Deep-dive from How Foundations Get Fixed: Repair Methods in Plain English
Crack injection is the cheapest foundation repair on the menu, and for the right problem, it's an excellent long-term fix. It's also the repair most likely to be done wrong — either by using the wrong material for the situation, or by treating a symptom while ignoring the cause underneath.
The two materials — epoxy and polyurethane — are the entire toolkit. They solve different problems. Choosing between them is straightforward once you understand what each is actually doing to the crack. Getting the choice wrong is a common way to spend a few hundred dollars fixing nothing.
This guide covers what each material does, when each is right, what the process looks like, what it costs in Maryland, and how to spot the corner-cutting that separates a lasting repair from a temporary patch.
The fundamental question crack injection has to answer
Before you can choose between epoxy and polyurethane, you have to know what you're actually trying to solve. There are only three real reasons to inject a foundation crack:
- Stop water from coming through it (moisture management)
- Restore some structural continuity across the crack (strengthening)
- Seal the crack against radon or other gas entry (foundation cracks are the primary radon pathway)
Most homeowner injections are trying to solve #1 (and often #3 as a side benefit). The occasional injection is trying to solve #2 as part of a structural repair. The material choice follows from the goal.
The catch: injection assumes the crack is stable and the underlying cause has been (or will be) addressed. Injecting an active crack — one that's still moving because the pressure or settlement causing it hasn't been resolved — is a temporary win at best. The material will fail or the crack will simply reopen next to the repair.
Epoxy: the structural bond
Epoxy is a two-part chemical adhesive that cures into a hard, rigid, high-strength solid. When injected into a foundation crack, it fills the crack completely, penetrates the concrete pores on both sides, and — once cured — bonds the two faces of the crack back together with a strength often greater than the surrounding concrete.
Think of it as gluing the crack back together. The two sides of the wall that had separated are now, structurally, one piece again.
When epoxy is the right choice
Structural crack repair. When a crack is stable but you want to restore some load-carrying capacity across it — the wall was compromised structurally, and you want it to behave more like a monolithic wall again — epoxy is what you want.
Cracks in poured concrete that aren't actively moving. Epoxy needs a stable substrate to bond to. In a stable, dry crack, it excels.
Where later movement isn't expected. Because epoxy cures rigid, it doesn't flex. If the crack starts moving again after repair, the epoxy bond can break (usually adjacent to the original crack rather than at it, because the epoxy is stronger than the concrete).
Pre-repair for other work. Sometimes epoxy is used to seal cracks before applying carbon fiber straps or other structural bracing, so the underlying wall has continuity before the strap goes on.
When epoxy isn't the right choice
Active or leaking cracks. Epoxy needs a dry surface for a good bond, and it takes time to cure. Trying to inject epoxy into a crack that's currently letting water through is a bad match — the water interferes with the bond, and the rigid cure means any future movement fractures the repair.
Cracks in block walls or masonry. Epoxy works best in solid poured concrete. Hollow-core block and mortared masonry are less friendly substrates because the crack often passes through multiple mortar joints and hollow cavities.
Cracks with any ongoing movement. Rigid epoxy plus flexing crack equals broken bond. If there's any question about stability, polyurethane is safer.
Rough Maryland cost for epoxy injection
- Typical single crack: $350–$800, depending on length and access
- Complex or long cracks: up to $1,500 for a single crack repair
- Structural epoxy for pre-repair work as part of a larger job: usually rolled into the larger repair cost, not billed separately
Costs run at the higher end of that range in Maryland due to elevated labor rates and the Baltimore-area premium.
Polyurethane: the flexible seal
Polyurethane injection uses a different chemistry entirely. When it enters a crack, it reacts with moisture (which is why it works in wet cracks) and expands as it cures, foaming to fill the crack completely and squeezing into every void. The cured material is a flexible closed-cell foam — rubbery rather than rigid.
Think of it as expanding rubber that inflates to fill the crack watertight. The two sides of the wall aren't bonded together the way epoxy bonds them — they're sealed against water, but they can still flex slightly relative to each other without breaking the seal.
When polyurethane is the right choice
Leaking cracks. The go-to for water problems. Polyurethane cures in the presence of water — it actually needs moisture to react properly — so it works exactly when epoxy doesn't. A crack that's actively weeping when you inject it can be sealed watertight with polyurethane.
Cracks with any expected future movement. The flexibility handles small movements without breaking. If the crack has to breathe seasonally (which many Maryland cracks do with clay-soil wet/dry cycles), polyurethane handles that better than epoxy.
Non-structural water sealing. When the goal is purely to stop water — not to restore structural continuity — polyurethane is often the correct answer even if the crack is dry at the moment.
Block walls and masonry. The expanding nature helps polyurethane fill the irregular void patterns of block construction, hollow cores, and mortar joints better than epoxy.
Radon and gas sealing. Because polyurethane fills completely and stays flexible, it's excellent for sealing cracks against gas entry — a benefit worth considering in Maryland given radon prevalence in many counties. Full breakdown of the crack-radon connection here.
When polyurethane isn't the right choice
When you actually need structural restoration. Polyurethane seals; it doesn't bond structurally. If the wall needs to be rejoined mechanically, epoxy is the choice.
Very wide cracks. Polyurethane fills well but has practical limits — a crack wide enough that the expanded foam can't span it well is better addressed with other methods.
Rough Maryland cost for polyurethane injection
- Typical single crack: $250–$700, sometimes slightly less than epoxy for equivalent length
- Complex or long cracks: up to $1,200–$1,500
- Multi-crack jobs (several cracks on same visit): economies of scale often apply
Costs in Maryland typically run at the higher end of national ranges due to labor market factors.
