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Crawl spaces

Crawl space dehumidifiers

By Robert Stulpt Last reviewed

Short answer

Most of the time, if your crawl space is closed and you live in a humid climate, you want a dedicated crawl space dehumidifier. Use the code as a starting point for sizing: in Virginia, it’s 70 pints per day for every 1,000 square feet of crawl space when a dehumidifier is what dries the space. Give it a permanent drain and set it around 50% to 55%. It won’t cure water getting in, and it won’t dry a crawl space with the vents open.

On this page
  1. Do you need one?
  2. How to size it
  3. Crawl space units vs. portable dehumidifiers
  4. Crawl space dehumidifier installation
  5. Settings and maintenance
  6. Common mistakes
  7. Who installs them

Do you need one?

I suggest a dehumidifier in most closed crawl spaces in Virginia, especially if summers are humid, your home is near the coast or other water, the soil takes a long time to dry out, you have a history of high humidity, or your home’s AC doesn’t control the crawl space. I won’t use a dehumidifier to cover up a water issue. If there’s water on the ground, I take care of that first; see water in the crawl space. And there’s no sense buying a good dehumidifier if your crawl space has its vents open; see crawl space vents.

Why not use the home’s AC? Your house may not need cooling while the crawl space is still very damp. We have a lot of issues in April, May, September and October for that reason. Here in Hampton Roads, we also have mild days and nights with a high dew point. I like to control the crawl space independently. I’ve seen good designs that use the home’s conditioned air, but it’s not my first choice when I’m fixing an existing house.

How to size it

Use the code as a starting point. Then look at what you have working against you in drying the crawl space. Don’t size it just by looking at the label.

The 2021 Virginia Residential Code (R408.3) sets the minimum for crawl spaces dried by dehumidification at 70 pints per day for every 1,000 square feet of floor. Your state may use a different edition.

Crawl space floor areaMinimum capacity under the Virginia code
1,000 sq ft (93 m²)70 pints/day
1,500 sq ft (139 m²)105 pints/day
2,000 sq ft (186 m²)140 pints/day

After that, I look at:

  • The shape of the space and how tall it is
  • Separate compartments
  • Air leaks
  • The history of humidity in the space
  • The water table and drainage
  • Whether the house is near the coast

Crawl space units vs. portable dehumidifiers

Using a portable bedroom dehumidifier is the number one mistake. These units are designed for a bedroom. They won’t keep up with a crawl space, and they shut off when the bucket is full. If you’re sizing a dehumidifier for a basement instead, check out our basement dehumidifier guide and calculator.

Crawl space dehumidifier installation

Place the dehumidifier where it gets good airflow, which isn’t always the center of the crawl space. Leave room to service it, keep anything wet out from under it, and make the filter easy to reach. I like to use a gravity drain if I can. If I can’t, I use a condensate pump. Never use a bucket the homeowner has to empty. I add ducting if the crawl space has separate wings, interior walls or poor airflow.

Crawl space dehumidifier with a gravity drain Schematic section. 1: dehumidifier on a stand above the liner, with room to service the filter. 2: drain line sloping down through the foundation wall to the outside. Schematic, not to scale. 12
  1. Dehumidifier on a stand above the liner, with room to service the filter
  2. Drain line sloping down through the wall to the outside, where the ground outside is lower
Schematic, not to scale.
Crawl space dehumidifier with a condensate pump Schematic section. 1: dehumidifier on a stand above the liner, with room to service the filter. 3: condensate pump next to the unit. 4: discharge line run up and out through the rim joist to the outside. Schematic, not to scale. 134
  1. Dehumidifier on a stand above the liner, with room to service the filter
  2. Condensate pump next to the unit
  3. Discharge line run up and out through the rim joist
Schematic, not to scale.

Settings and maintenance

I set it at 50% to 55%. There’s no reason to go lower. Trying to get down to 35% won’t do you any good.

  • Check the filter every few months, and clean or replace it depending on use
  • Make sure the drain is working
  • Check the condensate pump, if there is one
  • Clean the coils
  • Have a general service once a year

For what it costs to run, see crawl space encapsulation and repair costs.

Common mistakes

  • A portable bedroom dehumidifier
  • A unit that’s too small for the space
  • A bucket that has to be emptied by hand, or no permanent drain
  • Vents left open
  • The unit sitting directly on wet ground
  • A filter you can’t reach
  • Drain water dumped back into the crawl space
  • One unit in a corner, expected to dry four separate compartments
  • The setting left at 70%
  • Using it to make up for a leak

Who installs them

Crawl space companies install these, and so do most HVAC companies. If the dehumidifier needs a new circuit, that work should be done by a licensed electrician.

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