How to pest-proof your home

Pests that exclusion keeps out

House Mouse Mus musculus The house mouse is a small, adaptable rodent that lives close to people in homes, schools and food businesses, eating and contaminating food, gnawing wiring and carrying disease. Norway Rat Rattus norvegicus The Norway rat, also called the brown or sewer rat, is a large, burrowing rat found throughout the contiguous United States wherever people live. Roof Rat Rattus rattus The roof rat, also called the black or ship rat, is a sleek, agile climber that nests above ground in trees, ivy, attics and wall voids. German Cockroach Blattella germanica The German cockroach is the most common cockroach living indoors in California and perhaps worldwide, especially in apartment buildings, and it has the fastest reproductive cycle of the common pest cockroaches. Penn State and Kentucky call it the most common cockroach in homes, apartments and other buildings, and Kentucky says it does not live outdoors. Arizona Bark Scorpion Centruroides sculpturatus The Arizona bark scorpion is a slender, climbing scorpion of the desert Southwest that often lives in and around homes. It is the only scorpion in the United States whose sting is considered a medical concern. Boxelder Bug Boisea trivittata Boxelder bugs feed mainly on the seeds of female boxelder trees and become a nuisance in fall, when large numbers gather on and around homes looking for a place to spend the winter. Indoors they can stain fabrics and smell when crushed, but they don't damage the structure and don't breed inside. Brown Marmorated Stink Bug Halyomorpha halys The brown marmorated stink bug is an invasive insect from east Asia that damages fruit and vegetable crops and invades homes in fall looking for overwintering sites. Minnesota Extension says it feeds on about 200 plant species and, unlike the native stink bugs there, prefers to spend the winter in homes. Clemson notes that the bugs gather on outside walls before they move in. Raccoon Procyon lotor Raccoons are masked, ring-tailed mammals that adapt very well to towns and suburbs. They raid garbage and gardens, den in attics, chimneys and under decks, and can carry rabies and raccoon roundworm.

Where pests get in: what to inspect

CDC suggests checking inside and outside the house for both large and small gaps. Outside, look in the roof among the rafters, gables, eaves and soffits; around windows and doors, especially those without weather stripping; where the foundation meets the ground; around attic and crawl space vents; and around holes for electrical, plumbing, cable and gas lines.^[6]

Inside, CDC's list covers under and behind kitchen cabinets, closet floor corners, around the fireplace, behind appliances, around the pipes under sinks and washing machines and those leading to water heaters and furnaces, around floor air and dryer vents, inside the attic, in the basement or crawl space, around floor drains, and where the floor meets the wall.^[6]

ICWDM adds sill plates, door thresholds and utility lines to the inspection, inside and out. For bats, UC IPM says entry points are often beneath eaves, around the chimney, around air and plumbing vents, near loose boards, beneath roof caps, or in openings squirrels or other animals have made; stains from body oils, urine and droppings below a gap are a sign it's being used.^[7][10]

Seal the openings

Exclusion works on more than rodents. UC IPM says sealing cracks and using door sweeps and weather stripping keep cockroaches from coming indoors, Kentucky's entomologists recommend tight-fitting screens and door sweeps for spiders, and Arizona's extension calls pest-proofing by far the most effective way to reduce scorpion contact and stings.^[16][17][19]

• Seal cracks in the foundation and openings around water pipes, vents, wires, sewer pipes and downspouts with metal or concrete.^[1][2]
• Make doors, windows and screens fit tightly; seal exterior doors at the threshold and between double doors; cover door and window edges with metal where rodents gnaw.^[1][2]
• Cover ventilation grills with 1/2-inch galvanized mesh (18-gauge expanded metal or 16-gauge wire cloth) to keep rats out.^[2]
• Steel wool scouring pads make good temporary plugs but may rust over time.^[1]
• Keep siding, eaves and soffits in good repair, and window screens free of holes.^[4]

How small a gap each pest can use

Sources don't all draw the line in the same place. The sizes below are the ones each publisher gives; where they differ, both are listed. Sealing to the smallest size that applies to your pests covers the larger ones too.^{[1][8][15][11]}

