Every crawlspace I open in Roswell in July tells the same story: foundation vents wide open, 82 percent relative humidity under the floor, condensation beading on the ductwork, and brick piers with white salt bloom halfway up. The vents were supposed to prevent exactly this.
Why venting backfires in a humid climate
The theory behind crawlspace vents is that outdoor air is drier than crawlspace air, so exchanging them dries the space. That holds in a dry climate. In north Georgia in summer it is exactly backwards.
Outdoor air at 90 degrees and 65 percent RH holds a lot of water. Push it into a crawlspace where the ground and the surfaces sit at 65 to 70 degrees, and the relative humidity of that same air climbs past 90 percent. On any surface below the dew point — a cold supply duct, a water line — it condenses.
So the vents are pumping moisture in, not out, for the several months of the year when it matters most. The wood framing takes on moisture, the piers stay damp, and the salt in the brick and mortar goes into solution and migrates to the surface.
What it does to the masonry
Brick piers and a perimeter foundation wall in a persistently damp crawlspace suffer the same way any wet masonry does, just out of sight:
- Efflorescence and salt crystallization eroding mortar joints and softening brick faces.
- Mortar deterioration at the base, where moisture is highest and evaporation is lowest.
- Rising damp into sill plates and band joists, which is where the expensive repairs are.
- Pier movement where the soil under a pier footing softens and the footing settles unevenly.
- Corroding shims and straps. Steel shims between a pier cap and the beam rust and lose thickness in a damp crawlspace, and the beam drops a fraction of an inch at each one.
None of this is visible from inside the house until a floor starts to feel soft or a door stops latching, which is typically several years after the mechanism started.
A pier that has lost its joints and is bearing on crumbling mortar can be rebuilt for $600 to $1,800 each. A rotted band joist and sill run into five figures across a house.
Encapsulation, and what it actually involves
The modern approach is to stop treating the crawlspace as outdoors:
- Fix bulk water first. Gutters, downspout extensions, grade, and a French drain if water enters from a slope. Nothing else works until this is done.
- Ground vapor barrier. 10 to 20 mil reinforced sheet across the whole floor, lapped and sealed, run up the walls and piers and mechanically fastened.
- Close and seal the vents.
- Insulate the perimeter walls rather than the floor above, with rigid foam or closed-cell spray, detailing for termite inspection strips as required locally.
- Condition the space — a dedicated dehumidifier draining to the exterior, or a small supply from the HVAC system with a return path.
Cost for a typical Roswell crawlspace: $4,000 to $12,000, with the dehumidifier $1,200 to $2,500 of that. It is not cheap, and it is usually less than the repairs it prevents.
The reason this ends up on the mechanical contractor’s plate is that the symptoms show up as comfort and duct problems long before anyone looks at the masonry. If ducts in a crawlspace are sweating, that is a moisture reading, and it is worth having a Roswell HVAC contractor put a hygrometer down there for a week before anyone spends money on equipment.
Frequently asked questions
Is it against code to close crawlspace vents? No. The building codes have permitted unvented, conditioned crawlspaces for years, subject to requirements for a ground vapor retarder and either mechanical drying or a supply of conditioned air. Local amendments vary, so confirm the specifics before you start.
Will a dehumidifier alone fix it? It will lower the humidity while it runs, and it will run constantly and expensively if the vents are open and the ground is bare. Sealing and a ground barrier are what make the dehumidifier a small appliance rather than a permanent industrial load.
What humidity should a crawlspace hold? Below 60 percent RH year round is the working target; below 55 is better where there is wood in contact with masonry. Buy a $20 hygrometer and leave it down there — it is the cheapest diagnostic tool in the house.