Concrete is porous. No matter how solid it appears, there will be minute fissures that will accommodate moisture. Houses age and settle. That means concrete basement walls may crack, allowing moisture in. The ground also shifts and settles, creating holes or pockets that can hold water, which can seep through concrete. There is no one absolute foolproof solution for basement moisture, but there are things to look for and ways to correct many moisture problems.
Find the Source
Identify the source of the moisture. Look for obvious holes or cracks around any areas with puddles of water or just damp spots. Examine the area both inside and outside; sometimes the moisture problem may be outside where water is being funneled down an outside wall. Make sure the ground slopes away from the foundation all the way around the house. Fill any obvious holes with dirt or mortar if there is a puddling gap between a sidewalk and a foundation wall. Use extenders on downspouts to move water farther away from the foundation.
Make a Moisture Test
Test for the source if it is not obvious. Sometimes basement dampness can have an internal origin. Tape a 12-inch square of aluminum foil to a damp wall, sealing it around the edges as tightly as possible. Leave it for a couple of days and examine it; if it is wet on the outside, moisture is seeping through the wall, if only the inside is wet, moisture is coming from condensation within the basement. Seal plumbing fixtures, water heaters and any other internal source of water. Make sure all floor drains are open and stay dry.
Seal All Cracks and Seams
Seal every crack or hole in the concrete walls and caulk around any windows or other openings; even heating vents or plumbing pipes can allow small amounts of water to seep in. Use concrete patching compound on areas that are dry. Get a special hydraulic patching material that hardens in water for any areas that have obvious running leaks or constant dampness. This material can be spread like plaster with a mason's trowel for larger damp areas. You can increase the holding power of the patch by installing "key" that keeps the hardened patch material in place.Undercut the edges of small cracks with a rotary tool equipped with a diamond wheel accessory. Undercut larger cracks with a grinder or chisel hammer.
Investigate all Waterproofing Options
Investigate various waterproofing material options. These range from solid membranes, usually some type of plastic that is installed tightly against the wall, to liquid membranes, similar substances applied as liquids, to cement-type waterproofing paints. Check recommendations of all manufacturers and choose the system that best applies to your basement. Another option is a silica-based sealer, a clear liquid which penetrates deep into the pores of the concrete and hardens. It may then be covered with a waterproofing paint for double protection.