How to Seal Foundation Cracks from Inside (4-Step Guide)

TL;DR: Most hairline-to-⅛" foundation cracks can be sealed from inside using epoxy injection (for dry, structural repair) or polyurethane foam (for active water leaks). Clean and chisel the crack, install injection ports every 8-12 inches, pump in the resin under low pressure, and remove ports after cure. Skip DIY if the crack is wider than ¼", shows stair-stepping, or runs diagonally—those need a structural engineer first.

_Last reviewed: July 2026 · 7 min read_

A thin crack in your basement wall doesn't always mean disaster, but it does mean water has a path in—and ignoring it turns a $50 DIY fix into a $3,000 excavation. Most non-structural foundation cracks under ¼" wide can be sealed from the inside using injectable epoxy or polyurethane foam, stopping water and preventing the crack from widening. You'll skip digging up the yard and get a permanent repair in a weekend if you prep the surface correctly and choose the product that matches whether the crack is dry or actively leaking.

Okoniq Property Hub lets you log the crack location, upload photos, and set a reminder to inspect it annually—so you catch movement before it becomes a structural problem.

Is the crack structural or cosmetic, and does that change the repair method?

Determine whether the crack needs engineering before you seal it. Hairline cracks (under ⅛") that run vertically are usually settlement shrinkage and pose no structural risk—epoxy or polyurethane will handle them. Cracks wider than ¼", horizontal cracks, stair-step patterns in block walls, or diagonal cracks are red flags for foundation movement; sealing them without addressing the cause will fail within months. Measure the width with a crack gauge or a dime (1.35 mm; anything that swallows the dime's edge is ¼"+). Mark both ends of the crack with pencil and check again in 30 days—if the mark has moved, call a structural engineer before you seal.

If the crack is actively weeping, choose polyurethane foam; it expands on contact with water and remains flexible. If the crack is dry and you want a rigid, load-bearing bond, use epoxy resin. The difference matters: epoxy cures rock-hard and restores the concrete's original strength, while polyurethane stays rubbery to accommodate slight movement and block hydrostatic pressure. For borderline cases—cracks that show dampness but no pooling—epoxy works if you dry the surface with a heat gun for 15 minutes before injection.

What tools and materials do you need for an interior injection repair?

Assemble the correct injection kit and hand tools before you start. You'll need an epoxy or polyurethane injection kit (Emecole MetroChem or Simpson Strong-Tie kits run $80-$150 for 10 linear feet), a caulking gun or low-pressure injection pump (manual pumps cost $30-$60), a wire brush, a cold chisel, a shop vacuum, injection ports (plastic nozzles that stick into the crack every 8-12 inches), surface-seal paste (comes in most kits), mixing paddles, disposable gloves, safety glasses, and a respirator rated for organic vapors. If the crack is above shoulder height, rent a scaffold—ladders shift and you need both hands free during injection.

Buy 20% more resin than the manufacturer's coverage chart suggests; cracks rarely run straight and voids behind the wall consume more material than the surface width implies. For a typical ⅛" × 8' vertical crack in an 8" poured wall, budget 16 ounces of mixed epoxy. Polyurethane foams expand 15-20 times their liquid volume, so you'll use far less—4 ounces can fill the same crack if it's actively wet. Read the technical data sheet for your product; cure times vary from 30 minutes (fast-set epoxy) to 24 hours (flexible poly), and you cannot inject a second crack until the first has fully cured or you'll blow out the surface seal.

How do you prep the crack and install injection ports?

Clean and widen the crack at the surface so the resin can penetrate. Use a cold chisel and hammer to create a ¼" × ¼" V-groove along the entire length of the crack on the interior face—this gives the surface-seal paste a mechanical key and prevents resin from leaking out during injection. Vacuum all dust and loose concrete with a shop vac, then wipe the groove with a damp rag (not soaking—you want the concrete damp, not wet, unless you're using polyurethane foam on an active leak). If the wall is painted, grind or wire-brush the paint 2 inches on either side of the crack; resin won't bond to latex or oil coatings.

Drill holes for injection ports at a 45° angle into the crack, spacing them 8-12 inches apart and staggering them left-right if the crack wanders. Use a ¼" masonry bit and drill 2-3 inches deep, aiming to intersect the crack's interior path. Insert the plastic ports (they look like small grease fittings) and tap them flush with a rubber mallet—don't use a steel hammer or you'll crack the fitting. Mix the surface-seal paste (usually a two-part epoxy putty or hydraulic cement that comes in the kit) and trowel it over the entire V-groove, burying the base of each port but leaving the injection tip exposed. Let the paste cure for the time specified in the instructions—usually 3-6 hours for epoxy paste, 30 minutes for fast-set hydraulic cement. The surface seal must harden completely or the injection pressure will blow it out and waste your resin.

