Best Crawl Space Dehumidifier: Sizing, Pumps and Encapsulation

Use the maker's square foot rating and the AHAM pint figure, not the saturation number. The crawl space units here fall into three size classes: 35 pints AHAM for up to about 1,000 sq ft, 55 pints for 1,300 to 1,500 sq ft, and 60 pints for up to 1,800 sq ft. Past about 2,000 sq ft, buy the Santa Fe Compact70 or two smaller units placed at opposite ends.

The International Residential Code (2018 and later) allows an unvented crawl space to be dried by a dehumidifier sized at 70 pints a day for every 1,000 sq ft of floor. The code does not name a test condition, so ask your local inspector which rating they accept if the crawl space is part of a permitted job.

Size up one class if the crawl space has a dirt floor that is not fully covered yet, standing water after rain, or a musty smell already. A larger unit just reaches the set humidity sooner and cycles off.

AHAM pints are measured at 80 F and 60 percent relative humidity. Saturation pints are measured in hot, nearly saturated air (Argendon lists 90 F and 90 percent), where any dehumidifier pulls far more water, so the saturation figure runs two to two and a half times the AHAM figure on every unit here: 35 and 70, 55 and 120, 60 and 145.

Portable dehumidifiers use a third rating, the DOE test at 65 F and 60 percent. The Santa Fe Compact70 is listed by Energy Star at 43.7 pints on that cooler test, which is why its number looks smaller than the 55 and 60 pint AHAM units even though it is rated for more square feet. Never compare a saturation number to an AHAM or DOE number.

A crawl space unit has no bucket, so it must drain. If there is a floor drain, sump pit or an exterior wall where a hose can run downhill, a gravity unit is simpler and has one less part to fail. If water has to go up, over a footing or out a rim joist, buy a unit with a built-in pump or add a separate condensate pump.

Raise the unit on risers so the hose falls continuously and does not trap water under the unit. The built-in pumps on the Argendon and AlorAir pump models lift 19.6 ft per their makers, enough to reach a sink or a laundry standpipe from most crawl spaces.

Bare soil gives off water vapor every day, and open vents bring in humid summer air. A dehumidifier in that space runs constantly and never reaches its set point. Encapsulation means covering the ground (and ideally the walls) with a sealed vapor barrier, taping the seams, and closing the foundation vents, so the dehumidifier only has to handle what is left.

Lay the barrier with the seams overlapped and taped, run it up the walls and fasten it, and seal around piers. A 20 mil woven barrier resists tearing when you crawl across it later to service the unit. Fix any outside drainage problem, such as gutters dumping at the foundation, before you seal anything.

Crawl spaces run cool, and a cold coil frosts over. Every unit in this guide has auto defrost, which pauses the compressor and keeps the fan running until the ice melts. The Galaxy 60P is rated to run down to 37.4 F and the Compact70 to 49 F before it defrosts; standard portable dehumidifiers are built for living-space temperatures and lose capacity as the air cools.

Ducting lets one unit pull air from a far corner or push dry air down a long, chopped-up crawl space. Santa Fe sells ducting, hang and pump kits for the Compact70, which is also short enough to hang between floor joists. The compact 35 to 60 pint units are designed to sit in the open space and move air on their own.

Efficiency decides the bill more than wattage. At 2.4 liters per kWh, the rated efficiency of the Galaxy 60P, removing 20 pints (9.5 liters) a day takes about 3.9 kWh. At the 17.3 cent US average that is about $0.68 a day, or roughly $20 a month, which works out to 3.4 cents per pint of water removed.

Wattage tells you the worst case. The HD55S draws 471 W, so if the compressor ran 12 hours in a day it would use 5.7 kWh, about $0.98. The Sandidry Pro 35C draws 266 W, about $0.55 for the same 12 hours. A sealed crawl space usually needs far less run time after the first weeks of drying out, and the cost to run calculator shows the math at your state's rate.