How to Fix Foundation Cracks by Yourself with Foam Injection

A foam injection kit fills the crack through the full thickness of the wall, so water can’t follow it in. Done in the right order, it’s an afternoon of work for most homeowners.

What do you need to inject a foundation crack?

A polyurethane foam injection kit, a standard caulk gun and a few cleaning tools. The 10 ft kit has the foam, paste, ports, hose and gloves; you supply the gun and something to mix the paste on.

The two kit sizes differ mainly in how much crack they cover and what comes in the box. Both use the same foam and the same steps.

Each kit's technical data sheet

How do you inject a foundation crack, step by step?

Clean the wall, set the ports, paste over the crack, let it harden, then inject from the bottom up and let the foam cure. These steps follow the 10 ft kit’s instructions, and they match the order the American Concrete Institute (ACI) gives in its crack injection bulletin (opens in a new tab).

1. Clean the crack and the wall around it

   Wire-brush loose concrete, white efflorescence, paint and coatings off a strip 1 to 2 inches wide on each side of the crack, then wipe off the dust. The surface has to be dry. Don’t grind or chisel a groove: ACI recommends wire brushing because mechanical grinders may fill the cracks (opens in a new tab) with dust.

2. Set the ports, lowest one near the floor

   Mark port spots along the crack, spaced about the thickness of the wall apart, which the kit puts at usually about 8 inches. Mix a small batch of paste, dab it on each port base and press the port centered over the crack, starting at the bottom. Keep paste out of the port hole and the crack under it.

3. Paste over the crack

   Trowel the paste along the full length of the crack, 1/8 inch thick and 2 inches wide, and mound extra around each port base. Paste over the surface only; don’t push it into the crack. With the peel-off kit, stick a 4 inch strip of duct tape at the top of the crack first and paste over 2 inches of it, so you have a tab to pull later.

4. Let the paste harden, then flush with water

   Wait until the paste is fingernail hard, but not overnight: the 10 ft kits say about 2 to 4 hours for the epoxy paste and about 1 hour for the peel-off, and the 30 ft kits say about 20 minutes. Then pour 1 to 2 cups of water into the top port. Water should come out of every port below it, which tells you the crack is connected and no port is blocked.

5. Inject from the bottom up

   Load the foam cartridge in the gun, fit the mixing nozzle and push the hose onto it. Press the hose into the lowest port and squeeze slowly at low pressure until foam comes out of the port above, then close the valve, cap the port and move up. Hairline cracks can take 3 to 4 minutes at each port.

6. Let it cure, then remove the ports

   The foam finishes foaming in about 3 or 4 hours. Knock the ports off with a hammer. Leave the epoxy paste in place (it’s paintable) or chisel, sand or grind it off; with the peel-off kit, pull down on the tape tab and the paste and ports come off together.

The two rules that matter most are in steps 4 and 5. ACI says to make sure the paste and port adhesive have properly cured so they can sustain the injection pressures (opens in a new tab), and that higher pressure should be managed with care to prevent a blowout of the paste or ports. The kit’s instructions put it more simply: the secret is patient, low-pressure injection.

ACI’s bulletin describes a heavier paste layer, about 1 inch wide by 3/16 inch thick, for contractor epoxy injection at higher pressure. For these foam kits, follow the kit’s 1/8 inch by 2 inch layer and mound it thicker around the ports, where the pressure is highest.

How long does each stage take?

Plan on about half a day, plus the wait before you knock off the ports. The paste needs 1 to 4 hours to harden depending on which kit you have, the injection itself takes minutes per port, and the foam is done in about 3 or 4 hours.

The kit's instructions don't give a minimum temperature for the foam. Cold slows the peel-off paste: its data sheet lists 1 to 1 1/2 hours to tack-free at 40°F, against 25 to 30 minutes at 73°F. Do the job in a heated basement, and for hairline cracks warm the foam cartridge in a pail of hot tap water for 15 to 20 minutes first.

What mistakes make foundation crack injection fail?

Most failed injections come from rushing the paste, pushing the foam too hard, or sealing the crack shut before the foam gets in. Each one is avoidable.

• Grooving the crack. The kit says paste over the surface, and ACI warns grinding packs dust into the crack. A V-groove also makes the paste sink in instead of bridging.
• Pasting into the crack or the ports. Paste in the crack or a port hole blocks the foam. The water flush in step 4 shows you which port is blocked before you start.
• Injecting too soon. Paste that isn’t fingernail hard lets ports pop off and foam push through. Wait the full time for your kit.
• Waiting overnight. The kit’s instructions say not to. Inject the same day the paste hardens.
• Squeezing hard and fast. High pressure blows out ports and paste. Squeeze slowly and give hairline cracks 3 to 4 minutes per port.
• Skipping the flush. The kit says the water also helps the foam react, so don’t skip it on a dry crack.

If something does go wrong mid-job, the injection troubleshooting guide covers popped ports, foam leaking through the paste, paste that won’t stick and foam that won’t reach the next port.

Is polyurethane injection foundation repair or epoxy injection crack repair better for a DIY fix?

Polyurethane, for almost every homeowner. Polyurethane injection foundation repair seals a leaking or damp crack and stays flexible, while epoxy injection crack repair glues a dry, stable crack back together for strength.

ACI’s bulletin says that when a crack does not compromise the structural integrity (opens in a new tab), polyurethane may be more suitable. Epoxy is a structural repair, and the need for one is a call for an engineer, not a weekend kit. The epoxy vs polyurethane comparison explains the difference in full.

The “epoxy” in an epoxy paste kit is only the surface seal that holds the ports and keeps the foam in the wall. The crack itself is filled with polyurethane foam.

Is a foundation crack repair kit the right fix for your crack?

Yes, if the crack is vertical or diagonal, in a poured concrete wall, 1/4 inch wide or less, with flush edges and not growing. That describes most shrinkage cracks, and those are the ones that leak.

The foam is a water seal. The kit’s maker says the foam is not intended for repairing structural cracks, and the kits are sold for cracks up to 3/8 inch wide, but this site sends anything over 1/4 inch to an engineer first. A crack that wide, or one with offset edges, can mean the wall moved.

Fix the water problem outside too. ACI’s material on leaking cracks notes that hydrophobic foam typically retains volume after curing (opens in a new tab), which is why it keeps sealing between rains, but it can’t stop water from finding a new crack if the gutters dump against the wall. For a crack that’s leaking today, start with the leaking crack guide; for one you’re not sure about, the vertical cracks guide and when to worry help you read it.

If you’re still comparing brands, the best foundation crack repair kit roundup lays out the options, and hydraulic cement covers when a plug works instead.

The sealing after crack repair guide explains why an injected wall doesn’t need a sealer, and which outdoor slabs do.