• House mice: UC IPM says they can squeeze under gaps 1/4 inch tall and through openings 3/8 inch wide. CDC puts it as a hole the width of a pencil. UC IPM, CDC and NPIC all say to seal openings larger than 1/4 inch.^[1][6][9]
• Rats: UC IPM says rats, especially young ones, can squeeze beneath a door with only a 1/2-inch gap, and recommends sealing all holes larger than 1/4 inch to exclude both rats and mice. ICWDM's Norway rat guidance sets the line at openings larger than 1/2 inch.^[2][8]
• Bats: UC IPM says bats can squeeze through openings as small as 1/4 inch. Delaware's wildlife agency says bats need only 1/8 inch to get back in once they've been excluded.^[10][11]
• Snakes: ICWDM says all openings 1/8 inch and larger should be sealed to exclude snakes. Snakes don't gnaw, so ICWDM says closures that keep out mice will also keep out snakes.^[15]
• Tree squirrels: UC IPM says that when even a small opening is found, squirrels can enlarge it by gnawing, and that without an obvious entrance they can gnaw their own way into an attic.^[13]
• German cockroaches: UC IPM says adults can hide in a crack 1/16 inch wide and the young in even smaller ones, so sealing cracks and hollow spaces such as false-bottom cupboards and hollow walls matters indoors as well as at the outside walls.^[16]

Materials that hold up

For rodents, the material matters as much as the gap. UC IPM says plastic screening, rubber, vinyl, insulating (expanding) foam, wood and other gnawable materials are ineffective for plugging holes used by house mice, and its rat notes say door and window edges can be covered with sheet metal if gnawing is a problem.^[1][2]

Expanding foam is where sources differ. ICWDM says foam that expands is useful to reduce heat loss but will not prevent rodent entry. CDC's seal-up steps use it differently: fill small holes with steel wool, then put caulk around the steel wool to keep it in place or use spray foam. If you use foam, treat it as a holder for the metal plug, not the plug itself.^[7][6]

• Small holes: CDC suggests steel wool held in with caulk. ICWDM suggests stainless steel or copper wool, or covering the opening with 1/4-inch wire mesh.^[6][7]
• Larger holes: CDC lists lath screen, metal, cement, hardware cloth or metal sheeting, cut to fit around pipes. For rats, ICWDM names concrete mortar, 24-gauge galvanized sheet metal and 18-gauge hardware cloth as heavy materials that resist gnawing.^[6][8]
• Skip chicken wire. UC IPM says never to use lightweight chicken wire, since rats can pass through the wire openings.^[2]
• Squirrel entrances: UC IPM lists sheet metal or 1/4-inch wire hardware cloth.^[13]
• Where animals don't gnaw, lighter closures work. UC IPM says bat entryways can be sealed with caulking, putty, duct tape, self-expanding polyurethane foam or 1/4-inch hardware cloth once the bats are out. For insect pests, Kentucky's entomologists suggest caulk for gaps and cracks and door sweeps or weather stripping for gaps under doors and windows.^[10][18]

Doors, windows and screens

Gaps under doors are a common way in. Kentucky's entomologists point to them for spiders and recommend door sweeps; NPIC recommends door sweeps and weather stripping to keep mice from crawling under doors; UC IPM recommends both for cockroaches; and Arizona's extension says door sweeps should fit tightly with no gaps for scorpions.^{[17][9][16][19]}

For rodents, UC IPM adds a caution: typical weatherproofing materials may not be sufficient to exclude rats and mice. It says exterior doors should be tight-fitting and sealed at the threshold, and double doors sealed at the astragal gap, the space between the two doors. On wooden doors, UC IPM notes a rat may gnaw to enlarge a gap.^[2]

• Weather stripping around doors and windows with a snug fit (Arizona extension, for scorpions); a scorpion in one of its figures likely came in through a gap in a door's weather stripping.^[19]
• Window screens that are in good condition and fit tightly in the frame. UC IPM says every window that can be opened should be screened.^[19][2]
• Windows that open near the ground can be protected from rats with 19-gauge, 1/2-inch mesh galvanized wire cloth, per UC IPM.^[2]
• Tighter-fitting screen doors help keep single bats out, according to UC IPM, which says bats get in through open doors and windows and doors with loose screens.^[10]

