Crawl Space Dehumidifiers: Sizing, Reviews & Installers

What a crawl space dehumidifier is, and why you would want one

A crawl space dehumidifier is a refrigerant dehumidifier built to run unattended in the space under a house: a low, cool, often dirty area that is 2 to 4 ft high and reached through a hatch. It pulls water vapor out of the crawl space air, condenses it on a cold coil, and sends the water out through a drain hose or a built-in pump. Because nobody is down there to empty a bucket, there is no bucket; because the space is cool, makers publish capacity at 65°F or 73°F alongside the 80°F rating so you can see what it does in cool air; and because it may run for a decade without being looked at, the units we cover carry five- and six-year warranties.

The reason to own one is relative humidity. The EPA's mold guidance says indoor relative humidity should be kept below 60 percent, ideally between 30 and 50 percent, and notes that crawl spaces where relative humidity is high are common sites of hidden mold growth, particularly if the floor is bare earth. In the humid South that is not where an untreated crawl space sits: in Advanced Energy's North Carolina field study, vented crawl spaces stayed above 70 percent relative humidity almost all of the summer of 2003. That is the range where mold grows on joists, insulation sags and falls, ducts sweat, floors cup, and termites find damp wood. Air from the crawl space also moves up into the living space, so the musty smell in the hallway is usually a crawl space problem.

A dehumidifier is only half the fix. It removes moisture that gets in; it cannot stop moisture from getting in. A sealed vapor barrier over the soil and closed foundation vents come first, and the dehumidifier holds the sealed space at 50 to 55 percent afterward. We explain that pairing in dehumidifier vs. encapsulation.

How it differs from a basement or portable dehumidifier

A basement unit lives in a space you walk into. It may have a bucket, it is rated at 65 to 70°F, it needs a floor drain or an occasional trip downstairs, and it is priced for a room. A crawl space unit is rated on the same pint-per-day scale but at higher capacities (55 to 130 pints per day on the 80°F AHAM basis), ships with a drain fitting and often a condensate pump, can be hung from the floor joists or ducted, carries a MERV filter that can be reached through the hatch, and is sized in a 12 in. or 14 in. cabinet so it fits through the access door. Portable units from the big-box store are not built for any of that, and a space you never visit needs a unit that drains itself and keeps its setpoint without attention.

Sizing in one paragraph

The building code gives the base rate. IRC R408.3 lists dehumidification "sized to provide 70 pints (33 liters) of moisture removal per day for every 1,000 square feet (93 m2) of crawl space floor area" as a way to condition an unvented crawl space. Multiply your floor area in square feet by 0.07 for the baseline in pints per day, add about 30 percent for a wet crawl space or a humid climate, and buy the next size up. A damp 1,200 sq ft crawl space in Charleston lands at about 100 pints; the same crawl space in a moderate climate lands at 85. Our size calculator does the arithmetic, shows its assumptions, and lists the units that meet the result.