Crawlspace and Basement Moisture, Finding the Source and Drying It Out

Why the Lowest Level Gets Wet

Basements and crawlspaces sit in the soil, and soil holds water. Water reaches these spaces in three main ways, and each needs a different fix.

• Bulk water. Rain and snowmelt from roofs and yards soak the soil next to the foundation and push through cracks, joints and the gap where the wall meets the floor.
• Capillary wicking. Concrete and block act like a sponge and pull ground moisture up and through, leaving damp walls and white crusty deposits.
• Condensation. Warm, humid air meets cool foundation walls, pipes and floors and drops its water on them. This is common in summer.

Plumbing leaks and sewer line cracks add a fourth source, so do not rule them out.

A damp lower level affects the whole house. Air moves upward through homes, so moisture, musty odor and mold spores from a crawlspace or basement rise into the living space. Wood framing that stays wet can rot, and wet insulation stops working. Fixing the source protects the floors above you, not just the space below.

The Plastic Sheet Test

Before you spend money, find out whether the water is coming through the wall or forming on it. This simple test takes two days.

1. Cut a square of plastic sheeting about 2 feet by 2 feet.
2. Tape all four edges tightly to a damp section of foundation wall or floor.
3. Leave it for 24 to 48 hours.
4. Peel it back and look.

Also watch when the problem shows up. If the space gets wet within hours of rain, it is bulk water. If it is damp all summer even in dry spells, condensation is likely. If one spot stays wet no matter the weather, look for a plumbing leak nearby.

Fix the Outside Water First

A large share of wet basement and crawlspace problems start at the roof edge. One inch of rain on a typical roof sheds hundreds of gallons, and a clogged gutter or short downspout drops much of it right next to the foundation.

• Gutters. Clean them in fall and spring. Check that they slope toward the downspouts and do not overflow in heavy rain.
• Downspouts. Extend them so water lands at least 4 to 6 feet from the house, and farther on clay soil. Buried lines should be tested with a hose to confirm they flow.
• Grading. Soil should fall away from the house. A common target is about 6 inches of drop over the first 10 feet. Build low spots up with compacted soil, not mulch.
• Window wells. Clear leaves and debris and add covers where water pours in.
• Sprinklers and hoses. Aim heads away from the foundation and repair leaking hose bibs.

Give these changes a few storms before judging them. Many owners find the problem shrinks or stops without any interior work at all. If water still comes through during heavy rain, the next step is a drainage system, and that is contractor work.

Crawlspaces, Vented Versus Closed

Many older crawlspaces have bare dirt floors and vents in the foundation walls. The old idea was that outside air would dry the space. In humid summers the opposite often happens. Warm outdoor air flows in, hits the cool soil and framing and condenses, leaving wood wet and moldy.

There are two broad approaches.

• Vented with a ground cover. Keep the vents but cover the soil with heavy plastic. This alone cuts the moisture rising from the ground and is a reasonable DIY upgrade.
• Closed or encapsulated. Seal the vents, line the floor and walls with heavy plastic, and condition the space with a dehumidifier or a small supply of air from the HVAC. This is usually the more effective fix in humid climates, and it is usually pro work.

For a ground cover, use polyethylene sheeting of at least 6 mil thickness. Many installers prefer 10 to 20 mil because it resists tears when people crawl over it. Overlap seams by at least 6 inches, tape them, and run the edges several inches up the walls and piers.

Do not seal a crawlspace that has fuel burning equipment in it, such as a furnace or water heater, without a pro checking the combustion air supply. Starving an appliance of air can create carbon monoxide.

Basements, Walls, Cracks and Drainage

In a basement, look closely at the walls and floor before you choose a fix.

• White powder or crust is efflorescence, salt left behind as water moves through concrete. Brush it off dry. It shows water is passing through, but it is not mold.
• Thin vertical cracks in poured walls are common from shrinkage. If they leak, a pro can inject them.
• Horizontal cracks, bowing walls or stair step cracks in block can mean soil pressure is pushing the wall in. Have a structural engineer look before any waterproofing company.
• Water at the wall and floor joint usually means water pressure under the slab. The fix is typically an interior drain channel at the footing leading to a sump pump.