Vents, chimneys, roof and walls

Roof rats and squirrels come in high. UC IPM says roof rats are excellent climbers, so openings above ground level must be plugged too, and that tree squirrels traveling on utility lines and fence tops often find entrances at about those heights.^[2][13]

• Roof and attic vents: UC IPM says to repair or replace damaged foundation ventilation screens and the internal screens on roof and attic air vents and under the eaves.^[2]
• Rooftop plumbing vents: UC IPM recommends covering vent pipes more than 2 inches in diameter with screens over their tops.^[2]
• Bathroom and exhaust vents: University of Nebraska–Lincoln Extension suggests 1/2-inch mesh screens to keep birds from nesting, and cautions about dryer vents, where lint can build up in the vent and catch fire.^[20]
• Chimneys: UC IPM recommends a spark arrester that meets your area's fire code, secured tightly so raccoons can't pull it loose; it keeps raccoons, tree squirrels, rats and birds out. For bats, UC IPM suggests 1/2-inch or smaller welded wire mesh over the chimney top and checking local spark arrester rules with the fire department.^[14][10]
• Crawl spaces: UC IPM says to provide a tight-fitting cover for the crawl space. Kentucky's termite guidance says to keep crawl space vents free of leaves, dirt, debris and vegetation.^[2][3]
• Weep holes in brick veneer: screen them, don't plug them. Arizona's extension says to use coarse steel mesh, because the holes are important for ventilating the wall spaces; ICWDM suggests weep vent screens or exclusion cloth to keep snakes out.^[19][15]
• Roof eaves: Arizona's extension also suggests caulking around roof eaves, pipes and other wall penetrations against scorpions.^[19]

Pipes, wires and other utility penetrations

Every pipe, wire and cable that passes through a wall or the foundation leaves a possible gap. UC IPM says to seal all openings around pipes, cables and wires that enter through walls or the foundation, and CDC says to cut the patch material to fit around the pipes.^[2][6]

Look on both sides of the wall. CDC's inside checklist includes the pipes under sinks and washing machines, the lines to water heaters and furnaces, and all electrical, water, gas and sewer lines. ICWDM says to check the corners of doors and windows, around water pipes and where utility lines enter for snakes as well.^[6][15]

If roof rats are traveling along overhead utility wires, UC IPM says to contact a pest professional or the utility company for help preventing it.^[2]

Before you seal: make sure nothing is inside

Closing a hole with an animal behind it traps it in the house. UC IPM suggests a simple test for squirrels: plug the entrance loosely with a wad of newspaper; if any squirrels remain inside they'll remove it. It also says to be sure no animals are in a chimney before covering it.^[13][14]

Bats need the most care. UC IPM says to avoid excluding bat colonies when flightless young could be present, usually May through September in California, because the young starve and cause odor and fly problems; it calls winter an ideal time for exclusion in most areas, since most bats leave in late fall.^[10]

State rules vary. Delaware's wildlife agency says complete exclusions shouldn't take place during the maternity season, generally May 15 to August 31 there, and that exit points should be sealed five to seven days after one-way devices let the bats out. Ohio requires written permission from its Division of Wildlife to exclude bats from May 16 through July 31 when 5 or more bats are present, and from October 15 through March 31 if any bats are present, to protect hibernating bats; it names April 1 to May 15 and August 1 to October 15 as the ideal periods. That winter restriction differs from UC IPM's advice that winter is ideal, so check your own state wildlife agency. Ohio also makes it illegal to seal all entry points while bats are inside, or to use glue traps, chemicals, toxicants or foggers to kill bats. See the bat exclusion guide for the one-way door method.^[11][12]

Remove food, water and shelter

• Collect garbage and garden debris often, use cans with tight lids, and keep cans clean of food residue.^[2]
• Feed pets only what they'll eat at one feeding and store pet food in rodent-proof containers.^[2]
• Good sanitation alone seldom eliminates mice, but poor sanitation attracts them and lets them build up.^[1]
• Remove overhanging tree limbs within 6 feet of the roof so rats and squirrels can't reach it.^[2]
• Store pet food and bulk foods in heavy-duty containers with tight lids. ICWDM says galvanized steel containers in good condition resist rats better than vinyl or plastic. CDC suggests thick plastic, metal or glass containers with tight lids for food inside.^[8][6]
• Reduce clutter and nesting material. NPIC says clutter lets mouse activity go unnoticed and suggests removing cardboard boxes and newspapers mice could nest in.^[9]
• Move firewood, stacked items and debris away from the foundation, and trim shrubs, vines and limbs that touch the house; Kentucky's entomologists say these give spiders shelter and a bridge to the structure.^[17]

