Actively leaking crack or hole
Water coming through right now? Hydraulic cement sets in minutes, even against running water.
It's rigid, so it can crack again if the wall moves.
Our pick: Quikrete Hydraulic Water-Stop Cement
Caulk by room
Basement & foundation
Going deep… below your house
The basement is where caulk faces its most Herculean challenge: hydrostatic pressure, efflorescence, freeze-thaw cycles, and the general ambitions of water to be somewhere it shouldn't. For active leaks, hydraulic cement goes in first. For cracks after the water's stopped, polyurethane crack fillers do the heavy lifting. For joints and penetrations, use a waterproofing-grade sealant.
Your guideFlavio, wet-zone specialist
Where to caulk in the basement & foundation
Every joint in the room, what goes in it, and why. The color bar matches the formulation.
Dry, non-structural wall cracks
Polyurethane sealant stays flexible and bonds to concrete. Leaky poured-wall cracks are often sealed with injection kits.
Our pick: Sikaflex-1a
Pipe and wire penetrations
Where a pipe passes through the foundation, seal it on the outside where you can and on the inside as backup.
Our pick: Sikaflex-1a, Soudal Fix ALL
Rim joist and sill plate
A big source of drafts. Seal gaps between the sill, the foundation, and the rim joist; foam works for the bigger ones.
Our pick: DAP Alex Plus
Penetrations between floors
Many codes require fire-rated sealant or fireblocking where pipes and wires pass between floors.
Our pick: 3M Fire Barrier CP 25WB+
Basement windows and wells
Seal the frame to the foundation, and keep the well clear so water drains away.
Our pick: Sikaflex-1a
Seal a dry foundation crack
Rule out a structural problem
Hairline vertical shrinkage cracks are common. Horizontal cracks, stair-steps with displacement, or anything wider than about 1/4 inch or growing: call a structural engineer first.
Clean it out
Wire brush, vacuum, and scrub off efflorescence (the white chalky stuff).
Make sure it's dry
Polyurethane won't stick to a wet crack. If it's leaking, plug it with hydraulic cement first.
Backer rod if it's deep
Push rod into wider cracks so the sealant is the right depth.
Fill and tool
Press polyurethane into the crack, tool it flat, and let it cure fully, which can take days in a cool basement.
Mistakes we see
- Expecting caulk to hold back water under pressure.
- Sealing the floor-to-wall joint over an interior drainage system. That gap may be how the water is supposed to reach the drain.
- Ignoring structural cracks. Sealant hides the symptom.
- Using acrylic on below-grade concrete.
How long it lasts
- Check it
- After heavy rain and each spring thaw.
- Redo when
- Dampness, new cracks, or efflorescence returns.
- Typical life
- Depends on the wall. Moving cracks will keep moving.
Basement & foundation questions
Is a crack in my foundation serious?
Thin vertical cracks from concrete shrinkage are common and usually cosmetic. Horizontal cracks, bowing walls, cracks wider than about 1/4 inch, or cracks that grow are a job for a structural engineer.
Will sealing cracks fix radon?
Sealing helps, but the EPA says sealing alone hasn't been shown to lower radon significantly or consistently. Test, and use a proper mitigation system if levels are high.
Hydraulic cement or caulk?
Hydraulic cement for active leaks and holes; flexible sealant for dry cracks and joints that move.
More answers in the full FAQ, or let the Caulk Finder pick for you.
