Epoxy vs Polyurethane Crack Injection: Which One?
Updated June 2026
The choice comes down to what the crack is doing. Epoxy is a rigid structural resin that cures hard and glues a dry, non-moving crack back into one solid piece, so it is used when the wall needs its strength restored. Polyurethane is flexible, foams to fill a damp crack, and flexes with seasonal movement, so it is used when water is the problem. In a poured wall, a leaking crack is usually a polyurethane job and a dry structural crack is usually epoxy. The honest way to know which one your wall needs is a free site visit and a written quote.
If a contractor quotes you a foundation crack injection without saying which resin he plans to use, he has skipped the only decision that matters. There are two resins for the job, epoxy and polyurethane, and they do opposite things. One restores strength. The other stops water. Pick the wrong one and the repair fails, no matter how clean the work looks the day it goes in. This guide walks through which crack gets which resin, and where injection stops being the answer at all.
The one question that picks the resin
Before anything else, the crack has to be read for what it is doing. There is really one question: is this crack a strength problem or a water problem?
A strength problem is a crack that has weakened the wall and needs the two sides bonded back into one piece. The wall is dry, the crack is not moving, and the point of the repair is to make the concrete whole again. That is an epoxy job.
A water problem is a crack that is letting water through, or will the next time the ground gets wet. The wall does not need to be made stronger so much as it needs to be made dry, and the crack may flex a little as the seasons change. That is a polyurethane job.
Most cracks in a poured wall are water problems, not strength problems. So most injection work is polyurethane. But the only way to be sure which one you have is to look at the crack in the wall, which is why a real quote starts with a site visit and not a price.
Epoxy: the structural one
Epoxy is a two-part resin that cures into a rigid, high-strength solid. Once it sets, it is as hard as the concrete around it, and it bonds the two faces of the crack together so the wall acts as one piece again. In the right crack, cured epoxy is stronger than the original concrete.
That strength is the whole point. When a crack has actually reduced what the wall can carry, epoxy welds the crack shut rather than just plugging it, and puts the strength back.
Epoxy has two firm requirements. The crack has to be dry, because epoxy will not bond properly through water, and the crack has to be dormant, meaning it is not still moving. Epoxy cures hard and has no give, so if the wall shifts after the resin sets, the rigid repair can crack again right beside the old line. Use it on a dry, stable, structural crack and it lasts. Use it on a wet or moving crack and it lets go.
Polyurethane: the flexible one
Polyurethane is the water-stopping resin. It is injected as a liquid, then it reacts and foams, expanding to fill the crack through the full thickness of the wall and push into the damp, irregular spaces a rigid resin would skip. It seals against water rather than welding the wall together.
The two things that make polyurethane the common choice are moisture tolerance and flex. It cures fine in a damp or actively leaking crack, which is exactly the condition most basement cracks are in. And it stays flexible after it cures, so when the wall expands and contracts through the freeze-thaw seasons, the seal moves with it instead of fracturing. A crack that opens a hair in winter and closes in summer does not break a polyurethane seal.
What polyurethane does not do is add structural strength. It is a gasket, not a weld. If the wall genuinely needs strength restored, polyurethane alone is not the answer, and that is the line where the two resins stop being interchangeable.
Epoxy vs polyurethane at a glance
The two resins line up almost as opposites.
| Factor | Epoxy | Polyurethane |
|---|---|---|
| What it does | Bonds the crack into one solid piece | Seals the crack against water |
| Cured property | Rigid and high-strength | Flexible, foams to fill |
| Restores wall strength | Yes | No |
| Tolerates a wet crack | No, needs a dry crack | Yes, cures through moisture |
| Tolerates slight movement | No, can re-crack | Yes, flexes with the wall |
| Typical use | Dry, dormant structural crack | Leaking or active crack |
Read down the table and the rule falls out on its own. Strength and a dry crack is epoxy. Water and a damp or moving crack is polyurethane.
Why a leaking poured-wall crack is usually polyurethane
Walk into most Southern Ontario basements with a cracked poured wall and the story is the same. A thin vertical or diagonal crack, dry most of the year, that weeps during the spring melt when the ground around the house is saturated. The wall is sound. The crack is just an open path for water.
That crack is a textbook polyurethane job, for two reasons. First, it is wet, or it gets wet, and epoxy will not bond reliably through that moisture. Second, the crack lives through the freeze-thaw cycle, so the wall moves a little every year as the temperature crosses zero, and a flexible seal rides that movement where a rigid one would fracture beside it.
Forcing epoxy in to make that wall stronger solves a problem it does not have. The wall does not need strength. It needs to stop leaking. That is why the leaking shrinkage crack, the most common foundation crack there is, lands on polyurethane far more often than epoxy.
When the crack really does need epoxy
Epoxy earns its place on a narrower set of cracks, with a different feel from the ordinary leaker. The wall is dry. The crack is wider, often runs through a load-carrying part of the wall, and matters because the concrete’s strength has actually been compromised, not because water is getting in. And the crack is dormant, with no sign it is still opening over time.
In that situation the goal flips. You are not gasketing a leak, you are gluing the wall back into one piece, and only a rigid, high-strength resin does that.
The honest catch is that a truly structural crack is not always an injection job at all. If the crack is structural because something is still pushing on the wall, gluing it shut with epoxy does not remove the force, and the wall can crack again. That is the point where the fix moves past resin choice to reinforcement.
When injection is not enough, and carbon-fibre
Both resins share one blind spot. Injection fills a crack. It does not hold a wall against a force that is still acting on it. So the real question is not always epoxy or polyurethane. Sometimes it is whether injection alone can fix this at all, and the crack is a symptom of a wider problem.
