How to Seal Foundation Gaps to Keep Pests Out

Foundation gaps are the primary highway for ants, spiders, mice, and crickets to enter your home. Over time, concrete shrinks and settles, creating hairline cracks and wider voids where the foundation meets the sill plate or siding. Sealing these is the single most effective way to cut down on household pests without relying on chemical sprays. Done well, this project creates a permanent, flexible, and impenetrable barrier. You aren't just filling holes; you are reinforcing the thermal and physical envelope of your home. Focus on the transition points where different building materials meet, as these are the weak spots where pests find easy access.

1. Find Every Hidden Entry Point. Walk the entire exterior of your house with a bright flashlight. Check the seam between the concrete foundation and the wooden sill plate, and look for cracks wider than a dime.
2. Strip Away Old Barriers. Use a stiff wire brush to scrape away loose concrete, dirt, old crumbling caulk, and spider webs from inside the cracks. Vacuum out the debris or use compressed air to ensure the sealant has a clean surface to bond to.
3. Armor Every Large Gap. Cut pieces of stainless steel mesh to fit tightly into any gaps larger than a quarter-inch. Stuff the mesh deep into the gap using a putty knife, ensuring it creates a physical barrier that rodents cannot chew through.
4. Lock Down the Line. Load an exterior-grade polyurethane sealant into your caulking gun. Apply a steady, overlapping bead into the crack, pushing the tip of the nozzle into the gap to fill it from the back forward.
5. Perfect the Seal. Drag a finger dipped in soapy water over the wet caulk to smooth it out. This ensures the sealant makes full contact with both sides of the gap, creating a water-tight and pest-tight seal.
6. Verify Your Work Holds. Allow the sealant to cure for at least 24 hours before expecting full pest-repelling performance. Check back after a week to ensure no gaps have reopened due to movement.