Fix moisture and wood contact

• Divert water with gutters, downspouts and splash blocks; repair leaking faucets, pipes and AC units; grade soil to slope away from the house.^[3]
• Reduce crawl space moisture: keep vents clear and consider polyethylene sheeting over about 75 percent of the soil.^[3]
• Eliminate wood-to-soil contact, keep firewood and lumber away from the foundation, and use mulch sparingly, a couple of inches deep and never against siding.^[3]
• Keep wood siding and door and window frames at least six inches above ground level, per Kentucky's termite guidance, and don't let vines, ivy and dense plantings touch the house.^[3]
• Moisture draws more than termites. Kentucky's entomologists list minimizing moisture and debris around the home, along with sealing entry points, as ways to keep centipedes and millipedes out.^[18]

Fall invaders

Boxelder bugs, multicolored Asian lady beetles and brown marmorated stink bugs gather on warm south- and west-facing walls in fall and squeeze through cracks to overwinter. Exclusion is the best strategy: tight windows and doors, intact screens, and sealed openings around the foundation, pipes, wires and chimneys.^[4]

Perimeter insecticides give limited control because these insects are mobile, and indoor insecticides are generally not recommended; vacuum up any that get in.^[4]

Next steps by pest

Sealing comes first for every pest below; the linked guides cover what to do about animals or insects already inside. NPIC notes that integrated pest management puts prevention, sanitation and exclusion first and uses pesticides only as a last resort.^[5]

• Mice and rats: seal first, then trap what's already inside. UC IPM calls exclusion the most successful and permanent form of house mouse control. See the mouse and rat guides for trapping and cleanup.^[1][2]
• Bats: watch at dusk to find the exits, fit one-way devices, then seal after the bats are out and outside the maternity season. The bat exclusion guide covers the steps.^[10][11]
• Squirrels and raccoons in the attic or chimney: confirm the animal is out, then close the hole with sheet metal or hardware cloth and cap the chimney. See the attic squirrel and raccoon guides.^[13][14]
• Snakes: seal to 1/8 inch and cut down the rodents, insects and cool, damp hiding spots that ICWDM says attract them; the snake guide covers fences and what doesn't work.^[15]
• Cockroaches, spiders, centipedes and scorpions: door sweeps, weather stripping, tight screens and caulked cracks, plus less clutter and moisture. Kentucky's entomologists note that perimeter barrier sprays for spiders may need to be reapplied monthly or every other month in warm weather.^{[16][17][18][19]}
• Boxelder bugs, lady beetles and stink bugs: seal before they gather in fall and vacuum any that get in; see the fall home invaders guide.^[4]

Common questions

Can I use expanding foam to seal mouse holes?

No. UC IPM lists insulating foam, plastic, rubber, vinyl and wood among the materials mice chew through; use metal or concrete.^[1]

What size gap can a mouse fit through?

House mice can squeeze under gaps 1/4 inch tall and through openings 3/8 inch wide; seal anything larger than 1/4 inch.^[1]

Do door sweeps and weather stripping keep out mice and rats?

They help: NPIC recommends door sweeps and weather stripping to keep mice from crawling under doors. UC IPM cautions that typical weatherproofing materials may not be enough to exclude rats and mice, and suggests sheet metal on door edges where gnawing is a problem.^[9][2]

What size mesh should I use on vents?

For rats, UC IPM specifies 1/2-inch galvanized mesh: 18-gauge expanded metal or 16-gauge wire cloth on vent grills. ICWDM suggests 1/4-inch wire mesh where mice are the concern. Never use chicken wire, UC IPM says, and take care with dryer vents, where Nebraska Extension says lint can catch fire.^[2][7][20]

Can I seal up bats in the attic right away?

Not while young bats may be inside. UC IPM says to avoid excluding colonies when flightless young could be present, and states such as Delaware and Ohio restrict exclusion in summer (Ohio also in winter, when bats hibernate). Use one-way exits first and seal after the bats are out.^[10][11][12]

Should I plug the weep holes in my brick wall?

