Why open vents make Georgia crawl spaces wetter, not drier
Foundation vents were designed on the assumption that outside air is drier than crawl space air. In Metro Atlanta, that assumption is false for a large part of the year. The U.S. Department of Energy states the mechanism directly: in humid climates the air from outside is often more moist than the air in the crawlspace, which leads to structural, mould and pest problems. Atlanta receives roughly 50 inches of rain a year, and from June through August the dew point sits above 65 degrees on most days. A crawl space under an air-conditioned house holds surfaces in the low to mid 60s. When 78-degree air with a 70-degree dew point enters a foundation vent and touches a 64-degree floor joist, water condenses out of it onto your framing.
This is not a marginal effect. Ventilating a Georgia crawl space in July actively imports moisture. The vent that was meant to dry the space is the delivery mechanism for the water on your joists. The reason older houses survived it is that they were not air conditioned, so the crawl space and the outside air were closer in temperature and condensation was rarer.
What Georgia's building code actually says
What we found
All 20 Metro Atlanta jurisdictions we catalogued allow a sealed crawl space. Five of them changed how you file for one during 2025.
The code question is settled and identical statewide. The administrative question is not: of the 20 cities and counties on this site, three replaced their permitting software or their permitting jurisdiction during 2025 (Mableton, Dunwoody and Peachtree Corners), one accepts online submittals only (Smyrna), and one contracts plan review to a third-party firm (Stone Mountain). A separate five incorporated since 2005, so permit history from before then sits with the county rather than the city.
Method: We catalogued the permit authority, filing route and local quirk for all 20 Metro Atlanta jurisdictions we serve in August 2026, then counted. The full table is in our Metro Atlanta permit guide.
Georgia's residential code, based on the International Residential Code Section R408 with state amendments, addresses under-floor space ventilation in two ways that matter to homeowners.
First, the ventilation requirement can be reduced substantially when the ground is covered. Under the Georgia amendment, ventilation openings may be reduced to 1 square foot for each 1,500 square feet of under-floor area, with one opening within 3 feet of each corner, when the ground surface is 90 percent covered with a Class 1 vapor retarder. Without that ground cover, the base requirement is far larger.
Second, ventilation is not required at all when the crawl space is conditioned. Under R408.3, an unvented crawl space is permitted when a Class 1 vapor retarder covers the ground and an appropriately sized dehumidifier is provided. That is the code basis for the sealed crawl space, and it has been available in Georgia for years. It exists because of Department of Energy research: Building America found that unvented, conditioned crawlspaces use 15 to 18 percent less heating and cooling energy while reducing humidity by over 20 percent, and that work is what put R408.3 into the residential code in the first place.
What Class 1 vapor retarder means
A Class 1 vapor retarder has a permeance of 0.1 perm or less, which makes it effectively impermeable to water vapor. Standard 6-mil polyethylene meets that permeance threshold. Going thicker, to 12-mil or 20-mil reinforced liner, buys puncture resistance and durability rather than better vapor performance.
The three options, compared
| Strategy | What it involves | Typical cost | Summer humidity result | Best for |
|---|---|---|---|---|
| Leave vents open year-round | Nothing | $0 | Crawl space relative humidity commonly 70 to 85 percent | No Metro Atlanta house. This is the default that causes the problem. |
| Seasonal open and close | Manual vent covers, closed May to September, open in winter | $65 to $140 per vent | Improves summer humidity but rarely gets below 60 percent on its own | Homeowners who cannot seal yet and want the cheapest meaningful improvement |
| Sealed and conditioned | Class 1 vapor retarder, sealed vents, sealed rim joist, dehumidifier | $2,800 to $12,000 depending on scope | Holds 50 to 55 percent relative humidity year-round | Any Metro Atlanta crawl space where the budget allows it |
When should foundation vents be closed?
If you are running a seasonal strategy rather than a sealed crawl space, the practical rule for Metro Atlanta is to close vents when the outdoor dew point rises above your crawl space temperature, and open them when it falls below. In practice that means closed from roughly May through September and open from November through March, with April and October as judgment months.
The better version of that rule uses a measurement instead of a calendar. A $20 hygrometer left in the crawl space for two weeks tells you what is actually happening under your house. The EPA advises keeping indoor relative humidity below 60 percent, and ideally between 30 and 50 percent. If your crawl space reads above 60 with vents open, closing them is the right move regardless of the month.
Do crawl spaces need ventilation at all?
Not in a humid climate, and Georgia's code reflects that. A crawl space needs its moisture controlled, and ventilation is one way to attempt it, effective in dry climates and counterproductive in humid ones. The sealed crawl space achieves the same goal by removing the moisture source with a ground barrier and removing the residual humidity with a dehumidifier, which works in July as well as it works in January.