The choice, in one paragraph
Structural + stable + dry = epoxy. Water + flexible + wet = polyurethane.
More specifically: if the goal is restoring structural connection across a stable, dry crack in poured concrete, epoxy is the tool. If the goal is stopping water, sealing against radon, or handling a crack that might flex, polyurethane is the tool. Most residential Maryland crack injection is polyurethane, because most homeowner injection jobs are about water and moisture rather than structural restoration.
A contractor who understands the distinction and picks the material based on your specific crack is the right contractor. A contractor who "always uses" one material regardless of situation is a flag.
What the injection process actually looks like
For a typical single-crack injection:
1. Surface preparation. The crack area is cleaned. Any loose material is removed. Debris, paint, or previous patch material is scraped clear. Cleanliness matters for either material, but especially for epoxy bond quality.
2. Injection port installation. Small plastic ports are attached along the crack — typically every 6–12 inches. These are the ports material will be pumped through.
3. Surface sealing. The crack is sealed on the surface between the ports with a paste (a fast-set epoxy or paste sealer), so injected material stays inside the crack instead of oozing out. The ports are left open.
4. Injection. Material is pumped under low pressure through the ports, starting at the bottom port and working up. As material appears at the next port, that port is capped and injection moves up. This ensures the material fills the crack completely from bottom to top.
5. Cure time. Epoxy: typically 24 hours to reach handling strength, longer for full cure. Polyurethane: often 15 minutes to an hour, with full cure within a day.
6. Port removal and finishing. Once cured, the ports are removed and the surface paste is chipped off or ground down. The finished repair is a thin visible line where the crack was.
Total on-site time: typically 1–3 hours for a straightforward single-crack injection. Multi-crack jobs take proportionally longer.
The corner-cuts to watch for
Injection is one of the easier repairs to do badly and still have it look done. Watch for these:
Skipping surface prep. A contractor who shows up, pops ports on, injects, and leaves without any prep is applying material to a compromised bond. Especially problematic for epoxy.
Wrong pressure. High-pressure injection can push material through the crack too fast, missing voids. Low-pressure, patient injection is the technique that fills completely.
Not injecting to the full depth of the crack. Cracks pass through the full thickness of the foundation wall — 8" to 12" or more. A surface-only fill doesn't seal the interior. Proper injection fills all the way through.
Using the wrong material for the crack. Epoxy on a wet crack, polyurethane on a structural crack requiring bond restoration — both are misapplications that produce a "repair" that fails when tested.
Ignoring the cause. Injecting a crack caused by ongoing hydrostatic pressure without addressing the drainage means the crack is likely to reopen or the wall will crack again nearby. Any injection job on a Maryland foundation should include a conversation about what caused the crack and whether that's being addressed. Drainage guide.
When crack injection isn't the answer
Injection is designed for non-structural, stable cracks. It's not the answer for:
Bowing walls. A wall that's bowing needs carbon fiber straps or wall anchors — bracing, not gluing. Injection into the horizontal crack on a bowing wall might seal water, but it doesn't fix the pressure that's pushing the wall inward.
Settling foundations. A foundation that's sinking needs piers to transfer the load to stable soil. Injection into a settlement crack seals the visible symptom but does nothing about the sinking.
Active, growing cracks. Injection assumes the crack has reached its final state. If the crack is still growing, the underlying cause needs to be resolved first — otherwise the injected material fails when the crack continues moving.
Wide cracks with significant displacement. Once a crack is wide enough and offset enough, injection isn't the physical solution — you're beyond material-fill territory and into structural repair territory.
A contractor recommending injection for any of these situations either doesn't understand the diagnosis or is selling you the wrong repair.
The DIY question
Small crack injection kits are widely available at big-box stores for $50–$150. Can homeowners do this themselves?
Sometimes, with realistic expectations. For a very small, dry, stable, non-structural crack that you want to seal against moisture — a hairline in a poured concrete basement wall, for example — a DIY kit can work. The technique isn't rocket science: install ports, seal the surface, inject the material.
Where DIY frequently fails:
- Surface prep is often inadequate — homeowners tend to skip the grinding/cleaning that professionals do
- Pressure control is harder without professional equipment — kits use pressurized cartridges that don't give the fine control of a professional pump
- Fill depth is uncertain — without seeing material appear at successive ports, it's hard to know if the crack filled completely
- Diagnosis is skipped — a homeowner injecting a crack often doesn't know whether it's genuinely non-structural, whether the cause is being addressed, or whether the material choice is right
For a small, low-stakes, obviously cosmetic crack you want sealed against moisture, DIY is reasonable and often adequate. For anything larger, wetter, or where you're not confident about the diagnosis, the $500 professional job is usually the better investment. And any crack showing signs of being potentially structural — horizontal direction, displacement, growth over time — should be professionally assessed before injection either way.
The one-page summary
- Epoxy: structural bond, rigid cure, needs dry surface, best for stable poured-concrete cracks where structural continuity matters. Roughly $350–$800.
- Polyurethane: flexible seal, expanding foam, works wet, best for water/radon sealing and cracks that might flex. Roughly $250–$700.
- The choice: structural = epoxy, water = polyurethane. Contractor picking based on your crack is right; contractor "always uses" one is wrong.
- Process: clean, port, seal, inject bottom-up, cure, remove ports. Straightforward when done right.
- Injection isn't the answer for bowing walls, settlement, or actively growing cracks — those need structural repairs.
- DIY is possible for small stable cracks with realistic expectations. Professional for anything larger or uncertain.
- Address the cause too. An injection that seals a crack while ignoring the water pressure or settlement that caused it is a temporary win.