Sealing a basement wall crack starts with the right material: epoxy for dry structural cracks, polyurethane for active leaks.
One wrong material choice turns a basement crack repair into work you pay for twice. Epoxy bonds dry cracks back together but will not stop water; polyurethane stops leaks but does not restore strength. So the first step in how to seal basement wall cracks is deciding which job the crack actually needs.
For most homes the answer is straightforward. A dry, stable crack that has not changed in years can usually be repaired from inside the basement. An actively leaking, widening, or displaced crack is a structural warning sign, not a weekend project.
Epoxy or Polyurethane: Which One Fits Your Crack?
Basement wall cracks get one of three treatments, and the crack itself makes the call. Epoxy injection is for dormant, dry, structural cracks where the goal is bonding the concrete back into one piece; polyurethane injection is for wet or moving cracks where the goal is stopping water; cement-based patching or hydraulic cement handles small, unchanging surface cracks.
Epoxy works as a rigid adhesive, so it restores strength but cannot flex. Polyurethane expands into a flexible foam, so it blocks water but adds little structural value. Using epoxy on a wet crack fails because the bond never holds; using polyurethane on a structural crack stops the leak but leaves the wall no stronger. Routing and sealing — cutting the crack into a neat groove before patching — and dry-packing cover the same small-crack territory when you want a cleaner surface finish.
| Material | Best For | Skip It For |
|---|---|---|
| Epoxy injection | Dry, dormant structural cracks that need strength restored | Wet, leaking, or moving cracks |
| Polyurethane injection | Active leaks, wet cracks, and cracks that may move | Cracks that need structural strength restored |
| Cement patch or hydraulic cement | Small, dry, non-growing cracks | Wide, leaking, or structural cracks |
If your crack is small enough for a tube product, our tested basement crack sealer roundup compares the most durable options on the shelf.
When Does a Basement Crack Need a Professional?
Call a professional before sealing anything when the crack is wider than about a quarter inch, keeps growing, forms a network, shows vertical displacement, or seeps water — those are structural or drainage failures, not cosmetic flaws.
Canada’s federal basement guidance draws the same line: cracks larger than 5 mm (0.25 in.), cracks that keep growing, large networks of cracks, and cracks where water is seeping through should all be evaluated by a professional. Warranties can force the issue even earlier — NRMCA notes that some concrete warranties require repair once a crack leaks or exceeds 1/8 in. (3 mm).
Check exterior drainage before repairing any leaking crack. Gutters that empty against the wall, downspouts that stop too short, and grading that slopes toward the house will keep the crack wet no matter what you inject. Fix the water path first, then seal.
How Epoxy and Polyurethane Injection Actually Works
Epoxy and polyurethane are installed the same way: injection ports go in along the crack, the surface between the ports is sealed, and material is pumped in at low pressure until the crack is full.
Complete kits bundle everything — two-part epoxy or polyurethane cartridges, injection ports, a mixing nozzle, and a caulk-style gun. That keeps the job manageable for a first-timer and ensures the components are tested to work together.
- Fix the drainage problem and let the wall dry. Epoxy gets its best results when the crack is dry at injection time.
- Clean the crack. Remove old caulk, paint, or sealant, blow out dust and debris with clean compressed air, and wipe the surface dry.
- Install injection ports along the crack. Spacing depends on the system — TxDOT’s manual says port spacing should not exceed the crack’s depth, some systems call for ports about 8 in. apart, and the common rule of thumb is roughly 1 in. of spacing per inch of wall thickness. Follow the manufacturer’s instructions for the kit you bought.
- Seal the crack surface between the ports and let the sealer cure. If the crack runs all the way through the wall, seal the back side as well.
- Inject from the lowest port and work upward at low pressure — typically 20 to 40 psi — until material shows at the next port or you hit refusal.
- After the material cures, remove the ports and surface sealer and grind off any leftover residue.
Done right, the crack fills solid from the inside out, and the wall either behaves like one piece again or stops moving water through it. For pressure-injected structural repairs, TxDOT’s pressure-injected epoxy procedure spells out the full sequence.
The whole decision fits in one line: dry and stable cracks take epoxy, wet or moving cracks take polyurethane, and small unchanging cracks take a cement patch. Anything wider than a quarter inch, anything still growing, and anything that leaks goes to a professional. That ordering is the difference between a repair that lasts for years and one that leaks again next spring.
FAQs
Can I seal a basement wall crack from the inside?
Yes. Epoxy and polyurethane injection are both done from the inside, which makes them the standard approach for finished basements. The catch: a leaking crack usually points at outside drainage too — fix gutters and grading first, or the water finds a new path through the wall.
How can I tell if a basement crack is structural?
A structural crack is usually wider than a quarter inch, keeps growing over months, shows one side higher than the other, or runs roughly horizontal. Hairline vertical cracks that hold the same size for years are far more often cosmetic. When in doubt, a contractor’s visit costs far less than a failed repair.
Is hydraulic cement as good as epoxy for basement cracks?
Hydraulic cement is a fast-setting patching material for small, dry, non-moving cracks, often part of a routing-and-sealing job. It does not bond the crack faces together the way epoxy does, and it will not stop an active leak the way polyurethane injection can, so treat it as a surface repair.
References & Sources
- TxDOT. “Crack Repair — Pressure Injected Epoxy.” Bridge repair manual covering port spacing, bottom-up injection, and Type IX epoxy specifications.
- Government of Canada. “Foundation Maintenance.” Federal guidance on the 0.25 in. professional-evaluation threshold and prep steps before repair.