How to Seal Concrete Cracks to Reduce Radon

Which Cracks and Gaps Matter Most

Priority 1: The floor-wall joint

The gap where the basement floor slab meets the foundation wall runs the full perimeter of the basement. It's often 1/8"–1/2" wide and accounts for a large percentage of total radon entry in many homes. This is the highest-value sealing target.

Priority 2: Visible slab cracks

Control joints (the intentional straight-line cuts in the slab) and settlement cracks (irregular, often wider in the middle) are entry pathways. Wide cracks (1/8"+) are more significant than hairline cracks.

Priority 3: Pipe and wire penetrations

Any pipe or conduit passing through the slab (floor drain, water supply, sewer) creates a gap that allows soil gas to enter. The annular space around these penetrations should be sealed.

Priority 4: Porous concrete (block walls)

Hollow concrete block walls allow radon to travel through the block cores and exit into the basement. Surface sealing helps but doesn't address the interior core pathway — this requires more involved approaches.

Materials and Their Applications

Polyurethane sealant (e.g., Sikaflex, OSI QUAD):

For cracks up to 1/4" wide and joints where slight movement is expected. Flexible — doesn't crack when the concrete moves slightly with temperature. Apply with a caulk gun; tool smooth with a wet finger.

Hydraulic cement:

For the floor-wall joint and wider gaps (1/4"+). Sets rapidly (3–5 minutes), bonds to wet concrete, and is rigid enough to resist soil pressure. Mix per instructions to peanut butter consistency; pack firmly into the joint.

Epoxy injection:

For structural cracks where rigidity is needed. Two-part epoxy is injected under pressure to fill the full depth of the crack. Stronger than polyurethane but doesn't flex. Best for static, non-moving cracks.

RadonSeal concrete sealer:

For overall concrete porosity reduction. Applied to the full slab and wall surfaces; penetrates and chemically hardens the concrete matrix. Not a crack filler — works on diffusion through intact concrete.

Step-by-Step: Sealing the Floor-Wall Joint

The floor-wall joint is the most impactful single sealing target. Here's how to do it:

1. Clean the joint: Remove loose concrete, dust, and old caulk with a wire brush and vacuum. The joint needs to be clean and dust-free for adhesion.

1. Widen if needed: If the gap is very narrow (<1/8"), use an angle grinder or oscillating tool to widen it to 1/4" — this gives sealant enough depth for a durable bond.

1. Apply backer rod (for gaps >1/4"): Foam backer rod pushed into the joint creates a backing surface for the sealant and controls application depth.

1. Apply hydraulic cement or polyurethane:

• Hydraulic cement: Mix to thick consistency, press firmly into the joint, and smooth before it sets (3–5 minutes)
• Polyurethane: Apply from a caulk gun in a continuous bead; tool into the joint and smooth

1. Allow full cure: Hydraulic cement: 24 hours before painting or coating. Polyurethane: 24–48 hours to skin over; 7 days for full cure.

What Sealing Cannot Do

Sealing addresses visible entry points but doesn't:

• Create negative pressure beneath the slab (only a fan does that)
• Stop radon diffusion through porous concrete (use RadonSeal for that)
• Address radon entering through the sump pit (use an airtight cover)
• Prevent radon that's already in the sub-slab air from entering through new pathways as sealed ones close

For homes above 4.0 pCi/L, sealing is a supplement — not a solution. A sub-slab depressurization system addresses all entry pathways simultaneously.

Shop RadonSeal → | Find a certified mitigator →