How to Seal Pipe Penetrations Against Pests

Pests view the plumbing penetrations under your kitchen sink as a grand entrance. Even a gap the size of a pencil is enough for mice, spiders, or roaches to squeeze through, bypassing your walls and entering your living space. Sealing these gaps isn't just about pest control; it stops moisture and drafts that create the perfect breeding ground for unwanted guests. Done well, this project is invisible but indestructible. You are looking to create a physical barrier that rodents cannot chew through while ensuring the seal remains airtight. Once you have blocked the physical path, you eliminate the highway that pests use to travel between your utility lines and your cabinets.

Find Every Gap First. Empty the cabinet under the sink and clean the area around the pipes with a vacuum to remove debris. Use a flashlight to identify every gap where the pipes exit the wall or floor, checking behind cabinets where lines often run.
Pack the Mesh Tight. Cut pieces of copper wool or mesh to fit the gaps around your pipes. Push the mesh firmly into the holes using a flathead screwdriver until the space is tightly packed.
Fill with Expansion Foam. Apply a pest-blocking expansion foam over the copper mesh to fill the remainder of the hole. Hold the nozzle at the back of the gap and fill toward the front, letting the foam expand to seal the edges.
Trim Flush with Wall. Wait for the foam to cure completely, which usually takes about one to two hours depending on the brand. Once fully hardened, use a utility knife to trim the excess foam so it sits flush with the wall.
Seal with Silicone. Run a bead of silicone caulk around the base of the pipe where it meets the newly trimmed foam or the wall plate. Smooth the bead with a damp finger to ensure a continuous seal.
Verify All Gaps Sealed. Check the pipe seals from the other side if accessible, such as in the basement or utility room. Ensure there are no gaps left exposed that could allow pests to move between floor levels.