills the ductwork and subfloor further, which widens the gap between surface temperature and outdoor dew point, which means condensation forms faster and on more surfaces. Longer runtimes also increase duct leakage into the crawl space and pull more humid air into the house through the gaps every floor system has. Some homeowners then add a portable dehumidifier upstairs, which does nothing for the space below and can mask a problem that is quietly wetting framing. The common tell is a crawl space that looks worse in August than in February — that seasonal pattern points to airborne condensation rather than a plumbing leak or groundwater.
What Actually Changes the Humidity Under the House
The only durable fix is to stop treating the crawl space as outdoor space. That means separating it from outside air rather than trying to ventilate the moisture away — sealing the vents, covering the exposed soil with a continuous vapor retarder, sealing the seams and piers, and then controlling the remaining moisture mechanically. South Carolina builds on the International Residential Code, and Section R408.3 sets out what an unvented crawl space requires: continuous ground cover with lapped and sealed joints, plus one of several accepted approaches to conditioning the space, including dehumidification. Note that this is R408 in South Carolina, not the R409 numbering used across the North Carolina line — a detail worth confirming with anyone quoting code to you. In flood-zone homes, required flood openings change how sealing is handled, and that gets addressed separately rather than ignored.
“Frequently Asked Questions”
Your Air Conditioner Does Not Condition the Space Under Your Floor
This is the part that surprises most homeowners. A traditional vented crawl space is deliberately open to the outdoors — the foundation vents exist to let outside air move through. That means the space under your floor is not part of the area your HVAC system serves. It is closer, thermally, to your yard than to your living room. What the AC does affect is the temperature of everything that passes through that space: the supply ducts, the insulated floor above, the plumbing lines, and the joists in contact with them. Those surfaces get cold. The air surrounding them does not. That temperature split is the whole mechanism, and no thermostat setting resolves it, because the thermostat has no sensor and no supply register anywhere in the crawl space.
What Grand Strand Dew Points Do to an Open Crawl Space
Humidity percentage is a distraction here — dew point is the number that matters, because it tells you the temperature at which air has to give up its moisture. Along the Horry County coast, summer dew points routinely hold in the low to mid 70s Fahrenheit for stretches of days, and they stay elevated overnight rather than dropping the way they do inland. When outdoor air at a 74°F dew point moves through a vent and contacts a duct line or a subfloor surface sitting at 65°F, condensation forms on that surface immediately. It is the same physics as a glass of iced tea on a porch in July. Homes closest to the ocean, the Intracoastal Waterway, and the marsh systems at Cherry Grove, Little River, and Murrells Inlet tend to see the effect run longest, because those areas hold elevated dew points later into the evening.
Why Running the System Harder Can Make It Worse
Homeowners often respond to a musty, clammy house by dropping the thermostat, which is a reasonable instinct and usually the wrong lever. A lower setpoint chills the ductwork and subfloor further, which widens the gap between surface temperature and outdoor dew point, which means condensation forms faster and on more surfaces. Longer runtimes also increase duct leakage into the crawl space and pull more humid air into the house through the gaps every floor system has. Some homeowners then add a portable dehumidifier upstairs, which does nothing for the space below and can mask a problem that is quietly wetting framing. The common tell is a crawl space that looks worse in August than in February — that seasonal pattern points to airborne condensation rather than a plumbing leak or groundwater.
“The Bottom Line”
If your crawl space is humid while the AC runs nonstop, the two facts are related rather than contradictory. The system is chilling surfaces inside a space that is still open to outdoor air, and coastal Horry County supplies that air at a dew point high enough to condense on them. Sealing the space and controlling it mechanically is what changes the outcome. A free inspection provides an accurate assessment of the specific conditions under your home.