Sealed the Vents? Then Your Crawl Space Needs a Dehumidifier.

The question we hear most about sealing a crawl space is some version of: “If you close it all up, how does it breathe?”

It is a fair question, and the answer surprises people. In central North Carolina a crawl space does not need to breathe. The vents that were supposed to let it breathe are, for much of the year, the main way water gets in. What a sealed crawl space needs instead is something that actively takes moisture out of the air — and that is the dehumidifier.

Why Vents Make a Triangle Crawl Space Wetter

Foundation vents come from a time and a climate where outdoor air was usually drier than the air under the house. Our summers are the opposite. From June into September the outdoor air here routinely carries more moisture than the crawl space can hold at its cooler temperature. The ground keeps a crawl space cool, and the ductwork and cold-water pipes running through it are cooler still.

So warm, humid air comes in through the vents, meets those cool surfaces, and gives up its water. That is why so many vented crawl spaces here have sweating ducts, insulation sagging out of the joists because it is wet, a musty smell that reaches the house, and floors that cup in late summer. The vents are not airing the space out. They are feeding it.

Closing the Vents Is Only Half of It

Sealing the vents and laying a liner over the ground stops the two biggest sources of moisture: humid outdoor air and water vapor rising out of the soil. It does not make the space dry on its own. Moisture still arrives — through the house above, through small air leaks around pipes and wiring, from the concrete and block themselves, from the odd plumbing drip. In a sealed space with nothing removing it, humidity climbs.

That is why building codes treat a closed crawl space as one that must be actively dried, by a dehumidifier, by conditioned air from the house system, or by an exhaust fan, depending on how it is designed. In our work it is almost always a dedicated dehumidifier, because it controls the humidity directly and does not depend on the house's heating and cooling running.

Sealing a crawl space without a way to dry it trades a damp, vented crawl space for a damp, sealed one. The second is not an improvement.

It Works as One System

People reasonably want to do this in stages: the liner this year, the dehumidifier next. We push back on that, because each part depends on the others:

• A sealed liner over the ground and up the walls, so the soil stops feeding moisture into the space.
• Sealed vents and rim openings, so the summer air stops coming in.
• Treatment of existing surface growth on the framing where there is any, so it is not left in place to restart.
• A crawl space dehumidifier sized for the space, on its own circuit, with a drain line that takes the water it collects out of the crawl space.

Leave any one out and the others underperform. Liner without a dehumidifier still leaves humid air under the house. Dehumidifier with the vents open is trying to dry the whole neighborhood.

Living With a Crawl Space Dehumidifier

• The target is relative humidity below about 60 percent. That is roughly the line below which mold and wood-rot fungi stop growing. Most units let you set it and leave it.
• Change the filter on schedule. A clogged filter starves the coil of air and the unit runs longer to do less. Use the filter made for your model; one that fits loosely lets dust straight into the coil.
• Watch the drain. A unit that is leaking or spitting water is usually telling you about its drain line or condensate pump, not about the dehumidifier itself.
• Register it with the manufacturer. The manufacturer's warranty depends on registration, and it is the homeowner, not the installer, who registers. Do it the week it goes in.
• Have it looked at once a year. It runs out of sight, so a failure can go unnoticed for months.

Fall Is the Time to Check

By October a crawl space dehumidifier has done its hardest work of the year. It is a good moment to check the filter, make sure the drain is clear and running where it should, and look at the humidity reading. If the unit has been running nonstop and the reading still sits high, something is letting moisture in faster than it can take it out — an open vent, a torn liner, or water arriving after rain — and that is worth finding before next summer.

Triangle Reconstruction has sealed and dried crawl spaces across Wake and Durham counties and every county bordering them since 2012. If your crawl space smells musty, your ducts sweat in summer, or you want to know whether sealing yours makes sense, call (919) 924-7600.