The one thing you must not do
Do not close the vents and stop there. A crawl space with closed vents, bare soil and no dehumidifier is worse than a vented one, because the ground is still evaporating moisture into the space and now nothing is removing it. If you are going to close vents as a first step, the vapor barrier is the step that has to accompany it, not follow it a year later.
The termite consideration
Georgia's structural pest control standards treat under-floor ventilation that does not meet the residential code as a condition conducive to wood-destroying organism infestation. If you seal your vents, tell your pest control provider, and keep an inspectable band of foundation wall visible at the top of any wall liner so an inspector can still read the masonry for termite tubes.
How to tell which situation you are in
- Put a hygrometer in the crawl space and leave it for two weeks in summer. Above 60 percent is a problem.
- Look at the ground. Bare soil anywhere means you do not have the Class 1 vapor retarder the code reduction and the unvented option both depend on.
- Check the insulation. Batts hanging down or on the ground mean the space has been running humid for years.
- Feel the ductwork on a July afternoon. Condensation on the trunk lines means the air around them is above its dew point at that surface.
- Look at the joist bottoms with a flashlight. Dark staining, white growth or a fuzzy surface means the wood has been wet long enough to support growth.
Frequently Asked Questions
Should I open my crawl space vents in the winter in Georgia?
If your crawl space is sealed and dehumidified, no. Never open them. If your crawl space is vented, bare-ground and has no dehumidifier, then yes, winter is when ventilation actually helps, because outside air in January is genuinely drier than the air under your house. The seasonal approach only makes sense for crawl spaces that have not been sealed.
Is it against code to close crawl space vents in Georgia?
No. Georgia's residential code explicitly permits an unvented crawl space when a Class 1 vapor retarder covers the ground and an appropriately sized dehumidifier is provided. Closing vents without those two elements is not a code strategy, it is just closing vents, and it can make the space damper.
How many vents does a Georgia crawl space need?
It depends on whether the ground is covered. Under the Georgia amendment, when 90 percent of the ground is covered with a Class 1 vapor retarder, ventilation can be reduced to 1 square foot of opening per 1,500 square feet of under-floor area, with one opening within 3 feet of each corner. Without ground cover the requirement is much larger. If the crawl space is sealed and dehumidified under R408.3, no ventilation openings are required.
Do automatic crawl space vents work?
Temperature-actuated automatic vents open and close based on temperature, not humidity, and humidity is the variable that matters. On a 75-degree, 95 percent humidity Atlanta morning in June, a temperature-triggered vent will happily open and admit air that condenses on your joists. They are better than a vent nobody ever touches, and considerably worse than a sealed crawl space or a hygrometer and a manual cover.
Will closing my vents cause radon or moisture to build up?
Closing vents alone can raise moisture, which is precisely why the code pairs the unvented option with a ground vapor retarder and a dehumidifier. A properly sealed crawl space is drier than a vented one, not wetter. On radon, the Class 1 vapor retarder on the ground also reduces soil gas entry, and if radon is a specific concern the barrier can be detailed with a passive vent stack.
Sources
Primary sources only. This site never cites a home-services marketplace, cost aggregator or competing contractor, because those businesses compete with this one and their figures rarely trace to anything.
- Georgia Department of Agriculture: IRC Section R408 Under-Floor Space. R408 text as applied in Georgia: ventilation openings reduced to 1 sq ft per 1,500 sq ft with 90 percent Class 1 vapor retarder ground cover; unvented option under R408.3.
- Georgia Structural Pest Control Commission Notice 14-08: Under-Floor Space Ventilation Fact Sheet. Class 1 vapor retarder defined at 0.1 perm or less; 6-mil polyethylene complies; dehumidifier permitted in place of ventilation.
- U.S. Department of Energy, Building America Solution Center: Vented to Unvented Crawl Space. In humid climates, outside air is often more moist than crawl space air, so ventilation imports moisture.
- U.S. Department of Energy: Unvented, Conditioned Crawlspaces, Building America Top Innovation. Unvented conditioned crawl spaces use 15 to 18 percent less heating and cooling energy and reduce humidity by over 20 percent. Research influenced IRC R408.3.
- U.S. EPA: A Brief Guide to Mold, Moisture and Your Home. Keep indoor relative humidity below 60 percent, ideally 30 to 50 percent.
- Georgia Structural Pest Control Commission Rules, Chapter 620-6: Wood Destroying Organisms. Conditions conducive to infestation include cellulosic debris under a building, wood-to-soil contact, and under-floor ventilation not meeting the residential code.
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