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Seal a cracked basement wall from the inside by injecting polyurethane foam, which fills the crack through the full thickness of a poured wall, so you don't have to dig outside. It works on vertical and diagonal cracks up to 1/4 inch with flush edges, wet or dry. Digging out and waterproofing the outside is a contractor job, and horizontal, bowing or offset cracks go to an engineer first.
Before you start
An inside repair seals a crack. It doesn't hold a wall in place. Stop and get the wall looked at if any of these fit.
- The crack is horizontal, or the wall bows or leans in
- It's a stair-step crack along the mortar joints of a block wall
- One side of the crack sits farther in or out than the other
- The crack is wider than 1/4 inch, or it's getting wider
If any of these fit, find out who to call first: a structural engineer or a repair contractor.
What do you need for basement wall crack repair?
A polyurethane foam injection kit, a caulk gun and a few cleaning tools. The 10 ft kit covers one crack up to about 10 feet long and includes the ports, two foam cartridges and an epoxy paste to seal the surface.
- Standard caulk gun
- Wire brush or wire wheel on a drill
- Shop vacuum
- Hair dryer or heat gun, if the wall is damp
- Putty knife
- 1 to 2 cups of water and a small cup
- Hammer, to knock off the ports
- Gloves and safety glasses
Cracks add up to more than 10 feet? The kit sizing guide explains when to step up to the 30 ft kit, and epoxy vs polyurethane explains why foam is the DIY pick.
How do you fix a cracked basement wall from the inside?
Glue ports over the crack, seal the face with paste, then inject foam from the bottom port up until the crack is full. The foam travels through the crack to the outside face of the wall, so one repair from the basement seals the whole crack.
Here’s how to fix a cracked basement wall from the inside, summarized from the 10 ft kit’s instructions:
Clean 1 to 2 inches each side
Wire brush off paint, coatings, efflorescence and loose concrete, then vacuum. The surface must be dry where the ports and paste go.Place the ports
Center the ports on the crack, spaced about the wall’s thickness apart (usually about 8 inches). Start with the one closest to the floor, and don’t cover the port openings.Seal the surface
Spread the paste about 1/8 inch thick and 2 inches wide along the crack and mounded around each port. Keep it out of the crack itself.Wait for the paste
Give it about 2 to 4 hours, until it’s hard to a fingernail. The instructions don’t recommend waiting overnight.Flush with water
Pour 1 to 2 cups of water into the top port. It should come out of every port below it, and the moisture helps the foam react.Inject from the bottom up
Inject slowly at low pressure into the lowest port until foam shows at the next port up. Close and cap it, then move up one port at a time.Remove the ports
After about 3 or 4 hours, when foaming is done, knock the ports off with a hammer.
A hairline crack takes patience: the instructions allow 3 to 4 minutes per port. The full method, with what to do when foam leaks through the paste, is in the foundation crack injection guide.
Should you repair a basement wall crack from the inside or the outside?
From the inside, for one crack or a few in an otherwise dry wall. Injection fills the crack from face to face, so it seals the outside as well, with no digging. Outside repair makes sense when the whole wall leaks, the drain tile has failed, or there are too many cracks to inject.
| Inside injection | Outside excavation | |
|---|---|---|
| Who does it | You, or an injection contractor | A waterproofing or foundation contractor |
| Digging | None | Down to the footing along the wall |
| What it fixes | The crack, through the full wall thickness | The whole wall face, plus drain tile if it's replaced |
| Best for | One or a few cracks in a poured wall | Many leaks, failed drain tile, or a wall you can't reach inside |
| What it won't fix | A wall that's moving, or water pooling outside | A wall that's moving |
The ACI’s guide to leaking cracks says both polyurethane and epoxy can be injected from the interior or exterior (opens in a new tab), while a flexible membrane on the outside face may require excavation. That’s the honest trade: inside injection seals cracks, outside work waterproofs walls.
The NRMCA says positive side waterproofing (exterior) is generally better (opens in a new tab) than inside coatings for keeping water out, which is why a painted-on interior sealer isn’t a fix for a leaking crack. Injection is different, because the foam reaches the outside face through the crack.
