Closing openings around pipes under a kitchen sink is very important for homeowners both to save on energy costs and to prevent pest issues. Exclusion is one of the first steps in a program to deny pests the opportunity to establish themselves within your home. Pests can take advantage of surprisingly small spaces to gain entry. During construction, try to keep holes around pipes as small as possible. Where you can, cut your cabinet backing and flooring to fit as closely as possible around pipes. Once you finish treating the kitchen, seal around pipes and openings in the bathroom and around your home's exterior.
Repair any water damage or leaks in the sink area. Damaged wood can make it easier for pests to enter your home, and most patches will need a solid backing to hold properly.
Seal smaller cracks using caulk. Select a caulk appropriate to the type of material you need to seal. This will exclude insects and snakes, but won't prevent mice from gnawing their way in. Spray expanding foam sealant into larger cracks. The foam will stop insects and cold air and can be used indoors and outdoors, but is not rodent-proof.
Stuff steel wool into small cracks using a screwdriver to exclude rodents. Mice can't chew through the metal fibers. Remember that steel wool may rust in damp locations and may require replacement.
Block large openings by creating a patch using welded wire and patching compound, suggests the University of Massachusetts Extension website. Use wire cutters to trim roughly a square of wire in the size you'll need to cover the hole plus an approximate 1-inch overlap. Apply the patching compound over or around the perimeter of the wire using a putty knife to smooth the surface.
Install hardware cloth or flexible metal screen material around oddly shaped holes. These materials will exclude insects and rodents. Cut the screen to fit the hole, and use caulk or patching compound to seal the edges. The University of Massachusetts Extension website warns that you must use woven hardware cloth, as unwoven versions may break. Use 1/8-inch mesh to prevent snakes and mice from entering through the material.
Cut a can lid to fit small- to medium-sized holes using metal snips. Nail the metal in place or attach it using patching compound. Holes need to be less than 1/4-inch in diameter to exclude mice and to exclude snakes. Such a patch will help prevent rodents and snakes from entering around pipes.