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Use polyurethane foam for most basement wall cracks, especially damp or leaking ones. It stays flexible and seals the crack through the wall. Use epoxy only for a dry, stable crack that needs the two sides bonded back together for strength, and let a structural engineer make that call.
Polyurethane foam is a flexible water seal, and epoxy is a rigid structural glue. That one difference decides which goes in your crack.
Is epoxy or polyurethane better for foundation cracks?
For most homeowners, polyurethane. The typical basement wall crack is a shrinkage crack that leaks or might leak, and the goal is to keep water out. Polyurethane does that in wet or dry cracks and flexes if the crack moves a little.
Epoxy is better only when the wall needs its strength back. The American Concrete Institute’s repair bulletin says that if the crack does not compromise the structural integrity (opens in a new tab) of the structure, polyurethane or other nonstructural materials may be the better choice.
| Polyurethane foam | Epoxy | |
|---|---|---|
| What it does | Seals the crack against water | Bonds the two sides back together |
| After it cures | Flexible | Rigid |
| Wet or leaking crack | Works, and is the usual choice | Needs a dry crack, or a moisture-tolerant epoxy |
| Crack still moving | Can flex with small movement | May not be suitable |
| Adds strength | No | Yes |
| Who it's for | Leaking or damp shrinkage cracks in poured walls | Dry, stable cracks an engineer says need a structural repair |
American Concrete Institute repair bulletin on epoxy injection and ACI material on leaking cracks
What does epoxy crack injection do?
Epoxy crack injection pumps a two-part resin into the crack, where it hardens and glues the concrete back into one piece. ACI says it can restore structural integrity and reduce moisture penetration (opens in a new tab) through cracks 0.002 inch wide and wider.
The catch is that epoxy is rigid. ACI’s bulletin warns that if a crack is subject to later movement, an epoxy repair may not be applicable (opens in a new tab). A rigid bond in a moving crack either cracks again next to the repair or lets water back in.
Any concrete crack repair epoxy comes with a few rules:
- The cause has to be fixed first. ACI says that when a structural repair is needed, the conditions that caused the crack must be corrected before injecting.
- The crack must be dry. If it’s damp and can’t be dried out, ACI says to consider a moisture-tolerant epoxy.
- No rusting rebar cracks. ACI says cracks caused by corroding reinforcing steel should not be repaired by epoxy injection (opens in a new tab), because the rust keeps cracking the concrete.
Those rules are why epoxy is mostly a contractor and engineer tool. If a crack needs a structural bond, it needs someone to confirm why it cracked.
What does polyurethane crack injection do?
Polyurethane crack injection fills the crack through the wall with a resin that foams and cures into a flexible plug. It doesn’t make the wall stronger. It stops water from following the crack in.
There are two kinds, and the difference matters if your crack dries out between rains. In ACI material on leaking cracks, 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 tend to shrink when they dry out and may not regain enough volume to stop the leak when the wall gets wet again.
ACI lists chemical grout injection for both active cracks, whose width changes with time (opens in a new tab), and dormant ones. Epoxy is listed for dormant cracks only.
When should you use epoxy instead of polyurethane?
When an engineer says the crack needs a structural repair and the cause has been fixed. That’s the order ACI gives: find out why it cracked, correct it, then bond it.
For a homeowner, the honest answer is that epoxy is rarely a first-step DIY choice. If you think your crack needs strength, it’s worth an engineer visit before buying any kit. If it just needs to stop leaking, polyurethane is the simpler fit.
Why does a polyurethane kit come with epoxy paste?
The epoxy in a polyurethane foam kit isn’t the crack filler. It’s the surface sealer, a paste spread over the face of the crack and the injection ports so the foam stays in the wall instead of squirting out.
ACI’s bulletin calls this the cap seal and describes applying it 1 inch wide by 3/16 inch thick (opens in a new tab) over the length of the crack. It has to cure before injecting, or the pressure can blow it off.
Kits come with one of two surface sealers:
- Epoxy paste: the 10 ft foam kit with epoxy paste ($204.95). It holds well and is usually ground off or left in place after.
- Peel-off sealer: the 10 ft foam kit with peel-off sealer ($239.95). It pulls off after the foam cures, for a cleaner wall where it will stay exposed.
The peel-off vs epoxy paste comparison covers which finish suits your wall, and the injection guide walks through the whole job.
Which cracks shouldn’t get either one?
Neither injection fixes a wall that is still moving. Injection seals a crack; it doesn’t hold back soil or stop settling.
Skip the kit and have a structural engineer look first if you see:
- A horizontal crack in a basement wall, or a wall bowing inward.
- Stair-step cracks in a block wall.
- A crack wider than 1/4 inch.
- One side of the crack sticking out farther than the other.
- Rust stains coming out of the crack, which can mean corroding rebar.
The horizontal crack guide and the when to worry about foundation cracks guide show how to read these. For a vertical crack that’s stable and leaking, the vertical cracks guide is the next step, or use the product chooser to match a crack to a kit.
Questions
Can you inject epoxy into a wet crack?
Standard epoxy needs a dry crack to bond. ACI's epoxy injection bulletin says that if a crack is damp and can't be dried out, a moisture-tolerant epoxy should be considered. For an actively leaking crack, polyurethane is the usual choice.
Does crack injection work on a block wall?
No. Injection is for poured concrete walls, where the crack runs through solid concrete. Block walls are hollow, so the material has nowhere to stop, and stair-step cracks in block are often structural. Have an engineer look at those.
Is epoxy stronger than polyurethane?
Yes. Epoxy is a rigid adhesive that can restore a wall's structural integrity, while polyurethane foam is a flexible seal that adds no strength. Strength only helps if the crack has stopped moving, though.