Polyurethane Crack Injection: Sealing a Leaking Foundation Crack

Polyurethane crack injection is the fix for a crack that leaks. It's a flexible, moisture-activated foam that's injected into the crack, reacts with the water already there, expands to fill the void, and cures into a seal that keeps water out — and, crucially, stays flexible, so it accommodates the thermal and minor seasonal movement that would re-crack a rigid epoxy repair. That flexibility, and its tolerance for a wet substrate, are what make it the right tool for an active, weeping, non-structural crack. What it does not do is restore structural strength, and it does not stop the water at its source. This page covers what polyurethane injection actually does, how it differs from epoxy, when it's the right call, the things it can't fix, and how the DIY kit compares to a professional job.

What Polyurethane Injection Does

Polyurethane crack injection does one job well: it waterproofs a crack. The resin is pumped into the crack through a line of surface ports, and three things happen in sequence — it reacts, it expands, and it stays flexible.

• It reacts with water. Polyurethane is moisture-activated. Where epoxy needs a dry, clean substrate, polyurethane reacts with the water present in the crack — which is why a wet or actively leaking crack is the ideal candidate, not a problem to dry out first.
• It expands to fill the void. As it reacts, the foam expands — DIY-grade formulations can expand many times their liquid volume — pushing into the full depth and the fine branches of the crack and filling voids a rigid paste would bridge over.
• It stays flexible. Cured polyurethane remains flexible rather than setting hard. A foundation crack moves a little with temperature and with the seasonal wet-and-dry cycle of the soil around it — the kind of ongoing movement the concrete-cracking literature (ACI 224R-01, Control of Cracking in Concrete Structures) treats as routine rather than exceptional. A flexible seal moves with it instead of fracturing, which is the property that makes polyurethane durable on a crack that isn't perfectly static.

Add those up and the use case is specific: hairline and leaking cracks in solid poured concrete. High-pressure injection only works on poured concrete — on hollow concrete block (CMU) the resin flows into the cores instead of filling the crack, so block walls need exterior waterproofing or an interior drainage approach instead. And the entire job is waterproofing. Sealing the crack closes the leak; it does not, on its own, do anything structural.

Polyurethane vs Epoxy (the Key Choice)

The first decision on any crack-injection job is which resin, and it comes down to two questions: is the crack leaking, and is it structural? Polyurethane and epoxy are the two injection chemistries, and they're built for opposite conditions.

Polyurethane is the waterproofing resin: flexible, moisture-tolerant, for active or wet leaks in non-structural cracks. Epoxy is the structural resin: a rigid, two-part adhesive whose cured tensile and compressive strength exceeds the surrounding concrete, so it effectively welds a dry, stable crack back into a monolith — but only on a clean, dry surface, and it will not flex if the crack keeps moving.

Clean rule: a leaking, non-structural crack that needs to stay flexible = polyurethane; a dry, stable crack that needs its strength back = epoxy. They're not ranked — they solve different problems, and the wrong one fails predictably (rigid epoxy re-cracks on a moving crack; polyurethane adds no strength to a structural one). For the structural side of the decision, see our epoxy injection guide.

When Polyurethane Is Right

Polyurethane is the right call when a crack scores as non-structural and wet. In practice that means:

• The crack is actively leaking, weeping, or damp. Water through a basement or stem-wall crack is the signature case. Because the resin reacts with water, the moisture that disqualifies epoxy is exactly what polyurethane needs.
• The crack is hairline or non-structural. Most foundation cracks are not structural — they're thin, vertical or in a fine web, with no offset between the faces, and not growing. A leaking version of that crack is a clean fit for a flexible waterproof seal. (For where these thin cracks sit on the cosmetic-versus-structural spectrum, see our hairline cracks guide.)
• The crack sees minor movement. A crack that opens and closes slightly with temperature or the seasonal soil cycle wants a flexible seal, not a rigid one — polyurethane's flexibility is precisely what handles that small, ongoing motion.

The dividing line is the one an engineer draws between cosmetic and structural cracks. A hairline or leaking, non-structural crack is a polyurethane fit. A crack that is horizontal, offset out of plane, widening, or paired with other movement signs is structural — and that's an engineering question, not a sealing one. Triage which side of that line your crack falls on before you reach for a kit; if there's any doubt, an independent engineer's report settles it.

