Crawl Space Dehumidifier Size Helper

What size dehumidifier belongs under the house? Capacity depends on how sealed the crawl is, whether liquid water is present, and how manufacturers rate their units. Use this form to check those conditions, see how ratings work, and compare them with a published closed-crawl code minimum.

How to use it

1. Enter floor area and average height.
2. Say whether the crawl space is sealed (vents closed) or still open and leaky.
3. Say whether standing water is present.
4. Optionally note climate moisture and an indoor humidity target to discuss with a contractor.

When a pint-per-day number is not useful

If standing water is present, or vents are still open, a capacity recommendation would mislead you. Liquid water needs drainage first, and an open crawl keeps exchanging moist outdoor air. Start with water in the crawl space, when not to encapsulate, and the encapsulation guide.

How capacity is rated

Portable dehumidifier capacity in the United States is rated in pints per day under U.S. Department of Energy test procedures that ENERGY STAR also uses — about 65°F dry-bulb and 60% relative humidity for portable units. ENERGY STAR groups portable products into capacity classes of 25 pints/day or less, 25.01 to 50, and 50 or more.

One published closed-crawl example, the North Carolina Residential Code dehumidifier method (R409.5.1), requires a permanently installed dehumidifier with a minimum rated capacity of 15 pints per day, drained to daylight or an interior condensate pump. That is a jurisdictional floor, not a sizing equation for every sealed crawl in every state. Compare manufacturer AHAM-rated charts for your area and climate.

Worked example

A sealed 1,200 sq ft crawl that averages 3 ft high has about 3,600 cubic feet of volume, no standing water, and a discussion humidity target of 50% (within the EPA’s whole-home indoor guidance of ideally 30–50%, generally below 60%). You will see that volume, the North Carolina 15 pints/day floor as one code example, and ENERGY STAR capacity classes — then manufacturer charts for a final pick.

Limits and safety

• Not a substitute for a licensed HVAC or moisture professional.
• Cooler crawl air often means less water removal than the rated sticker suggests.
• Continuous drain is usually essential; full buckets get ignored.

Also useful: Dehumidifier guide, Humidity and moisture control, and Vented vs unvented crawl space.

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Sources

The U.S. Environmental Protection Agency’s mold and moisture materials describe indoor relative humidity ideally between 30% and 50%, and generally below 60%, as whole-home guidance. ENERGY STAR’s dehumidifier key efficiency criteria define portable capacity classes and point to the Department of Energy test procedure in 10 CFR Part 430, Appendix X1 (portable rating conditions of 65°F dry-bulb and 60% relative humidity). The North Carolina Residential Code closed-crawl dehumidifier method (R409.5.1), as published in public building-department excerpts, states a minimum rated capacity of 15 pints per day for that option — verify the edition your jurisdiction adopted.