What kind of sealant works best for sealing cracks in a poured concrete foundation wall?

Polyurethane-based crack sealant is the best choice for sealing cracks in poured concrete foundation walls in the GTA, but the right product depends on whether the crack is actively leaking, hairline or wider, and above or below grade.

For non-leaking hairline cracks (under 3mm wide) on above-grade foundation walls, an elastomeric masonry sealant is the most practical option. These sealants remain flexible after curing and can expand and contract with Toronto's extreme temperature swings — critical when your foundation wall endures over 50 freeze-thaw cycles every winter. Products like Sikaflex or Loctite PL Concrete & Masonry sealant bond well to concrete, remain flexible down to -30°C, and can bridge cracks up to 3mm. You will pay $15–$30 per tube at GTA building supply stores, and a single tube covers roughly 10–15 linear feet of crack depending on width and depth.

For wider cracks (3mm to 10mm) that are not actively leaking, a two-part polyurethane caulk applied with backer rod gives the best long-term seal. Press closed-cell backer rod into the crack to a depth roughly equal to the crack width, then apply the sealant over top. The backer rod prevents the sealant from bonding at the bottom of the crack (called a "three-sided bond"), which allows it to stretch properly as the crack moves with seasonal thermal cycling. This is especially important in the GTA where foundation walls can shift slightly between summer and winter due to clay soil expansion and contraction. Expect to spend $8–$15 per tube of polyurethane caulk plus $5–$15 for a roll of backer rod.

For actively leaking cracks in poured concrete, surface sealant alone will not solve the problem. The water pressure behind the wall will eventually push through or around any surface-applied product. In this case, crack injection is the proper repair. A professional injects either epoxy resin (for structural bonding — fills the crack solidly and restores structural integrity) or polyurethane foam (for waterproofing — expands to fill the crack and remains flexible). Epoxy injection is preferred for structural cracks and costs $300–$600 per crack in the GTA. Polyurethane foam injection is better for non-structural cracks where ongoing movement is expected, and runs $300–$800 per crack depending on length and accessibility.

Before applying any sealant, clean the crack thoroughly. Use a wire brush to remove loose concrete, dust, and any old failed sealant. For best adhesion, the concrete should be clean, dry, and free of paint or coatings along the crack edges. If you are sealing a crack that has been previously repaired and failed, grind the edges back to clean concrete before resealing.

One important caution: if you see step cracks, horizontal cracks, or any crack wider than 5mm that appears to be growing, do not simply seal it and move on. These patterns can indicate structural foundation movement caused by GTA clay soil pressure, hydrostatic pressure, or settling. A structural crack requires professional assessment — sometimes carbon fibre straps, wall anchors, or even underpinning are needed. Sealing a structural crack with caulk hides the problem and allows it to worsen unseen, potentially turning a $2,000 repair into a $20,000 foundation stabilization project.

For most homeowners dealing with typical hairline settling cracks on above-grade foundation walls, applying a quality elastomeric sealant is a straightforward DIY project. Clean the crack, apply the sealant with a caulking gun, tool it smooth with a wet finger or plastic spoon, and allow 24–48 hours to cure before painting. If the crack is below grade or actively leaking, bring in a professional for proper crack injection — it is one of the most cost-effective foundation repairs available in the GTA market.