What It Can't Do

Two limits define polyurethane injection, and missing either is how homeowners waste the repair.

It is not structural strengthening. Polyurethane seals water out; it does not restore the load-carrying capacity a structural crack has lost. A flexible foam plug in a crack that's actively moving because of soil pressure or settlement is waterproofing a symptom while the structure keeps failing behind it. If the crack is structural, the structure is the problem — and if that crack is dry and stable, the strength-restoring repair is epoxy, not polyurethane. Either way, the call belongs to an engineer.

It does not fix the source of the water. This is the one most worth internalizing: injection seals the leak's path through the crack, not where the water is coming from. The usual real sources are bad grading that pitches water toward the wall, clogged gutters and downspouts dumping roof runoff at one spot, or an under-slab plumbing leak wetting the soil from below. Sealing the crack while leaving those in place treats the symptom. On a slab home in particular, an under-slab plumbing leak can be the hidden driver — and no amount of crack sealing addresses a leaking pipe. The durable result is the seal plus the drainage, grading, or plumbing correction that stops the water reaching the crack in the first place.

And it is not a fix for a moving foundation. Polyurethane addresses the crack, never the cause of the crack — the cardinal rule of all crack repair. If the foundation itself is moving, you're back to diagnosis and the structural menu, not a tube of resin.

The Injection Process and DIY vs Pro

The mechanics are the same whether a contractor or a homeowner does it; the difference is judgment about whether the crack is a candidate.

1. Prep the crack. Wire-brush and vacuum the crack, removing loose concrete, paint, and efflorescence an inch or two on each side. Polyurethane tolerates moisture, so the surface doesn't have to be bone-dry the way it must for epoxy.
2. Set the surface ports. Place injection ports along the crack, spaced roughly 8 inches apart — a useful rule of thumb is about one inch of spacing per inch of wall thickness — starting near the lowest point.
3. Seal the surface. Trowel the supplied surface-seal paste over the crack between the ports, leaving the port openings clear, and let it set until it's hard.
4. Inject — and tolerate the moisture. Starting at the lowest port, inject slowly at low pressure (about 20–40 psi) until resin oozes from the next port up; cap that port and move up. The resin reacts with the water in the crack and expands to fill it. Patience matters: incomplete filling is the single most common cause of failure.
5. Cure and finish. Let it cure, then knock off the ports and dress the surface seal. Monitor the crack afterward to confirm the leak has stopped.

For DIY, low-pressure polyurethane kits are sold for non-structural, leaking cracks in poured concrete and run about $60–$150. They're a reasonable homeowner job once you've confirmed the crack is cosmetic and not active structurally — never a tool for diagnosing whether a crack is structural. A professional injection runs roughly $250–$1,500 per crack (multiple cracks are often priced per linear foot). The full kit-by-kit walkthrough and product picks live on our DIY products guide; for what a professional seal should cost against the alternatives, see the crack-repair cost page.

FAQ Note

The FAQ below covers what San Antonio homeowners ask most before sealing a leaking crack — polyurethane versus epoxy, whether you can inject a wet crack, whether it's structural, whether it stops the leak for good, how long it lasts, the DIY question, and how it differs from foam slab leveling. For the structural alternative, see epoxy injection; for the DIY kits and the cost of a professional seal, the DIY products and cost pages; and for the full menu of crack types and repairs, the crack-repair hub.

Get Matched With a Vetted San Antonio Crack-Repair Specialist

If a crack is leaking and you want it sealed right — or it's leaking and showing structural signs and you want a P.E.-led second opinion before anyone injects anything — we'll match you with a vetted San Antonio specialist who can waterproof the crack and flag whether the water source needs correcting too. The match is free, the quote is no-obligation, and we don't take a fee from you. We screen for the right resin for the crack (polyurethane for a flexible waterproof seal, epoxy for dry structural repair), an honest read on whether a crack is structural enough to need an engineer first, and a plan that addresses the drainage, grading, or plumbing leak driving the water — not just the leak path. If a quote seals a structural crack or ignores the water source, we'll tell you. That's the only way an editorial matching service should work.