Control Basement Moisture

Advice on how to control moisture in the basement.


Water in a basement can be caused by something as simple as clogged downspouts to a more complicated scenario, such as a rising water table. Fortunately, most cures for wet basements aren't costly. Here's a look at the possible problems and solutions.

Condensation or Leaks?
When warm air comes in contact with cool basement walls and floors as well as plumbing pipes, condensation can occur. If water problems seem to clear up in summer when windows and doors are closed and the air-conditioner is running, condensation could be the culprit. Water collecting on the floor or dampness on walls or pipes isn't always condensation, however, but could be signs of leaks or seepage. To determine the source of the water, tape squares of aluminum foil to different spots on the basement floor and walls, using duct tapes to secure the edges. Leave the foil in place for several days. Droplets collecting on the underside of the foil indicate water seeping in from outside; droplets atop the foil point to condensation.

Excess humidity—which can be elevated by such internal sources as a basement shower, washing machine, or unvented dryer—can lead to damp walls, dripping pipes, and surfaces covered in mildew. To alleviate this condensation, improve ventilation in the basement by installing ventilating fans or opening windows during mild weather. You can also seal interior walls, add a subfloor system, and install a dehumidifier.

For more information, go to Dehumidifiers and Sump Pumps

If condensation is forming on pipes, cover them with adhesive-backed insulating tape or foam sleeve insulation—both are affordable solutions and available at home improvement stores.

Water-soaked soil pressing on foundation walls is known as hydrostatic pressure. In some cases, the pressure is severe enough to crack concrete. While small cracks won't jeopardize the integrity of the foundation, they do provide an easy path for water to get inside. Because both poured and block concrete walls are porous, they can wick water into the basement as well.

To solve these problems, route water away from the house so it doesn't collect around the foundation and seep inside. Make sure that the driveway, patios, sidewalks, and exposed earth slope away from the house. The grade should drop 2 inches vertically within 1 foot from the house. Continue this rate of decline for at least 3 feet to create a slope that drops 6 inches.

Another way to ensure that water doesn't soak in around the foundation is to check that gutters and downspouts are clear of debris and in good condition, with no sagging spots that can allow water to overflow. Add extensions, if necessary, to make sure that water is carried at least 5 feet away from the foundation.

Button Up Bows
After a house is built, soil moves and settles around its foundation, adding pressure to even the sturdiest basement walls. Minor cracks don't indicate a weak foundation. If walls are bowed from pressure, it's possible to straighten them—a process that may call for steel bracing. To find out if your wall requires bracing, consult a licensed home inspector or an engineer. A licensed building or remodeling contractor can do the work. Search directories for "Foundation Contractors."

Repair Cracks
Use a cold chisel and a hammer to chisel minor cracks and holes so they're wider at the bottom than at the top. This helps prevent the patch from popping out after it sets. Make the hole at least 1/2 inch deep. Then vacuum out any dust and concrete fragments.

Mix hydraulic cement in a bucket, adding water to the dry mix until it has a puttylike consistency. Then work it by hand. When plugging a hole, roll the mixture into the shape of a plug. For a crack, roll the hydraulic cement into a long, snakelike shape. Press the material into the opening. Keep working and applying pressure to the patch to make sure it fills every tiny crevice. Most cements will set even if water is leaking through the hole at the time of the repair (in which case the water should stop running). Apply pressure to the patch for several minutes to allow it to set.

There are two types of interior drainage systems, which are also called dewatering systems. One requires a 1-foot-wide channel cut into the perimeter of the basement floor, all the way through the concrete. Perforated plastic drainpipe is fitted into the channel and covered with gravel. New concrete is then poured over the gravel to floor level. A slight space is left between the floor and the wall to allow weeping walls to drain directly into the channel. The drainpipe leads to a reservoir equipped with a sump pump. Excess water drains into the reservoir and is drawn outside the house by the sump pump. Because this type of dewatering system is installed below floor level, it's sometimes effective in preventing problems caused by rising water tables.

The second type of dewatering system does not need an opening in the basement floor. Instead, plastic channels are affixed to the basement walls with waterproof glue where the walls meet the floor, much like baseboard trim. The channels direct excess water to a sump pump. Though adding plastic channels is less costly than opening the basement floor, it's not as effective at intercepting rising water tables as the below-floor system.

When you occasionally discover damp spots in the basement, an interior cement-base sealer can help. Unfortunately, sealers work only on bare concrete, so if your block or poured wall has been previously painted, you'll have to have the basement wall sealed on the exterior of the house.

To apply concrete sealer inside on bare concrete, clean away dirt, grease, and dust from the walls using a stiff-bristle brush. Thoroughly wet the wall with a fine mist from a garden hose. Mix liquid and powder components of a cement-base sealer according to manufacturer's directions and apply with a stiff brush. As you brush, fill in all the pores in the wall. Go over cracks several times, if necessary, to fill them. If a crack is too large to fill with sealer, fill it first with hydraulic cement. Some sealers have to stay wet for several days to ensure bonding. Apply a second coat, if necessary.

Existing homes with extreme moisture problems may require exterior basement waterproofing—a costly proposition because dirt must be excavated from the foundation to allow sealers and/or membranes to be applied to the walls. If you're building a new home, be sure to have exterior waterproofing applied before the contractor backfills soil.

Investigate these potential problem areas around the foundation as well as the basement interior:

  1. Gutters clogged and downspouts too short.
  2. Windows positioned so close to soil level that water can enter.
  3. Inadequate grading of soil near foundation walls.
  4. High water table engulfing foundation.
  5. Cracked walls.
  6. Floor slab cracked by hydrostatic pressure.

To resolve the problems, try these suggestions:

  • Clean gutters and extend downspouts (or add splash blocks). Debris collecting inside gutters can dam downspouts, causing water to overflow and end up next to the foundation. Clean them in the spring and fall to avoid problems. Make sure all gutters are straight and slope gently toward downspout locations. Sagging gutters trap water in low spots and cause overflowing. Downspouts should extend at least 5 feet from foundation walls. Lengthen short downspouts or place concrete splash blocks beneath downspout openings to direct water away from foundation walls.
  • Install window wells. According to building codes, the outside bottom edge of a basement window should be at least 6 inches above the soil to prevent leakage from groundwater and to keep wood framing members from rotting.
  • Add or remove soil to achieve the proper-grade slope. To prevent moisture damage to the basement interior, soil should slope downward 6 vertical inches for a distance of 3 horizontal feet.
  • Install a perimeter drainage system to carry away excess water. An interior drainage system, or dewatering system, intercepts water where foundation walls meet the floor. From that point, water is directed to a sump pump so it can be removed.
  • Fill cracks in walls and apply sealer. The pressure of water-soaked soil cracks basement walls. The damage usually occurs during colder months when water-laden soil freezes and expands, pushing against basement walls with enough force to crack masonry. If you see a crack line running horizontally across the basement wall, it may be because builders poured part of the wall and allowed it to harden before pouring the rest. In this case, have the crack inspected by an engineer.
  • Fill cracks in the floor (use the same techniques and materials for filling cracks in walls and apply sealer). First gauge the severity of the cracks, and follow the guidelines for fixing basement flooring problems to determine the best remedy.

For more information, go to Dehumidifiers and Sump Pumps

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