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How do I keep a crawl space dry?

Keep water away from the foundation first: clean gutters, carry downspout water well away, keep soil sloping away and fix leaks. Then cover the soil with heavy plastic, keep crawl space humidity below 60 percent with a properly sized dehumidifier or a sealed, conditioned design, and check the liner, sump and readings every season.

Free to ask. Local shops send quotes, and you pay only when you book one.

By Kasra Taheri, Founder, LoclisUpdated

Researched and drafted with AI, then checked claim by claim against the sources listed below before it was published. Prices and rules change; the dates beside each source say when it was last checked.

Step by step

Read when to stop and call a pro, further down this page, before you start.

  1. Step 1: Keep gutters clear

    Inspect and clean gutters and downspouts at least twice a year, and make sure downspouts discharge into drains or at least 10 feet from the foundation.[9][1]

  2. Step 2: Keep the soil sloping away

    Look for soil that has settled against the house and aim for a 5 percent slope, which is 6 inches of fall over the first 10 feet.[2][3]

  3. Step 3: Aim sprinklers away

    Run lawn sprinklers and watch where they spray, and turn any that wet the foundation.[1]

  4. Step 4: Fix leaks promptly

    Repair dripping pipes and keep the air conditioner's drip pan clean and its drain line flowing.[4][5]

  5. Step 5: Send exhausts outdoors

    Make sure the dryer, bathroom and kitchen exhausts end outside, not in the crawl space.[2]

  6. Step 6: Cover the soil

    Keep the whole dirt floor covered with at least 6-mil polyethylene, lapping the seams well.[5]

  7. Step 7: Read the humidity

    Keep a humidity meter in the crawl space and check it through the summer, aiming below 60 percent and ideally 30 to 50.[4]

  8. Step 8: Size the dehumidifier

    Pick capacity from how damp the space is: a damp, musty space needs less than one where walls or floors sweat.[6]

  9. Step 9: Drain it safely

    Plug the dehumidifier into a properly grounded outlet and route its hose to a drain or sump, away from cords and connections.[6]

  10. Step 10: Watch for frost in cool weather

    Below about 65 degrees the coils can frost; switch the unit off to defrost or use a model rated for low temperatures.[6]

  11. Step 11: Keep the access door shut

    Keep the crawl space door closed and weather-stripped, at least 6 inches above the outside soil, with runoff kept away from it.[1]

  12. Step 12: Check the sump

    Make sure any sump pit has a tight lid and the pump runs when water rises.[1]

  13. Step 13: Add a water alarm

    Set a battery water alarm at the lowest spot so a leak does not go unnoticed for weeks.[1]

  14. Step 14: Inspect it regularly

    Check the liner for tears, the wood for stains or mold, and the floor for puddles, especially after heavy rain.

The short answer

A dry crawl space comes from stopping water in a set order: first water from outside, then moisture from the soil, then humidity in the air. Most of the work is routine maintenance a homeowner can do: clean gutters, keep soil sloping away, fix leaks, keep the ground covered, and run a dehumidifier or a sealed crawl space system that holds the humidity down.

It matters because damp crawl spaces do not stay under the house. The EPA's moisture guidance says "damp basements or crawlspaces may add as much water vapor to the air in a day as all the other internal sources combined".[3]

Water pooled under clear plastic sheeting on a crawl space floor, with a white drain pipe running overhead
Environmental Protection Agency, Public domain, via Wikimedia Commons

Keep water away from the foundation

Rain is the biggest source, and it is the cheapest to control.

  • Gutters and downspouts. Downspouts should discharge into drains, or at least 10 feet from the foundation.[1] A splash block that drops the water right at the corner of the house falls short of that. The National Park Service's maintenance advice is to inspect and clean gutters at least twice a year.[9] If your gutters overflow, see how to adjust gutters so they drain properly.
  • Grading. Soil settles against foundations over time. Aim for a 5 percent slope, which is 6 inches of fall over the first 10 feet.[2][3]
  • Sprinklers. All sprinkler flow should be directed away from the home.[1]
  • Leaks. The Virginia Energy Savers Handbook's list starts with this: "Fix all water leaks including plumbing leaks, faucet leaks, roof leaks, and any water entering the basement or crawl space."[5] The EPA adds: "Keep air conditioning drip pans clean and the drain lines unobstructed and flowing properly."[4]
  • Exhausts. Kitchen, bathroom and clothes dryer exhausts should end outdoors,[2] never in the crawl space.

If a leak or flood does wet the crawl space, act fast: the Virginia Department of Health advises: Clean and dry wet or damp building materials within 24 to 48 hours of a flood or leak[8][4] to keep mold from taking hold.

Cover the soil

Bare dirt gives off moisture all the time. For a vented crawl space, the Virginia Energy Savers Handbook's advice is to "install a 6-mil polyethylene ground cover",[5] lapping the seams 12 to 18 inches.[5] A crawl space that is being sealed needs more: a continuous liner with seams overlapped, taped or sealed, and the edges run up and sealed to the walls.[1] Either way, check it regularly, and after heavy rain, for tears, gaps and puddles like the one in the photo above, and patch damage with tape made for the liner.

