Quick answer
If your bare concrete or block basement wall is damp but has no water running through it, buy a masonry waterproofing paint with a stated pressure rating, and for most basements that is the KILZ Basement and Masonry Waterproofing Paint, rated 4.7 stars across 1,262 Amazon ratings. Its listing states 12 PSI, a 4-hour recoat and a 7 to 14 day full cure. Fill cracks with hydraulic cement and scrub off white salt deposits before the first coat. No paint stops water pouring through a crack, a high water table or soil that slopes toward the house, and KILZ says its paint is not for walls with active leaks.
Buyer's Guide
What to know before you buy.
What Waterproofing Paint Can and Cannot Do
A masonry waterproofer fills the pores of bare concrete and block so less dampness reaches the room. That helps a wall that looks dark, smells musty, or shows a faint tide line after rain. It is not a repair for water you can see moving. A crack that runs during storms, water at the joint where the wall meets the floor, and water rising through the slab are all driven by pressure outside the wall. A coat on the inside face, the side waterproofers call the negative side, is working against that push. KILZ prints the limit on its own listing: not for walls with active water leaks. So start outside. EPA advises cleaning and repairing gutters regularly and making sure the ground slopes away from the foundation so water does not collect around it. Carry downspouts 4 to 6 ft out, then judge how much damp is left.
Test the Wall First: Seepage or Condensation
Damp walls have two very different causes, and paint only helps one of them. Tape a square of clear plastic, about 2 ft across, to the bare wall with all four edges sealed, and leave it for a day or two. Beads of water on the wall side of the plastic mean moisture is coming through the masonry, which is the job a waterproofer is made for. Water on the room side means humid air is condensing on a cool wall. Paint will not change that; a dehumidifier will. EPA advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and a basement that runs above that range in summer will sweat on any wall, painted or not. Run the test in the season when the wall looks worst, and repeat it after a heavy rain.
What a 12 PSI Rating Means in Feet of Water
Two listings on this page state 12 psi, KILZ and INSL-X. Water exerts about 0.43 psi for each foot of depth, so 12 psi equals a water column close to 28 ft tall. A wall that sits 8 ft below grade, with soil saturated right to the surface, sees roughly 3.5 psi at the bottom. On paper, the rating is rarely the limit. In practice, a coating on the inside of a wall usually fails at its bond: salts and water moving through the masonry lift a coat that went onto dust, old paint or white deposits, and a crack that moves splits a rigid film. Neither listing says how its figure was measured. Treat a stated rating as a fair way to compare two paints, and treat the prep as the reason one of them is still on the wall in five years.
Prep Decides Whether the Coating Stays On
Start with a stiff wire brush on every white, powdery patch. That deposit is efflorescence, mineral salt left behind when water passing through the wall evaporates at the surface, and a coating laid over it lets go along with the salt. A splash of water tells it from mold: salt dissolves, mold smears. Strip old paint that flakes, because a waterproofer has to bond to the masonry, not to a failing coat. Chisel out loose mortar and patch holes and cracks with hydraulic cement, which KILZ asks for below grade. Wash and rinse the wall, then let it dry for several days with a fan or dehumidifier running. Mold gets cleaned off, never coated: EPA's guidance is to clean up mold and dry the surface before painting, because paint applied over moldy surfaces is likely to peel.
How Much Paint a Basement Takes
Measure the walls, not the floor. Add the lengths of all four walls, multiply by the height, then subtract doors and windows. A 30 by 40 ft basement with 8 ft walls has 140 ft of wall, or 1,120 sq ft. At the Ames listing's 100 sq ft per gallon per coat, two coats need about 22 gallons. Liquid Rubber's stated 50 sq ft per gallon for interior walls lands on the same total. KILZ, INSL-X and the Drylok listing give no coverage, so read it off the can before ordering. Bare block soaks up more than smooth poured concrete, and the first coat on any bare masonry uses more than the second, so buy about 10 percent extra. Keep one sealed can for touch-ups; a scuff from a moved shelf is easier to fix with the same batch.
Who Should Not Paint the Wall
Paint is the wrong buy if water runs in during storms through a crack or at the floor joint, if water comes up through the slab, or if the sump keeps cycling for days after rain, which points to a high water table. Those need water handled outside and under the floor: drain tile and a sump pump, or exterior waterproofing by a contractor. A horizontal crack, a wall bowing inward, or a stair-step crack in block is a structural question for an engineer, and paint would only hide the evidence. Finished basements are out as well, because the coating has to touch bare masonry and framing and drywall cover it. Renters should report seepage to the landlord rather than paint over it. And if the yard slopes toward the house, regrading will do more than any can.
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