Mold. Mildew. Rust. Rot. Yuck! A damp basement brings all these plagues into your house, and more. The excess moisture can damage tools and furnishings, make prime workshop or rec room space unusable, and even endanger your health. Studies show that the spores released by basement mold can trigger allergy attacks and cause respiratory problems.

Fortunately, it's easy to handle many of the things that cause damp basements. The two most common causes are condensation inside and poor drainage outside. The quick way to figure out which one you have is to perform this simple three-step test.
 

1. Tape a 24-inch-long piece of aluminum foil to the wall in an area where you think dampness may be accumulating or coming into your basement. Run duct tape around all four sides, sealing the foil against the wall.
2. Leave the foil in place for at least 48 hours.
3. Remove the foil and see which side is wet.

If the side facing the wall is wet, water is penetrating through the wall from outside. If the side facing the room is wet, the problem is condensation. Let's deal with condensation first, since it's easier to fix.

Controlling Condensation
Condensation happens when moisture condenses out of the air and collects on the coldest surfaces in the basement, including the walls. You're likely to notice it most during humid spring and summer months. There are two fixes for basement condensation problems: insulation and dehumidification.

Start by insulating surfaces that are likely to attract condensation—cold water pipes, bare air conditioning ducts and well tanks. Cover pipes with lengths of preformed foam pipe insulation, which is available to fit different pipe diameters. Wrap everything else with vinyl-faced fiberglass insulation. This comes in blanket form (like the insulation available for water heaters) and in rolls. The facing is important: it acts as a barrier and keeps moisture from soaking the fiberglass. When everything is wrapped, seal the open seams with duct tape.

The second part of this fix? Install a dehumidifier to remove excess moisture from the air. Locate the unit in the dampest part of the basement, at least 12 inches from walls or other obstructions that might interfere with airflow. For maximum dehumidification, buy a model that can run continuously during periods of high humidity, and connect its collection reservoir to a floor drain. Otherwise, the dehumidifier will turn off automatically when its collection pan or reservoir is full of water.

Plugging the Leaks
What if gutters, downspouts and regrading don't cure basement moisture problems? Then you may have poor drainage below ground level, or a water source too close to the foundation. Since outside excavation and waterproofing is work you'll want a professional to do, let's focus on what you can do inside to stop leaks and keep water at bay.

Sealing the inside involves repairing crack and plugging any holes in the basement walls and floor with hydraulic cement, and then waterproofing the walls. Here's how to proceed.

To repair cracks and holes:
 

1. Locate all large cracks and holes. Check the walls, the seam between the walls and floor, and the floor itself.
2. Widen the cracks and holes. Use a two-pound hand sledge and a masonry chisel to enlarge the openings. This improves the bond between the hydraulic cement and the existing concrete. Undercut the edges to create dovetail-shaped openings. These will hold cement better than V-shaped trenches.
3. Clean the openings. Remove loose concrete from the cracks and holes, and scrub them with a wire brush and water.
4. Fill the cracks and holes. Hydraulic cement is incredible stuff. It expands and tightly seals openings, and can even stop flowing water. But it dries quickly—between 5 and 15 minutes, depending on the manufacturer. So, only mix as much as you can use during that time. To work best the mix should have a buttery thickness. That's about 4 1/2 parts cement to 1 part water.

Use a trowel to layer the cement into the cracks and holes. Feather the edges so they're flush with the surface. Note: Hydraulic cement is caustic and the dust can irritate. Always wear gloves and a dust mask when working with it. And let the cement 1375>cure for several days before moving on to the next step.

To waterproof the basement walls:

1. Prepare the surface. Wash the walls to remove mold, grease, dust, dirt or other debris that will interfere with the waterproofing. Use a mild detergent and add a capful of bleach to the water to kill mold. If you see efflorescence on the wall or floor—a white, powdery crystal-like substance that often forms on concrete—remove it with muriatic acid and a wire brush. Read the label to determine whether and how much to dilute the acid and be sure to follow this safety rule: always add acid to water, not the other way around—so you don't get a chemical reaction. Muriatic acid is highly corrosive, so when you work with it, wear goggles, heavy-duty rubber gloves and long sleeves, and keep the basement ventilated as well as possible.
2. Seal the seams. With a masonry sealant seal the seam between the floor and the walls.
3. Apply the waterproofing cement. In a large bucket or cement-mixing pan, mix a batch of masonry waterproofing cement to the consistency of thick pancake batter—about 20 pounds of cement to 1 or 1 1/2 gallons of water. Let the mix set for 20 minutes before applying so all the cement dissolves. (Don't worry; the mix won't cure for two hours.) Then stir again and use a masonry brush to coat the walls. Apply at least two coats.

Kevin Ireland was formerly managing editor at both American Woodworker magazine and Rodale Woodworking and Do-It-Yourself Books. He was raised in a fixer-upper and has rebuilt three homes in the last 16 years.