Basement Crack Sealer for a Leaking Wall Crack: What Stops the Water

Why is water leaking through your basement wall crack?

Hydrostatic pressure: the weight of water sitting in the soil against your wall. When rain or snowmelt soaks the backfill, that water pushes on the wall, and a crack is the easiest way in. That’s why water leaking through basement wall cracks usually shows up after a storm or a thaw.

A Canadian building research digest explains that unless groundwater is drained away, a substantial pressure can then develop, forcing water through joints or cracks in the wall (opens in a new tab). It traces many wet basements to improper grading combined with inadequate disposal of run-off (opens in a new tab) from the roof.

So a leak has two parts: the crack that lets water in, and the water outside that pushes it. Seal the crack from inside and send the water away outside. Doing only one usually isn’t enough.

Which basement crack sealer works on a wet crack?

Polyurethane foam injection. It’s the one basement crack sealer that reacts with water, fills the crack through the full thickness of the wall and stays flexible. Epoxy needs a dry crack, and a hydraulic cement patch only plugs the face.

American Concrete Institute material on leaking cracks says flexible hydrophobic polyurethane foam grout typically retains volume after curing (opens in a new tab) and when the concrete around it dries. Some hydrophilic grouts shrink when they dry out, which matters in a basement where the crack is wet in spring and dry in August.

The foam in our pick is the hydrophobic kind. Its data sheet says it reacts with water, repels the excess and forms a closed cell foam that water can’t pass, and that it bonds to concrete wet or dry.

American Concrete Institute repair bulletin RAP-1 and material on leaking cracks, Bureau of Reclamation research, and the foam kit's data sheet

Rigid fills lose in a crack that moves. Bureau of Reclamation engineers found that rigid repair materials either crack adjacent concrete or can crack at the repair material (opens in a new tab) if there’s movement, while polyurethane grouts are flexible and can allow for some minor movement.

Epoxy’s limit is moisture. ACI’s epoxy bulletin says that if a crack is damp and can’t be dried out, an epoxy tolerant to moisture should be considered (opens in a new tab). The epoxy vs polyurethane comparison and the hydraulic cement guide cover the other two options in detail.

What do you need to seal a leaking crack?

A polyurethane foam injection kit, a standard caulking gun and a way to dry the wall face. The kit brings the foam, the surface sealer, ports and the injection hose.

The peel-off kit suits a wall that will stay bare, because the surface sealer pulls off in one strip once the foam has cured. The kit’s instructions list what’s in the box: two cartridges of foam, two of peel-off sealer, 15 ports, mixing nozzles, an injection hose with a shut-off valve, a wire brush, a trowel, gloves and safety glasses.

If you’d rather leave the surface sealer on the wall, the peel-off vs epoxy paste comparison explains the other version.

How to seal foundation cracks that leak

Wait for the leak to stop, dry the face of the wall, set ports and seal over the crack, flush it with water, then inject foam from the bottom up. This is the method in the peel-off kit’s instructions, condensed. The foundation crack injection guide walks through every step in detail.

1. Wait for the leak to stop

   The surface sealer won’t stick to a wet wall. The kit’s instructions say that if the surface is wet, wait a few days until it dries, or dry it with a hot air gun, hair dryer or oil-free compressed air. The inside of the crack can stay damp, since you’ll flush it with water anyway.

2. Check the crack

   Measure the widest point and run your hand across it. The kit is rated for cracks up to 3/8 inch, but a wall crack 1/4 inch or wider, or one with a step between the sides, should get an engineer’s look first.

3. Clean the face

   Wire brush 1 to 2 inches on each side of the crack to remove loose concrete, efflorescence, paint or coating, then wipe off the dust.

4. Set the ports

   Space the ports the thickness of the wall apart, usually about 8 inches, starting at the bottom of the crack. Stick each one down with a dab of mixed sealer, keeping the sealer out of the port’s opening.

5. Seal over the crack

   Put the duct tape tab at the top of the crack. Trowel the sealer along the crack about 1/8 inch thick and 2 inches wide, mounded around each port, and cover 2 inches of the tape. Mix only a third of a tube at a time; an 8 foot crack takes both tubes.

6. Let it harden

   Wait about an hour, until the sealer is fingernail hard. The instructions don’t recommend waiting overnight.

7. Flush with water

   Pour 1 to 2 cups of water into the top port. Water should come out of every port below it, which shows the crack is connected and the ports aren’t blocked. The water also helps the foam react.