Track your foundation inspection schedule and material purchases in Okoniq's maintenance log, where you can also attach before-and-after photos to compare crack width over time.

How do you inject the epoxy or polyurethane and finish the repair?

Start injection at the lowest port if the crack is vertical, or at one end if it's horizontal. Mix the two-part epoxy or load the polyurethane cartridge into your caulking gun or injection pump, attach the nozzle to the first port, and pump slowly—10-15 seconds of pressure, then pause. You'll feel resistance as the resin fills the crack; when resin begins to seep from the next port up (or along), stop pumping the first port, cap it with the included plug, and move to the second port. Repeat port-by-port until resin emerges from the top or final port, then cap that one. If you're using polyurethane on a wet crack, expect foam to expand out of the ports as it reacts with water—this is normal; wipe excess with a rag soaked in mineral spirits before it cures.

Let the resin cure per the manufacturer's schedule—typically 24-48 hours for epoxy, 24 hours for polyurethane. After cure, twist off the exposed port tips with pliers (they'll snap at the base you buried in the surface seal) and grind or chisel the surface seal flush with the wall. You can leave it as-is for an unfinished basement, or patch it with concrete patching compound and paint over it. Test the repair by running a hose against the wall for 15 minutes during the next rainstorm; if water reappears, you missed a branch crack or the seal failed—re-clean and re-inject that section. For active leaks that return, the problem is usually hydrostatic pressure from poor exterior drainage; consider adding a basement dehumidifier and ensuring downspouts drain at least 6 feet from the foundation.

When should you skip DIY and hire a contractor or engineer?

Call a professional if the crack shows any of these signs: width exceeding ¼ inch, horizontal or diagonal orientation, stair-step pattern in block walls, offset (one side of the crack has shifted relative to the other), or new cracks appearing in clusters. These patterns suggest ongoing foundation settlement, lateral soil pressure, or structural overload—sealing the crack without addressing the root cause will fail and may hide evidence an engineer needs to see. A structural engineer's inspection costs $400-$800 and will tell you whether you need underpinning, exterior waterproofing, or just drainage correction before you seal.

Hire a licensed foundation repair contractor (not a general handyman) if the crack requires injection from both sides, if it's in a crawlspace with less than 3 feet of clearance, or if the wall is bowing more than 1 inch. Contractors have commercial-grade pumps that deliver higher pressure for deep penetration and can inject from the exterior if interior access is blocked by finished walls. Expect to pay $300-$600 per crack for professional injection, or $3,000-$10,000 if excavation and exterior membrane work is needed. For minor cosmetic cracks that aren't leaking and haven't moved in a year, you can skip injection entirely and just apply a coat of hydraulic cement or masonry waterproofer—but log the crack location in Okoniq and re-inspect every 6 months to catch movement early.

FAQ

How long does an interior epoxy crack repair last?

A properly executed epoxy injection repair lasts 10-15 years or longer in stable soil conditions, often outlasting the surrounding concrete. Polyurethane foam repairs last 5-10 years and may need re-injection if the crack reopens due to foundation movement, but they handle wet conditions and minor settling better than rigid epoxy.

Can you seal a foundation crack from inside if water is actively pouring through?

Yes, but only with polyurethane foam, not epoxy. Polyurethane reacts with water to expand and cure, so it will fill and seal an active leak within 15-30 minutes. Epoxy requires a dry surface to bond and will not cure if the crack is wet—you'd need to stop the water with a temporary hydraulic cement dam before injecting epoxy.

Do you need to seal foundation cracks from the outside too?

Not for most hairline and vertical settlement cracks. Interior injection fills the crack's full depth and stops water migration. Exterior sealing is necessary only if the crack is part of a larger drainage or waterproofing failure, if you're already excavating for another reason, or if the crack is horizontal and caused by lateral soil pressure—in which case you need exterior drainage correction and possibly a French drain or sump pump upgrade.

Will sealing a crack stop the foundation from moving or settling further?

No. Crack injection stops water and restores some tensile strength to the concrete, but it does not address the soil, drainage, or structural loads that caused the crack. If the foundation is still settling or the soil is expanding, the crack will reopen or new cracks will form. Fix the underlying cause—improve grading, extend downspouts, install a sump pump—before or immediately after sealing the crack.

Can you paint over a sealed foundation crack or apply new waterproofing coating?

Yes, once the epoxy or polyurethane has fully cured and you've ground the surface seal flush. For epoxy repairs, wait 48 hours, then scuff-sand the patched area with 80-grit sandpaper so paint or masonry sealer will adhere. For polyurethane repairs, some products leave a slightly oily residue—wipe with denatured alcohol before painting. Any hydraulic cement or waterproof coating will bond to a properly prepped repair.

This is educational information, not structural engineering or contracting advice. For cracks wider than ¼", horizontal cracks, or signs of active foundation movement, consult a licensed structural engineer and obtain a soil report before attempting repair.