Quick Comparison
| Factor | Epoxy Injection | Polyurethane Injection |
|---|---|---|
| Flexibility | Rigid when cured | Remains flexible |
| Structural bonding | Yes, welds crack together | No, just seals |
| DIY kit cost | $50-150 | $40-100 |
| Best for | Dormant, dry cracks | Active or wet cracks |
| Cure time | 24-48 hours | Minutes to hours |
| Waterproofing | Good | Excellent |
What Epoxy Does Well
Epoxy is basically liquid plastic that hardens into something stronger than the concrete around it. When it cures, it actually welds the two sides of the crack together.
Structural Repair
The engineer who assessed my basement said epoxy is the standard for structural crack repair. It restores the wall to nearly original strength. The cured epoxy is actually stronger than the concrete, so if the wall were to crack again, it would crack somewhere else.
For structural cracks that an engineer wants repaired, epoxy is usually what they specify.
Complete Fill
Epoxy is low viscosity, which means it flows into every tiny void. It fills the entire crack from inside the wall to the outside surface. No air pockets, no gaps.
I watched my epoxy kit work. You inject it slowly, watching it seep through. Takes patience but fills everything.
Dormant Cracks
If your crack has been exactly the same width for years and years, epoxy makes sense. It's not going to move, so the rigidity doesn't matter. You get a permanent, structural repair.
What Polyurethane Does Well
Polyurethane foam expands as it cures and stays somewhat flexible afterward. Different strengths for different situations.
Active Water Leaks
Here's where polyurethane really shines. It can be injected into a crack that's actively leaking water. The water actually helps it cure and expand. Epoxy won't work in wet conditions; it won't bond properly.
Gary had a crack that leaked every time it rained. The contractor used polyurethane specifically because the crack was wet. Stopped the leak completely.
Flexible After Curing
Polyurethane stays slightly flexible. If your wall moves a tiny bit with temperature changes, the seal moves with it instead of cracking. This is why Rick said I should have used polyurethane near my window. That area sees more thermal movement.
Speed
Polyurethane cures fast. Sometimes minutes. You can see the foam expand and fill the crack almost immediately. Epoxy takes a day or two to fully cure.
Where Each One Falls Short
Neither product is perfect. Understanding the limitations matters.
Epoxy Failures
Epoxy fails when the crack moves after repair. A rigid bond can't handle movement. It'll crack right next to the original repair, or sometimes through the epoxy itself.
I've read horror stories about people epoxying active cracks, only to have new cracks appear next to the repair within a year. The movement didn't stop just because they filled the crack.
Polyurethane Limitations
Polyurethane doesn't provide structural strength. It seals the crack but doesn't weld the wall back together. If you need actual structural repair, polyurethane isn't enough.
The foam can also shrink slightly over time. Some contractors add a secondary sealant layer as insurance.
My Decision Process Now
After my experience and talking to both contractors and Rick, here's how I'd choose today.
Use Epoxy If
The crack is completely dry. The crack has been monitored for at least a year with no change. The crack is in the middle of a wall away from windows, corners, or other stress points. You want structural repair that restores wall strength.
Use Polyurethane If
There's any moisture in or around the crack. The crack is near a window, door, or corner. You haven't monitored long enough to know if it's active. The primary goal is waterproofing rather than structural repair. The crack is in a garage or unfinished space where slight foam appearance doesn't matter.
When In Doubt
Polyurethane is more forgiving. If you're not sure whether your crack is truly dormant, polyurethane handles uncertainty better because it stays flexible. You lose the structural bonding benefit, but most hairline and vertical cracks don't need structural repair anyway.
I'd probably use polyurethane on my next crack just to be safe.
DIY vs Hiring Someone
Both are doable as DIY projects. Both are also commonly hired out. Depends on your comfort level.
DIY Kits
Epoxy kits run $50-150 depending on crack length. Polyurethane kits are similar. Both come with injection ports, mixing supplies, and instructions.
My epoxy kit came with everything I needed. Watched YouTube, read the instructions twice, took my time. Total project was maybe 3 hours including setup and cleanup.
Professional Cost
Contractors typically charge $300-600 per crack for injection repair. You're paying for their experience, warranty, and the confidence that it's done right.
If you have multiple cracks or anything that looks complicated, hiring out might be worth it just for peace of mind.