The main signal is movement. A wall that is bowing inward, or a crack that keeps widening season after season, points to the soil outside loading the foundation, and sealing the crack does nothing about that load. This is where carbon-fibre comes in. Carbon-fibre straps are bonded vertically across the inside of the wall to lock it against further movement, often paired with a polyurethane injection so the crack gets sealed against water and braced against movement in one go. For ongoing movement, reinforcement is the fix, and injection on its own is not.
A few other cases where injection alone is not the answer:
- Cracks repeat in a pattern. A run of cracks along one wall, or matching cracks on opposite walls, usually means soil pressure or settlement, not isolated shrinkage. The cause has to be solved, not each crack in turn.
- Water is sitting against the outside of the wall. When the real issue is poor grading, a failed weeping tile, or a downspout dumping beside the foundation, the durable fix is exterior waterproofing and drainage, not an interior seal. Interior injection can stop a single leak, but if the whole wall is under water pressure, the water has to be managed from outside.
- It is a block or stone wall. Injection is built for solid poured walls. A hollow block wall leaks through its cores and joints, and the fix is usually waterproofing rather than a single resin injection.
An honest inspection names whether you have a crack problem or a wall problem before anyone fills anything.
What a proper injection job actually involves
When injection is the right call, the method matters as much as the resin. A real repair fills the crack through the entire thickness of the wall, not just the face you can see, and that takes a proper setup rather than a smear of product.
The crew cleans out the crack, then sets a row of injection ports along its length, spaced so the resin can travel from one to the next and confirm the crack is filling all the way through. The face of the crack between the ports gets a surface seal so the injected resin builds pressure and pushes deep into the wall instead of leaking back out. Then the resin, epoxy or polyurethane, is injected port by port under pressure until it carries the full depth and height of the crack. Once it has cured, the ports and the surface seal come off.
Compare that to the cheap fix, a trowel of hydraulic cement or a bead of caulk over the crack on the inside face. That seals the surface only. The crack still runs the full thickness of the wall, so water tracks through behind the patch and works it loose, usually by the next spring. You sealed the face, not the crack, and you pay again. The spec to look for on a real job is full-depth resin penetration through ports and a surface seal, not a skim of cement on the wall.
Getting the right call for your crack
The resin is a small part of the cost. Picking the right one, and knowing whether injection is even the answer, is what makes the repair hold. That call cannot be made from a photo or over the phone, because it depends on whether the crack is wet or dry, moving or dormant, isolated or part of a pattern, in a poured wall or a block one.
The honest way to get it right is a free site visit and a written quote that holds. Someone reads the crack in your basement, names whether it is epoxy, polyurethane, reinforcement, or waterproofing, and tells you why. Union-certified crews do the work, the quote you sign is the bill you get, and a lifetime warranty on labour stands behind it. If water is coming in right now, say so when you reach out, because an active leak moves to the front of the line.
Epoxy or polyurethane injection, which one do I need?
It depends on what the crack is doing, not which resin is better. Epoxy cures into a rigid, high-strength glue that bonds the two faces of a dry, dormant crack back into one piece, so it is the pick when the repair has to restore the wall's strength. Polyurethane foams and expands to seal a damp or leaking crack and stays flexible enough to move with the wall through the seasons, so it is the pick when water is the problem. A dry structural crack is usually epoxy. A wet, leaking crack in a poured wall is usually polyurethane. The crack tells you which one, and a site visit reads the crack.
Can you inject epoxy into a crack that is leaking water?
It is the wrong tool for it. Epoxy needs a dry, dormant crack to bond properly, because it cures into a hard, rigid solid. Water in the crack stops it from keying into the concrete and gives you a weak bond that lets go. A crack that is actively leaking, or one that gets wet every spring melt, is a job for polyurethane, which foams right through the moisture and seals against the water. If the wall also needs strength restored, that is a different conversation about carbon-fibre or excavation, not about forcing epoxy into a wet crack.
What is carbon-fibre and when do you need it instead of injection?
Injection fills a crack. It does not hold a wall against a force that is still pushing on it. When a wall is bowing inward or a crack keeps opening over the seasons, the soil outside is loading the foundation, and sealing the crack alone leaves that load unaddressed. Carbon-fibre straps are bonded vertically across the wall to lock it against further movement, and they often pair with a polyurethane seal so the crack stops leaking and stops moving at the same time. Ongoing movement is the signal for reinforcement, not just injection.
Are hairline cracks worth injecting at all?
Often only if they leak. A thin vertical or diagonal hairline crack in a poured wall is usually shrinkage from the concrete curing in its first year or two, and a dry one that stays put is cosmetic. The reason to inject a hairline is water. Even a hair-thin crack runs the full thickness of the wall and will weep during a wet spring, and polyurethane seals it before it stains the basement or feeds mould. A wide, growing, horizontal, or stepped crack is a different matter and needs a proper look, because that points to pressure rather than simple curing.
When is crack injection not enough to fix the problem?
When the crack is a symptom, not the problem. Injection is the right fix for an isolated crack in a sound poured wall. It is not the fix when a wall is bowing, when cracks repeat in a pattern around the foundation, or when the real issue is water sitting against the outside of the wall. Those cases need reinforcement, exterior waterproofing, or drainage, and injecting the crack alone just buys a season before it comes back. A honest inspection names whether you have a crack problem or a wall problem before anyone fills anything.
Does crack injection work on a block or stone foundation?
Injection is built for poured concrete walls, where the crack is a clean line through a solid pour. A concrete block wall is hollow and mortared, so a leak tracks through the cores and joints rather than a single crack, and the fix is usually different, often interior or exterior waterproofing rather than a resin injection. Older stone-and-mortar walls are different again. The first step on a block or stone wall is a look in the basement to confirm what you are actually dealing with, because the poured-wall injection method does not transfer cleanly.