The one decision that makes or breaks a crack repair — explained plainly, with a decision table.
Use polyurethane injection when the crack leaks or is wet — it’s a flexible, water-activated foam that permanently seals water paths. Use epoxy injection when the crack is dry and structural — it rigidly bonds the concrete back to full strength. The most common injection failure is epoxy pumped into a wet crack, where it cannot bond.
Epoxy is a structural adhesive. Injected into a crack, it cures rigid and bonds both faces back together — cured injection epoxy is typically stronger than the surrounding concrete. Think of it as welding the wall.
Polyurethane is a reactive foam. It meets water in the crack, expands up to ~20 times its volume, and fills the full depth plus every hairline branch. It cures flexible, so seasonal wall movement doesn’t break the seal. Think of it as permanent, expanding caulking that water itself activates.
| Your crack | Right material | Why |
|---|---|---|
| Actively leaking or damp | Polyurethane | Water activates the cure; epoxy cannot bond to wet concrete |
| Dry, hairline, non-structural | Either — usually polyurethane | Cheaper, faster, seals the future water path |
| Dry, wide (3 mm+), stable | Epoxy | Restores monolithic strength |
| Engineer-specified structural repair | Epoxy | Only epoxy rebonds the wall |
| Crack still moving seasonally | Polyurethane (after assessing cause) | Rigid epoxy re-cracks beside the repair |
| Horizontal crack / bowing wall | Neither alone | That’s a structural failure — assessment first |
Epoxy fails when it’s injected into wet cracks (no bond), into moving cracks (re-cracks alongside), or fed too fast (face-fill that leaves the depth hollow). Most “injection didn’t work” calls we take trace back to one of these three.
Polyurethane fails when it’s asked to do a structural job (it seals, it doesn’t bond), when a cheap single-component product shrinks in a crack that later fully dries, or when the real problem was never the crack — cove-joint or window-well water that injection was never going to stop. Those cases start at basement leak repair.
Almost every page comparing these two materials is written from opinion. There is a published document, and it is worth quoting.
ACI RAP Bulletin 1, Structural Crack Repair by Epoxy Injection, is the American Concrete Institute’s field guide to the method. It is short, it is free, and it sets three limits that decide most of these arguments:
RAP-1 also states plainly that the cause of the damage must be assessed and corrected before injection, and describes the procedure itself — surface ports at roughly 200 mm (8 in.) on centre, injecting from the widest point on a horizontal crack or from the bottom upward on a vertical one, continuing until the crack refuses to take more resin.
For epoxy injection specifically, ICRI also publishes Guide Specification 110.2. Neither ACI nor ICRI publishes an equivalent endorsement of polyurethane injection for foundation leak sealing — polyurethane is standard field practice with a sound engineering rationale, not a cited standard, and anyone telling you otherwise is overselling.
In practice the choice is rarely close. It comes down to these, in this order.
If water is moving through the crack, polyurethane. It cures because water is present. Standard epoxy needs a dry, clean substrate, and injecting it into a running crack is the single most common reason a repair fails within a year.
If the crack is structural — the wall needs the two faces bonded back together — that is epoxy’s job and polyurethane cannot do it. Cured epoxy is stronger than the concrete either side of it. Flexible foam is not a structural adhesive and should never be sold as one.
Seasonal movement rules out a rigid bond. A crack that opens in winter and closes in summer will break an epoxy repair. Polyurethane flexes with it.
Answer those three honestly and the material picks itself. The uncomfortable case is a crack that is both wet and structural — that is a staged repair: polyurethane first to stop the water, structural work once conditions allow. It is uncommon in houses, and it is not a reason to inject the wrong thing once and hope.
Polyurethane is typically the more economical repair for the same crack; epoxy carries a material and labour premium. Current per-crack ranges are published on our crack injection cost page.
If your crack leaks, the debate is over — polyurethane. If your crack is dry and an inspection says the wall’s strength matters, epoxy. If anyone quotes you epoxy for a crack that’s visibly wet, ask them one question: how will it bond? — and then call someone who checks moisture before choosing a material. Ours is checked at a free assessment.
Moisture checked, method matched, quote in writing — free.
