Foundation Crack Repair: Epoxy vs Polyurethane Injection
If you have identified a crack in a poured-concrete foundation wall — and confirmed [whether it is dangerous](/blog/foundation-cracks-which-ones-are-dangerous) — the repair is usually done by **injecting** the crack from the inside. The two materials used are **epoxy** and **polyurethane**, and they do genuinely different jobs. Choosing the right one matters, so it helps to understand the difference.
> This article is about *repairing* a known crack. To first identify what kind of crack you have and whether it is structural, start with [foundation cracks: which ones are dangerous](/blog/foundation-cracks-which-ones-are-dangerous).
How Crack Injection Works
For both materials, the process is similar: injection **ports** are fixed along the crack, the surface of the crack is sealed over, and the material is injected under pressure — usually from the bottom up — until it fills the full depth of the crack. The surface seal and ports are removed or trimmed once it cures. Done correctly, the material penetrates right through the wall, not just the visible surface.
Epoxy: Structural Repair
Epoxy is **rigid and high-strength**. When injected into a crack, it bonds the two faces of concrete back together and effectively "welds" the wall into a single piece again — the repaired crack can be as strong as the surrounding concrete.
Polyurethane: Flexible Water Sealing
Polyurethane injection is a **flexible foam** that expands as it cures, filling the crack and sealing it against water. It stays flexible afterward, so it can tolerate minor ongoing movement without re-cracking.
Which One? A Simple Rule
| Situation | Best choice |
|---|---|
| Structural crack, stable, restore strength | **Epoxy** |
| Leaking crack, water sealing | **Polyurethane** |
| Crack that moves seasonally | **Polyurethane** (flexible) |
| Actively leaking right now | **Polyurethane** (cures in wet) |
| Non-structural hairline letting water in | **Polyurethane** |
When a crack is both structural and leaking, a contractor may stabilize with epoxy and address water management separately — and will look at *why* the crack formed.
Injection Fixes the Crack — Not Always the Cause
This is the part DIY repairs miss. Injecting a crack seals **that crack**. If the crack is leaking because of **hydrostatic pressure** from saturated soil, sealing it may simply send water to the next weakest point. A lasting fix often pairs the injection with addressing the cause — [grading and downspouts](/blog/landscape-grading-downspouts-foundation-damage), or a drainage system. For widespread water or multiple cracks, [exterior waterproofing](/blog/aqua-bloc-vs-blueskin-foundation-wall-waterproofing) or [interior drainage](/blog/how-waterstop-interior-drainage-protects-basement) may be the better route.
**Note on block foundations:** injection is for **poured-concrete** walls. Concrete block (CMU) is hollow and cracks differently, so it is usually addressed with interior drainage, surface systems, or exterior waterproofing rather than injection.
What It Costs
Crack injection typically runs **$300–$800 per crack**, depending on length, depth and access. It is one of the most cost-effective foundation repairs when the problem is one or two specific cracks rather than general seepage. DIY kits exist, but for structural cracks or active leaks, professional injection (with the right material and full-depth penetration) is the reliable choice.
Frequently Asked Questions
Epoxy or polyurethane for a foundation crack?
Epoxy for structural repair — it is rigid and restores the wall's strength on stable cracks. Polyurethane for water sealing — it is flexible, tolerates movement, and can even be injected into an actively leaking crack. Many leaking, non-structural cracks are best sealed with polyurethane.
Can you inject a crack that is actively leaking?
Yes — with polyurethane. It reacts with moisture and expands, so it can be injected into a wet or actively leaking crack to seal it. Epoxy, which cures rigid and slowly, is better suited to dry, stable structural cracks.
Does crack injection fix the problem permanently?
It permanently seals that specific crack, but it does not change why the crack leaked. If the cause is hydrostatic pressure or poor drainage, you should address that too, or water may find the next weak point. For one or two isolated cracks, injection alone is often a complete fix.
How much does foundation crack injection cost?
Typically $300–$800 per crack, depending on its length, depth and how accessible it is. It is among the most cost-effective foundation repairs when you are dealing with specific cracks rather than general seepage across the wall.
Does crack injection work on block foundations?
No — injection is for poured-concrete walls. Concrete block is hollow and behaves differently, so leaks and cracks in block foundations are typically handled with interior drainage, surface systems, or exterior waterproofing instead.
Next Steps
[See Waterproofing Costs by City](/costs/waterproofing) | [Foundation Cracks Guide](/blog/foundation-cracks-which-ones-are-dangerous) | [Book Your Walkthrough](/start-project)