Waterproofing paints can help with light dampness on sound walls, but they cannot hold back water under pressure. Expect them to peel if the wall behind them stays wet.

If you plan to finish a basement, get it dry for a full year of seasons first. Framing, carpet and drywall over a damp wall is how many hidden mold problems start. Keep organic materials like cardboard boxes off the floor and away from walls.

Humidity in the Lowest Level

Even a basement with no leaks can be damp in summer from humid air. Set a hygrometer in the space and aim for below 60 percent, and ideally about 50 percent.

• Run a dehumidifier set to about 45 to 50 percent with a hose to a floor drain or sump, so it runs without emptying a bucket.
• Keep basement windows closed on hot, muggy days. Opening them lets humid air in to condense on the cool walls.
• Insulate cold water pipes that sweat and drip.
• Make sure the dryer vents outside and not into the basement.
• Store items on plastic shelving at least a few inches off the floor, in plastic bins rather than cardboard.

In a closed crawlspace, a dehumidifier made for the space or a small conditioned air supply keeps things dry. In a vented crawlspace with a ground cover, check a hygrometer a few times over the summer. If it stays high, talk to a pro about closing it in.

If humidity stays high after all this, go back to the plastic sheet test and the outside drainage. The machine should be a finishing step, not the whole fix.

Mold, Rot and When It Is Beyond DIY

Look at joists, beams and the subfloor with a flashlight. Press a screwdriver into any dark or stained wood. It should resist. If it sinks in easily or the wood crumbles, you have rot, and a contractor needs to assess it.

Light surface mold on framing in a small area can often be cleaned once the moisture is fixed. The EPA’s guidance treats about 10 square feet as a reasonable limit for DIY mold cleanup. Crawlspaces make even small jobs harder, because you work in a tight space with poor air and often with other hazards like rodent droppings, old insulation and wiring.

Bring in a pro for mold spread across many joists, sagging floors above, standing water that returns after rain, sewage from a broken line, or any material you suspect contains asbestos, such as old wrapped pipe insulation or some older floor tiles. When you get bids for drainage or encapsulation, get at least two, ask exactly what each includes, and ask whether the price covers a sump pump, a dehumidifier and a warranty.

Step by Step

1. Measure and Look

   Put a hygrometer in the space for a week and walk it with a flashlight. Note wet spots, stains, crust on walls and any musty odor.

2. Run the Plastic Sheet Test

   Tape a 2 foot square of plastic to a damp wall or floor for 24 to 48 hours. Water under it means seepage and water on top means condensation.

3. Clean and Extend Gutters

   Clear gutters and make sure every downspout drops water at least 4 to 6 feet from the house. Test buried lines with a garden hose.

4. Correct the Grading

   Build up low spots next to the foundation with compacted soil so the ground falls away from the house, about 6 inches over 10 feet.

5. Lay a Crawlspace Ground Cover

   Cover bare soil with polyethylene sheeting of at least 6 mil. Overlap seams at least 6 inches, tape them and run the edges several inches up the walls.

6. Control the Air

   Add a dehumidifier set to 45 to 50 percent with a continuous drain. Keep basement windows closed on humid days and insulate sweating pipes.

7. Check Wood and Cracks

   Probe stained wood with a screwdriver for rot and photograph wall cracks. Call a pro for rot, horizontal cracks or bowing walls.

8. Recheck After Storms

   Look again after the next few heavy rains and compare hygrometer readings. Only move to drainage systems if water still gets in.

Mistakes That Make It Worse

• Paying for interior waterproofing before fixing gutters and grading. Many problems go away with the cheap outside fixes.
• Opening basement windows on humid summer days to air it out. The damp air condenses on cool walls and makes it worse.
• Painting sealer over a wall that is actively wet. It peels and hides the problem.
• Finishing a basement before it has stayed dry for a full year of seasons.

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