How to Seal Foundation Cracks to Keep Pests Out

Foundation cracks are essentially welcome mats for rodents, ants, and spiders looking for a way into your home. Even a hair-thin fissure provides enough space for pests to squeeze through or burrow further into your structure, often leading to bigger issues behind your drywall. Sealing these gaps is a foundational chore that protects your perimeter from unwanted visitors and prevents moisture damage in the process. Done well, this repair creates a seamless, flexible bond that moves with your house as temperatures change. You aren't just plugging a hole; you are removing the primary entry vector for your local pest population. It takes patience to clean the cracks properly, but the result is a locked-down exterior that stays sealed for years.

  1. Scrub the Crack Clean. Use a stiff wire brush to scrub away dirt, loose concrete, and moss from inside the crack. Vacuum out all remaining debris with a shop vac to ensure the sealant has a clean, dry surface to grip.
  2. Widen Into a V-Shape. For cracks narrower than an eighth of an inch, use a masonry chisel and hammer to widen them into a V-shape. This creates a profile that allows the sealant to anchor firmly to both sides of the concrete.
  3. Insert Your Base Foundation. Push a foam backer rod into the crack until it sits about a quarter-inch below the surface. This creates a solid base for the sealant so it doesn't just sink into the hollow space behind the wall.
  4. Gun the Sealant Smooth. Cut the tip of the sealant tube at a 45-degree angle to match the width of the crack. Apply the sealant in a smooth, continuous bead, moving the caulk gun steadily along the length of the opening.
  5. Press and Smooth Flush. Use a putty knife or a gloved finger to press the sealant into the crack and smooth it flat against the surface. Ensure there are no gaps or pinholes where an insect could potentially crawl through.
  6. Wait and Inspect Thoroughly. Allow the sealant to cure for the time specified on the packaging, usually 24 hours. Once set, inspect the area to ensure the seal is tight and flush with the surrounding foundation.