Water or moisture in basements comes from two sources. One source is indoor humidity that condenses on cold surfaces, much like water droplets form on a cold drink on a humid day. The other is water—or water vapor—that comes from outside. Rainwater, melting snow or groundwater can saturate the soil around your foundation and leak in. Water can leak through cracks, or it can penetrate porous concrete or masonry walls in the form of water vapor. To figure out what’s causing the problem, tape aluminum foil to your basement wall and inspect it a few days later. Moisture on the outside surface of the foil indicates high indoor humidity. Moisture behind the foil means moisture is leaking through the walls. Eliminating the sources of humid air will help dry out your basement. Seal leaky dryer vents with foil tape to prevent unwanted humid air from entering your basement. Don’t just use duct tape; it’ll eventually fall off. Add a vent fan to your basement bathroom and make sure your family turns it on during showers. Keep your basement windows closed during humid weather. And if you’re still getting condensation on cool surfaces, run a dehumidifier to lower the indoor humidity. Insulate exterior walls to prevent condensation. In cold climates, insulating basement walls also saves energy and reduces your heating bill. But don’t cover the walls with insulation if water is leaking in from outside; you’ll just create a potential mold problem. Holes and cracks in your foundation can let moisture and water seep into your basement. Plugging them probably won’t solve basement leaks, but it’ll help. Hydraulic cement works great for patching holes in a foundation because it can set up even under water, and it expands as it sets to seal the hole and lock the plug in place. Use a cold chisel or an angle grinder fitted with a masonry-cutting disc or diamond blade to enlarge the hole or crack into an inverted “V,” with the narrow part of the “V” on the surface of the wall. Then follow the package instructions for mixing and using the hydraulic cement. To be effective, these coatings must be applied to bare concrete or masonry walls. Start by removing loose material with a wire brush. Then clean off any white powdery “efflorescence” with masonry cleaner. Follow the safety and application instructions carefully. A common mistake when using masonry waterproofing products is to spread them too thin. The goal is to fill every pinhole to create a continuous waterproofing membrane. Brush the coating in all directions to completely fill every pinhole. Add a second coat after the first dries. The best permanent fix for chronic basement leaks is to install drainage tubing below the basement floor that’s connected to a sump basket and pump. You can install a system like this yourself, but breaking out the concrete floor, burying the tubing, and patching the floor is a lot of backbreaking work. Materials to do an average basement will cost $600 to $1,000. Expect to spend $3,000 to $8,000 for a professionally installed system in a standard-size basement.