How to Keep Mice Out of Your House This Winter (Prevention Guide)

Mice don't hibernate. When temperatures drop below about 50°F, they start searching for warm, sheltered spots with easy access to food. Your house checks every box. The worst part? A mouse can squeeze through a hole the size of a dime — about ¼ inch in diameter. If you can slide a pencil through a gap, a mouse can get through it.

The migration typically starts in September/October and peaks in November. By the time you hear scratching in December, they've been inside for weeks. The goal is to beat them to it.

This guide is a seasonal task — do it once in early fall and you'll prevent 90% of winter mouse problems.

• Steel wool (coarse) — Mice can't chew through it. Stuff it into gaps before sealing.
• Caulk and caulk gun — For small gaps (¼ inch or less).
• Expanding foam — For larger gaps (½ inch to 1 inch). Note: mice can chew through foam alone, which is why you combine it with steel wool.
• Hardware cloth or metal mesh (¼ inch) — For covering larger openings like dryer vents and foundation vents.
• Door sweeps — For gaps under exterior doors.
• Weatherstripping — For gaps around doors and windows.
• Flashlight — For inspecting dark areas.
• Ladder — For checking roof-level entry points.
• A pencil — If a pencil fits through a hole, a mouse can too. Use this as your measuring tool.

Step 1: The Exterior Walk-Around

On a dry day in September or early October, walk the entire perimeter of your home with your caulk, steel wool, and flashlight. Check every inch from foundation to roofline. You're looking for any gap ¼ inch or larger.

Foundation level:

• Cracks in the foundation — seal with caulk or hydraulic cement.
• Where the siding meets the foundation — a common gap on many homes.
• Basement window wells and window frames.
• Where concrete steps meet the house.

Utility entries:

• Where pipes enter the house (water, gas, HVAC refrigerant lines).
• Electrical conduit entries.
• Cable and internet line entries.
• Dryer vent exit (use ¼-inch hardware cloth to cover, not screen — screen clogs with lint).
• Outdoor faucet (hose bib) — check the gap where it penetrates the wall.

Above ground level:

• Gaps where siding meets trim or soffit.
• Gaps around windows, especially older windows.
• Where the roof meets the walls (soffit gaps).
• Chimney base and chimney cap — ensure the cap has proper screening.
• Roof vents (attic, plumbing, exhaust) — should all have ¼-inch mesh screens.

Step 2: Seal Everything You Found

For each gap:

• Small gaps (under ¼ inch): Caulk alone.
• Medium gaps (¼ to ½ inch): Stuff with steel wool, then caulk over it.
• Large gaps (½ inch to 1 inch): Stuff with steel wool, then fill with expanding foam.
• Large openings (vents, weep holes): Cover with ¼-inch hardware cloth secured with screws or construction adhesive. Don't seal weep holes completely — they need airflow, so use mesh that allows air but blocks mice.

Step 3: Check Doors and Garage

• Install door sweeps on all exterior doors if daylight is visible under the door.
• Check the garage door seal — replace if worn, cracked, or has gaps at the corners.
• The door between your garage and house is an interior door but is a major mouse entry point. Ensure it seals properly.

Step 4: Interior Checks

• Under every sink — check gaps around pipes where they enter the wall.
• Behind the stove and refrigerator — check where gas lines or water lines enter.
• HVAC closets — check where ductwork and refrigerant lines pass through walls.
• Utility room / laundry — dryer vent connection, water heater area.

Step 5: Reduce Outdoor Attractions

• Move bird feeders away from the house — spilled seed attracts mice.
• Store firewood at least 20 feet from the house, elevated off the ground.
• Clear leaves and debris from foundation beds.
• Trim shrubs and ground cover away from the foundation — at least 12 inches of clear space.

• Do this inspection every September — Mark it on your calendar. New gaps form from settling, weathering, and seasonal expansion/contraction. A yearly check takes 1–2 hours and prevents most winter mouse problems.
• Store food in airtight containers — Glass and metal are best. Mice can chew through thin plastic, cardboard, and paper bags.
• Don't use cardboard for storage — Replace cardboard boxes in basements, garages, and attics with plastic bins with snap-on lids. Cardboard is both a nesting material and a hiding spot for mice.
• Keep garbage cans sealed — Both indoor and outdoor cans should have tight-fitting lids.
• Set a few traps preventively — Even if you don't think you have mice, a few snap traps along garage and basement walls act as an early warning system. If you catch one, you know something got through and can investigate.

• Using expanding foam without steel wool — Mice chew right through cured expanding foam. Always stuff steel wool into the gap first, then foam over it. The steel wool is the actual barrier.
• Only checking ground level — Mice are excellent climbers. They can scale brick, stucco, and wood siding. Entry points at roofline (soffits, roof vents, chimney) are just as vulnerable as foundation-level gaps.
• Forgetting the garage — The garage is the most common mouse entry zone. The large door has long rubber seals that wear out, and garages often have utility entries that aren't sealed as carefully as the house.
• Sealing up too late — If you do your sealing in November, mice may already be inside your walls. Do it in September before the fall migration. If you suspect mice are already inside, trap first, THEN seal — otherwise you trap them inside with you.
• Using copper mesh instead of steel wool — Copper mesh is marketed as a mouse-proof gap filler, and it works, but it's 3–4x more expensive than steel wool and no more effective. Regular coarse steel wool is the best value.