No. Arizona's extension says weep holes shouldn't be plugged or sealed because they ventilate the wall spaces; screen them with coarse steel mesh instead.^[19]

Sources

1. University of California Statewide IPM Program, “Pest Notes: House Mouse, Publication 7483 (revised July 2025).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnhousemouse.pdf (accessed 2026-11-10)
2. University of California Statewide IPM Program, “Pest Notes: Rats, Publication 74106 (revised June 2024).” ipm.ucanr.edu/legacy_assets/PDF/PESTNOTES/pnrats.pdf (accessed 2026-11-10)
3. University of Kentucky Entomology, “ENTFACT-605: Protecting your Home Against Termites (revised March 2026).” entomology.mgcafe.uky.edu/ef605 (accessed 2026-11-10)
4. University of Illinois Extension, “Fall house guests (Sept. 23, 2019).” extension.illinois.edu/news-releases/fall-house-guests (accessed 2026-11-10)
5. National Pesticide Information Center, “Minimizing Pesticide Exposures.” npic.orst.edu/health/minexp.html (accessed 2026-11-10)
6. Centers for Disease Control and Prevention, “How to Seal Up to Prevent Rodents.” www.cdc.gov/healthy-pets/rodent-control/seal-up.html (accessed 2026-11-10)
7. Internet Center for Wildlife Damage Management, “House Mouse Damage Prevention and Control Methods.” icwdm.org/species/rodents/house-mice/house-mouse-damage-prevention-and-control-methods/ (accessed 2026-11-10)
8. Internet Center for Wildlife Damage Management, “Norway Rat Damage Prevention and Control Methods.” icwdm.org/species/rodents/norway-rats/norway-rat-damage-prevention-and-control-methods/ (accessed 2026-11-10)
9. National Pesticide Information Center, “Mice.” npic.orst.edu/pest/mice.html (accessed 2026-11-10)
10. University of California Statewide IPM Program, “Pest Notes: Bats.” ipm.ucanr.edu/home-and-landscape/bats/ (accessed 2026-11-10)
11. Delaware Department of Natural Resources and Environmental Control, Division of Fish and Wildlife, “Best Management Practices for Bat Exclusions.” documents.dnrec.delaware.gov/fw/conservation/bats/Best%20Management%20Practices%20for%20Bat%20Exclusions%20-%20Dec%202010.pdf (accessed 2026-11-10)
12. Ohio Department of Natural Resources, Division of Wildlife, “Bat Exclusion Guidelines.” dam.assets.ohio.gov/image/upload/ohiodnr.gov/documents/wildlife/permits/Bat-Exclusion-Guidelines.pdf (accessed 2026-11-10)
13. University of California Statewide IPM Program, “Pest Notes: Tree Squirrels.” ipm.ucanr.edu/home-and-landscape/tree-squirrels/ (accessed 2026-11-10)
14. University of California Statewide IPM Program, “Pest Notes: Raccoons.” ipm.ucanr.edu/home-and-landscape/raccoons/ (accessed 2026-11-10)
15. Internet Center for Wildlife Damage Management, “Snake Damage Prevention and Control Methods.” icwdm.org/species/reptiles/snakes/snake-damage-prevention-and-control-methods/ (accessed 2026-11-10)
16. University of California Statewide IPM Program, “Pest Notes: Cockroaches.” ipm.ucanr.edu/home-and-landscape/cockroaches/ (accessed 2026-11-10)
17. University of Kentucky Entomology, “ENTFACT-623: Eliminating Spiders Around Homes and Buildings.” entomology.mgcafe.uky.edu/ef623 (accessed 2026-11-10)
18. University of Kentucky Entomology, “Centipedes & Millipedes in & Around the Home.” entomology.mgcafe.uky.edu/ef645 (accessed 2026-11-10)
19. University of Arizona Cooperative Extension, “Scorpions (AZ1768).” extension.arizona.edu/sites/extension.arizona.edu/files/pubs/az1768-2018.pdf (accessed 2026-11-10)
20. University of Nebraska-Lincoln Extension, “G2024 Urban Pest Birds: Controlling Damage.” extensionpubs.unl.edu/publication/g2024/html/view (accessed 2026-11-10)