8. Inject from the bottom up

   Start at the lowest port and inject slowly at low pressure until foam comes out of the port above. Close the valve, cap the port and move up. The instructions say hairline cracks take 3 to 4 minutes per port.

9. Peel it off

   After about 3 to 4 hours, pull down on the tape tab to strip off the sealer and ports.

For a hairline crack, the instructions suggest warming the foam cartridge in a pail of hot tap water for 15 to 20 minutes first so it flows more easily. If foam leaks around a port or a port won’t take foam, the injection troubleshooting guide has the fixes.

What if water is running out of the crack right now?

Wait for it to stop before you inject. The peel-off sealer needs a dry surface, and the kit’s product page says it won’t cure in the presence of moisture.

While you wait, go outside and look above the crack. A downspout emptying next to the wall or a low spot holding water there is often what’s feeding the leak, and fixing it may slow the water enough to work.

If the water never stops, the foam’s own data sheet describes the professional method: where high water flows are encountered, the crack face is first sealed with a fast-setting hydraulic cement or epoxy gel, then injected. That’s a job for a contractor with injection equipment, and the hydraulic cement guide explains where a plug like that fits.

How long does a crack injection take to cure?

About an hour for the surface sealer to harden, a few minutes per port to inject, and 3 to 4 hours before the seal comes off. Plan on most of a day for one crack.

The kit's instructions don't give a minimum temperature for the foam. Cold slows the surface sealer, so work in a heated basement if you can.

What keeps a sealed crack dry?

Getting the water away from the wall: soil sloping away from the house, clean gutters and downspouts that empty well away from the foundation. The National Ready Mixed Concrete Association says the drainage around the structure should be checked and corrected (opens in a new tab) before repairing leaking cracks.

The same NRMCA sheet recommends a finished grade sloping 1/2 to 1 inch per foot for at least 8 to 10 feet from the foundation, and says to keep it that way as the backfill settles. The Canadian digest adds that roof water should be carried away using extensions to the downspouts and splash blocks (opens in a new tab).

Check these on the outside of the leaking wall:

• Grade. Lay a straight board and a level on the soil against the wall. It should fall away from the house; fill low spots and sunken backfill with soil.
• Gutters. Clear them, and watch them in a heavy rain for overflow at the corners.
• Downspouts. Extend each one away from the wall onto a splash block or into a pipe. The digest warns roof water should not be led to the footing drain (opens in a new tab).
• Paving. A patio, walk or driveway that slopes toward the wall sends water straight to it. The gap between the driveway and the house needs sealing too.

What mistakes make a leaking crack come back?

Most repeat leaks come from patching the surface, sealing a wet face or ignoring the water outside. Each one has a simple fix.

• Patching the surface. Hydraulic cement or caulk smeared over the crack covers the inside face only. Water still fills the crack behind it and pushes the patch off.
• Sealing a wet wall. The surface sealer won’t cure on a damp face, so foam squeezes out around it. Dry the face first.
• Injecting too fast. The kit’s instructions call patient, low-pressure injection the secret to a good repair. Rushing blows foam out the next port before the crack is full.
• Skipping the flush. The water you pour in tests the ports and gives the foam water to react with.
• Using epoxy in a wet crack. Standard epoxy needs a dry crack. Foam is the right material for a leak.
• Leaving the downspout where it was. The crack can be sealed perfectly and the next weak spot starts leaking.
• Injecting a block wall. The kit is made for poured concrete walls. Hollow block has nowhere for the foam to stop.

When is a leaking crack a job for an engineer?

When water is coming through a horizontal crack or a wall that bows inward. Those usually mean soil pressure is pushing the wall, and a crack sealer doesn’t push back. The kit’s product page says it isn’t intended for structural cracks.

The pressure behind a leak can also load the wall. The NRMCA notes that cracks can grow if the walls are not properly designed, due to the continued horizontal pressures (opens in a new tab) applied by soils, water and temperature.

Have a structural engineer look before you seal if:

• The leak is through a horizontal crack, or the wall bows, leans or bulges.
• The crack is 1/4 inch or wider, or one side sits farther out than the other.
• It’s a stair-step crack in a block wall.
• A crack you sealed has reopened or keeps getting wider.

The horizontal crack guide and the bowing wall guide show how to check. For a thin vertical crack that’s stable and leaking, this page and the vertical cracks guide are the right place, and the foundation cracks hub covers every other crack type.