Dehumidifier for a crawl space: what to check before you buy

Why a crawl space stays damp

Open vents sound like they should dry a crawl space. In humid climates, the US Department of Energy's Building America guide says they can do the opposite. Warm, humid outdoor air comes in and condenses on floor joists that are cooler, especially when the rooms above are air conditioned.

Bare soil adds moisture from below. Building America's approach for a sealed crawl space covers the floor and the perimeter walls with a 6-mil polyethylene vapor retarder, a plastic sheet that slows moisture coming up from the ground, with the seams overlapped and sealed.

A sealed crawl space still needs a way to dry. The same guide lists an exhaust fan and conditioned air from the house as ways to do that, and one of its photos shows a dehumidifier keeping a sealed crawl space dry.

Know how whole-home units are rated

The crawl space models in this guide are whole-home dehumidifiers: larger units that usually connect to ducts and drain through a hose or pipe. AprilAire's specification sheets show them drying one area, such as a crawl space, or connected to a home's ducts through their 10-inch duct collars. ENERGY STAR certifies them in their own category, with their own efficiency minimums, separate from portable units.

ENERGY STAR's conversion chart covers whole-home units too. It turns an old 80-pint rating into about 70 pints, and an old 100-pint rating into about 85; actual models vary.

That is why a model sold as an 80-pint unit can show a lower figure in its ENERGY STAR record. Compare the certified figure, not the name on the box.

AprilAire also lists a minimum dew point for the air coming in. Dew point is the temperature at which moisture in the air starts to condense. In cool, dry air it is low, and the manual lists a dew point below 40°F as an error condition.

Plan drainage and access first

ENERGY STAR notes that most whole-home dehumidifiers only have a drain connection, no bucket. That drain should run to a floor drain or a condensate pump, a small separate pump that moves the water to a drain, set up during installation by an HVAC professional.

Think about service as well. Filters need regular attention, and a crawl space is not easy to get into. Plan where the unit will sit, how you will reach it and how you will see the humidity reading without crawling in.

• Where will the condensate drain, and does it need a pump?
• Is there a suitable outlet in or near the crawl space?
• Can you reach the filter without moving the unit?
• How will you check the humidity from outside the crawl space?

Get the space assessed before you buy

Sealing a crawl space changes how the house handles air and moisture. Before insulating a closed crawl space, Building America's guide says to check with local authorities that the materials meet code.

Ask a qualified contractor to look at the ground cover, any standing water and the drain route before choosing a unit. The models in this guide are certified whole-home units that their manufacturer markets for crawl spaces. They are options to discuss, not a substitute for that assessment.

Compare the documented specifications

Capacities come from each model's ENERGY STAR record. Drain and temperature details come from the manufacturer. These are models to check against your space, not a ranking from our own tests.