Keep the humidity down

Keep a humidity meter in the crawl space, because this is the number that tells you whether everything else is working. The EPA's advice for homes is to "keep indoor humidity below 60 percent (ideally between 30 and 50 percent) relative humidity".[4] Virginia Cooperative Extension's threshold is the reason: "Fungi (molds) grow in air over 60% relative humidity."[7]

Choosing a dehumidifier. ENERGY STAR sizes portable dehumidifiers by how damp the space is: about 20 to 30 pints a day for a space under 2,000 square feet that feels damp and smells musty at times (50 to 75 percent humidity), 25 to 40 pints for one that consistently feels and smells damp (75 to 90 percent), and 30 to 50 pints where walls or floors sweat or seepage is present (90 to 100 percent).[6] An ENERGY STAR certified unit uses 20% less energy than a similar conventional one.[6]

Running it in a crawl space. Crawl spaces are often cool, and frost can form on the coils below 65 degrees,[6] so choose a model rated for low temperatures. ENERGY STAR's safety advice applies doubly in a damp space: "Make sure the dehumidifier is connected to a properly grounded outlet. Keep drain hoses away from electrical cords and connections."[6] Use the hose fitting to drain to a floor drain or sump so the unit does not shut off with a full bucket. Most whole-home dehumidifiers drain through a line that an HVAC professional routes to a floor drain or condensate pump during installation.[6]

Vented or sealed: know which one you have

Vented crawl spaces are the hardest to keep dry in summer. Research for the Department of Energy found that "vented crawlspaces located in mixed or hot-humid climates tend to increase the moisture level within the space instead of keeping it drier".[1] The National Park Service's maintenance advice for older buildings is to seasonally close grilles when appropriate,[9] but check first that no furnace or water heater draws its air through those vents. Closing vents around such equipment risks "potentially creating a deadly situation if naturally ventilated types of combustion equipment are located in the crawlspace".[1]

Sealed crawl spaces stay dry only while their drying system works. "Crawlspaces must always have a drying mechanism."[1] If yours has a dehumidifier or a supply air vent, keep it running, watch the readings, and call the installer when they creep up. If you are deciding whether to convert, see is crawl space encapsulation worth it in Virginia and how to encapsulate a crawl space.

Doors and sumps. The access door should sit at least 6 inches above the outside soil, which keeps runoff and snowmelt from getting in,[1] and it should stay shut and be weather-stripped. Sump pits need tight lids: "Missing lids and an absence of sealing will allow moisture to enter and increase the relative humidity of the crawlspace."[1]

Stop and call a pro if

  • Water stands on the floor or runs in after rain even with gutters and grading fixed.
  • Humidity stays high with a dehumidifier running, or the smell comes back; see why a house smells musty from the crawl space.
  • Mold is growing on any of the framing. The Department of Energy guide says a mold remediation professional will need to be hired[1] to inspect and clean it up. Soft or sagging joists or subfloor need a pro too.
  • There is no grounded outlet near where the dehumidifier needs to go, or wiring looks damaged.
  • A gas furnace, water heater or gas line is in the crawl space and you are considering closing the vents.
  • The sump pump fails or the pit floods.

Licensed firms are listed in the Blacksburg restoration directory, and more is under restoration answers. When the crawl space needs more than maintenance, describe it on Loclis and licensed businesses will quote.

Common questions

What humidity should a crawl space be?
Aim below 60 percent, the level above which mold grows, and ideally between 30 and 50 percent, the range the EPA gives for indoor air. Check with a humidity meter left in the space for a few days, especially in summer.
Should crawl space vents be open or closed?
In a humid summer, open vents tend to let moisture in rather than out. Closing them for good is part of sealing the crawl space and needs a drying system first, and never close them if a furnace or water heater down there draws its air through them.
What size dehumidifier do I need for a crawl space?
ENERGY STAR sizes portable units by how damp the space is: about 20 to 30 pints a day for a space under 2,000 square feet that feels damp at times, 25 to 40 for one that is consistently damp, and 30 to 50 where walls or floors sweat. Pick one rated for cool temperatures.
Can I run a dehumidifier on an extension cord in the crawl space?
Avoid it. A damp crawl space is a poor place for an extension cord. Plug the unit into a properly grounded outlet, and have a licensed electrician add one if there is none nearby.
Is a sump pump enough to keep a crawl space dry?
It removes liquid water, not humid air. A crawl space with a sump still needs a ground cover and humidity control, and the sump pit needs a sealed lid so it does not add moisture of its own.

Sources

  1. [1] Guide to Closing and Conditioning Ventilated Crawlspaces (January 2013), U.S. Department of Energy, Building America Program (IBACOS for NREL). Checked October 26, 2026.
  2. [2] Designing Closed Crawl Spaces: A Quick Reference (February 2012), Advanced Energy, published by ENERGY STAR. Checked October 26, 2026.
  3. [3] Moisture Control Guidance for Building Design, Construction and Maintenance (December 2013), U.S. Environmental Protection Agency. Checked October 26, 2026.
  4. [4] A Brief Guide to Mold, Moisture and Your Home, U.S. Environmental Protection Agency. Checked October 26, 2026.
  5. [5] The Virginia Energy Savers Handbook (third edition), Virginia Department of Mines, Minerals and Energy. Checked October 26, 2026.
  6. [6] Dehumidifiers, ENERGY STAR (U.S. EPA). Checked October 26, 2026.
  7. [7] ENERGY SERIES: What about Moisture? (2908-9020), Virginia Cooperative Extension, Virginia Tech. Checked October 26, 2026.
  8. [8] Mold, Virginia Department of Health. Checked October 26, 2026.
  9. [9] Preservation Brief 39: Holding the Line: Controlling Unwanted Moisture in Historic Buildings, U.S. National Park Service. Checked October 26, 2026.

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