Choose epoxy when the goal is to rejoin the concrete and the crack is stable. Choose polyurethane when the goal is to stop water and the crack may move or is already leaking. Epoxy cures into a rigid, high-strength bond. Polyurethane stays flexible. Neither one treats the cause of the crack, so the first step is always to find out why it opened.

This comparison draws on the trade article Epoxy or Polyurethane Foam? by Jay Schmid (WATERPROOF! Magazine, 2010), the injection specialist Inblock, and two Quebec references: the APCHQ Performance Guide (edition consulted: 2006, in French) and CAA-Québec (in French).

What Is the Difference Between Epoxy and Polyurethane Injection?

Both methods inject a liquid resin into the crack, but they are built for different jobs. According to Schmid, epoxy restores the structural integrity of the concrete by filling the crack with a high-strength adhesive, while polyurethane foam is flexible enough to accommodate movement caused by changing soil pressure or minor settlement.

Question	Epoxy injection	Polyurethane injection
Main purpose	Rebond the concrete and restore its strength	Seal the crack against water
Once cured	Rigid, high-strength bond	Flexible
If the crack moves	Suited to a crack that will not change width over time	Tolerates minor movement
If the crack is wet or leaking	Depends on the formulation: some epoxies are made for damp conditions, so ask which product will be used	Reacts with moisture; often gives better results on wet or actively leaking cracks

The Quebec reference recognizes the same families of methods. The APCHQ lists three recognized ways to make a crack watertight: epoxy-based injection, injection of flexible products, and a waterproofing membrane.

When Is Epoxy Injection the Right Choice?

Epoxy fits a crack that has stopped moving and where the wall needs to be bonded back together. Schmid notes that it eliminates the movement that would otherwise occur from temperature swings and wet and dry seasonal cycles, and Inblock stresses that epoxy requires the crack not to change width over time. The catch is obvious: if the crack is still active, a rigid repair can simply crack again next to the original.

So before choosing epoxy, find out whether the crack is stable. Measure its width, mark both ends with a pencil, date it, and check again after each winter. The APCHQ recommends checking how cracks wider than 1/8 in. (3 mm) evolve after a freeze-thaw cycle. Our guide When Is a Foundation Crack Serious in Quebec? explains how to read what you see.

When Is Polyurethane Injection the Right Choice?

Polyurethane fits a crack that leaks, or that may keep moving slightly. Schmid says wet or actively leaking cracks often get better results from polyurethane foam, and that injecting it slowly gives the resin time to react with moisture and expand. Because it stays flexible, it can follow the small movements of the wall without losing its seal.

CAA-Québec describes this method as injecting a polyurethane liquid deep into the crack from the inside, and notes that it is often cheaper than work done from the outside. It adds an important condition: this sealing only lasts if the concrete is of good quality.

Does Injection Fix the Cause of the Crack?

No. Injection treats the crack, not what opened it. A crack caused by water pressure against the wall, by soil settlement or by poor drainage can reappear or let water through elsewhere if the cause stays. Read our articles on hydrostatic pressure and on the different types of foundation cracks to see what yours may be telling you.

The APCHQ also states that walls with cracks that exceed the expected minimum performance, or that let water in, must be repaired, and it asks for particular attention to any crack on a foundation wall. For structural problems, it calls for an assessment by an engineer.

Can a Program Help Pay for Crack Repair?

Possibly. In Montréal, the City’s RénoPlex program counts 370 $ per crack in its rate table to calculate the subsidy. That figure is a calculation basis, not a price quote. Our article How to finance foundation repairs in Quebec explains the conditions and the steps to take before work starts.

What Should You Do Before Choosing a Method?

Photograph and measure the crack, with a ruler in the frame, and note whether it leaks after rain or snowmelt.
Ask for the cause, not only the price. A good inspection looks at drainage, soil and the wall before proposing a resin. See what to expect during an inspection.
Ask which product will be used and why, since formulations differ, and what the contractor’s warranty covers.

Marco Réparation de Fondation repairs foundation cracks by injecting polyurethane or epoxy resin, and visits in person for every estimate and every job site inspection. The company holds a valid license from the Régie du bâtiment du Québec (RBQ No. 5775-3246-01) and serves Montreal, Laval, the South Shore (including Longueuil and Brossard), the West Island and Salaberry-de-Valleyfield. See our crack repair page.

Not sure whether your crack is active or stable?Contact Marco Réparation de Fondation for a free inspection and an honest opinion on which repair fits.

Frequently Asked Questions

Is epoxy stronger than polyurethane?

Yes, in the sense that matters here. Epoxy bonds the concrete back together and restores its strength, while polyurethane seals the crack without restoring structural strength. That is why they are not interchangeable.

Can polyurethane injection repair a structural crack?

Not on its own. Polyurethane is chosen to stop water and to follow movement. If the wall needs to be bonded back together, that calls for epoxy, after an assessment of why the wall cracked.

Will an injected crack stay sealed for good?

It depends on the cause and on the concrete. CAA-Québec warns that the seal lasts only if the concrete is of good quality, and a crack whose cause is still active can reopen. Watch it for a few seasons, and photograph it each year.