Why not just patch it with hydraulic cement or caulk?
Because a surface patch covers the face of the crack and leaves the inside open. Water from the soil follows the crack through the wall and around the patch. A rigid patch also cracks again when the wall flexes with the seasons.
Hydraulic cement does have a job here. The foam’s data sheet says to plug a wide crack or one with high water flow with hydraulic cement or epoxy gel first, so the foam stays in the crack while it cures. The hydraulic cement guide covers when to use it.
How long does a basement wall crack repair take?
Most of a day, and most of it is waiting. The paste needs 2 to 4 hours before you inject, and the foam needs about 3 to 4 hours before the ports come off.
| Material | Apply between | Light use | Full cure | Source |
|---|---|---|---|---|
| Epoxy paste surface seal | Dry surface, 6 to 7 minute working time | Inject after 2 to 4 hours, when fingernail hard | 1 to 3 days | Kit instructions, product page and paste data sheet |
| Polyurethane foam | Crack wet or dry, after a water flush | Ports off after about 3 to 4 hours | Foaming done in about 3 to 4 hours | Kit instructions |
The kit's instructions and data sheets don't list a temperature range. A cold basement slows any resin, so check the cartridge labels before you start.
What mistakes make a basement wall repair fail?
Most failures happen at the surface seal, not in the crack. If the paste doesn’t stick or blocks the ports, the foam leaks out the face or never gets in.
- Paste on a damp or dirty wall. The instructions say the surface must be dry and free of paint, coatings and efflorescence for 1 to 2 inches either side.
- Paste in the crack or over the ports. Keep paste on the surface and clear of the port openings, or the foam has nowhere to go.
- Waiting overnight to inject. The instructions don’t recommend it. Inject once the paste is fingernail hard.
- Skipping the water flush. The water shows the crack is open from port to port, and it helps the foam react.
- Injecting too fast. Go slowly at low pressure. The ACI warns that too much pressure can blow out the cap seal or ports (opens in a new tab).
- Starting at the top. Vertical cracks get injected from the bottom port up.
- Ignoring the water outside. The NRMCA says to check and correct drainage (opens in a new tab) before repairing leaking cracks, or the water finds the next crack.
If a repair already went wrong, see injection troubleshooting. If water is coming through right now, start with the leaking crack guide.
Does concrete wall crack repair work on block walls?
Not with an injection kit. The foam kits’ product pages say they’re made for poured concrete foundations, and in a block wall the foam disappears into the hollow cores instead of filling the crack. Block walls crack along the mortar joints, and a stair-step crack there goes to an engineer first.
For poured walls, concrete wall crack repair from the inside covers vertical and diagonal cracks. If yours is a different shape, the stair-step and diagonal cracks guide and the vertical cracks guide help you sort it out.
When does a basement wall need more than a crack repair?
When the wall moved. A horizontal crack, a wall that bows in, or a crack with one side pushed out means soil pressure or settlement, and a sealed crack will just open again. ASHI’s guide to basement cracks says an interior steel beam is a common repair for minimal displacement (opens in a new tab), and serious displacement needs an engineer’s design.
Get a structural engineer to look before anyone sells you braces, anchors or excavation. The horizontal crack guide and bowing wall guide explain what they’ll check, and the foundation crack repair guide covers every repair option side by side.
Questions
Can I paint or finish over the repair?
Yes. The kit's instructions say the epoxy paste surface seal can be painted once the ports are off. If you want the wall smooth, the paste can be taken down with a chisel, sander or grinder after it cures.
Does the crack have to be dry?
The foam doesn't care: it reacts with water and bonds to a wet crack. The surface around the crack does need to be dry for the ports and paste to stick, so dry it with a hair dryer or heat gun, or wait a few dry days.
How long does an inside repair last?
On a stable crack, as long as the wall stays put. The foam stays flexible and doesn't shrink much as it ages (its data sheet lists under 4% shrinkage). If the wall keeps moving, a new crack will open beside the repair, and that's a call for an engineer.
Will sealing the crack stop radon?
It closes one entry point, but rarely lowers radon much on its own. Test first, and see the radon and foundation cracks guide.