Foundation Crack Injection (Epoxy vs Polyurethane)

Why this matters

A poured concrete foundation crack is one of the most common foundation issues. The customer sees a vertical or diagonal crack 1/32 to 1/4 inch wide in the basement wall and assumes the worst. Most cracks are not structural; they're shrinkage cracks (the concrete cured too quickly) or stress cracks (settling), and they're treatable through crack injection. The two injection methods (epoxy and polyurethane) have different properties and serve different goals. Picking the wrong one produces a repair that fails - either the crack reopens, or water still leaks. Knowing which to use is the diagnostic skill.

When to inject

For cracks in poured concrete foundations:

• Vertical crack (most common; cosmetic and water-leak treatment)
• Diagonal crack (often related to settling)
• Horizontal crack with no significant displacement (less common)
• Wet cracks (water leakage)

When NOT to inject (recommend other repair):

• Wall is significantly bowed (over 1.5 inches)
• Wall is structurally compromised
• Cracks in block walls (different repair approach)
• Multiple parallel cracks (indicates major settlement)

The two materials

Epoxy injection

A two-part epoxy that bonds chemically to the concrete:

• Structural repair: makes the wall as strong as before the crack
• Bonds chemically to clean, dry surfaces
• Cures hard; doesn't expand
• Used when crack is dry and the goal is structural restoration
• Service life: 30+ years; effectively permanent

Polyurethane foam injection

A polyurethane foam that expands when injected:

• Fills voids and seals against water
• Flexible when cured (allows for some movement)
• Doesn't bond as strongly as epoxy
• Used when crack is wet (water leakage) and the goal is sealing
• Service life: 20+ years

Decision matrix

For most residential basement crack repair (water-leaking crack), polyurethane is the right choice.

Procedure for typical crack injection

Step 1: Assess the crack

• Width (how wide is the crack)
• Length (how long does it extend)
• Wet vs dry (is water actively leaking)
• Cause if known (settling, shrinkage)
• Adjacent cracks or wall movement

Step 2: Prepare the surface

• Clean the area around the crack
• Remove loose debris, paint, sealer
• For epoxy: surface must be completely dry
• For polyurethane: surface can be wet

Step 3: Install injection ports

Inject ports are mounted along the crack:

• Surface-mount ports (typical residential)
• Drill in ports (for deeper sealing)

Place ports approximately 6 to 12 inches apart along the crack length.

Step 4: Surface-seal the crack

Apply a surface paste over the crack between ports:

• Quick-setting epoxy paste
• Seals the crack surface so the injection material doesn't leak out
• Allow paste to cure (15 to 30 minutes typical)

Step 5: Inject

Starting at the lowest port:

• Inject material at moderate pressure
• Material fills the crack and travels along
• When material flows from the next port up, move to that port
• Continue up the wall

For polyurethane:

• Foam expands as it enters
• Some bubbling visible from upper ports
• The expansion drives the foam into all voids

For epoxy:

• Liquid epoxy fills the crack
• Less visible expansion
• Bonds chemically to surfaces

Step 6: Allow cure

• Polyurethane: 30 to 60 minutes
• Epoxy: 24 hours typically

Step 7: Remove ports and surface paste

After cure:

• Pry off injection ports
• Chip away surface paste
• Smooth the wall surface
• Customer can paint over

Common mistakes

Wrong material for the condition

• Epoxy on wet crack: doesn't bond properly; sealing fails
• Polyurethane on dry crack with structural concern: works for sealing but not structural

Inadequate surface preparation

• Dirty surface; epoxy doesn't bond
• Paint not removed; bond fails

Insufficient injection ports

• Material doesn't flow throughout the crack
• Voids remain; water still leaks

Wrong pressure

• Too low: material doesn't penetrate fully
• Too high: surface paste blows out

Skipping surface seal

• Injection material leaks out the crack surface
• Doesn't fill the deeper portions

Common crack types and recommendations

Vertical shrinkage crack

Most common in poured concrete. Typical width 1/32 to 1/8 inch. Caused by concrete shrinkage during cure. Usually not structural.

• Polyurethane injection for water sealing

Diagonal stress crack

Often near corners. Indicates some settling or stress. Width varies.

• Inspection for stability first
• If wall is not moving: polyurethane sealing
• If wall is moving: structural assessment first

Horizontal crack with displacement

A horizontal crack with the wall pushed in slightly above the crack is a serious structural issue. Don't just inject:

• Engineering assessment required
• Wall stabilization (carbon fiber straps, wall anchors)
• Then injection if needed

Multiple parallel cracks

Indicates significant settlement. Beyond simple injection:

• Foundation assessment
• May need underpinning (helical or push piers)
• Crack injection as part of a larger repair

Warranties

Most professional crack injection comes with:

• 10 to 25 year warranty (workmanship)
• Transferable to subsequent homeowners

Customer should keep documentation.

Customer expectations

• "Injection seals the crack; water shouldn't leak through it again"
• "If the cause of the crack is ongoing (settlement, water table), new cracks may form"
• "Cost is significantly less than excavation alternatives"
• "Crack is still visible (cosmetic); injection is internal"

When alternatives are better

Exterior membrane

For severe water issues:

• Excavate around the foundation
• Apply waterproof membrane to exterior
• Install drain at footing level
• Backfill with drainage stone

Interior drainage system

For basement that gets wet from multiple sources:

• Install perimeter drain inside the basement
• Sump pump system
• Doesn't address the crack but manages water that enters

Carbon fiber wall reinforcement

For walls that are bowed but not severely:

• Apply carbon fiber strips to interior wall
• Prevents further bowing
• Cosmetic but effective

References

• ACI 224 (Causes, Evaluation, and Repair of Cracks in Concrete Structures).
• ASTM C881 (Standard Specification for Epoxy-Resin-Base Bonding Systems).
• Manufacturer literature: SikaFix, Prime Resins, Polylevel injection, Concresive, Epoxy Industries.
• Manuall internal: Foundation Repair Foundation Settling Diagnostics, Foundation Repair Bowed Wall Remediation.