How to seal mouse entry points before cold weather
Mice can squeeze through a 1/4-inch hole, about the width of a pencil. To seal mouse entry points before cold weather, check the foundation, pipes, vents and doors, then fill gaps with steel wool held in by caulk, or with metal or cement for larger holes. Foam, wood and plastic alone won't stop them.
When to check and seal mouse entry points
| Job | When |
|---|---|
| Full outside walk-around | Once a year, before cold weather |
| Check cabinets and behind appliances for droppings | Regularly, and after any plumbing or cable work |
| Recheck steel wool plugs | Each year. It can rust over time. |
| Trim branches near the roof | As they grow back |
What you need to mouse proof your house
- A bright flashlight and a pencil. If the pencil fits in a gap, a mouse can too.
- Steel wool, plus caulk or spray foam to hold it in place
- Hardware cloth, metal sheeting or cement for larger holes
- Snap traps
- Rubber or plastic gloves, and disinfectant or fresh bleach solution for cleanup
Material matters. The University of California's pest guide says plastic screening, rubber, vinyl, expanding foam and wood don't work as plugs, because mice chew through them. CDC does use foam and caulk, but to hold steel wool in place. Use metal or concrete for anything you want to last.
How to seal mouse entry points, inside and out
- Look for signs indoors
Check under and behind kitchen cabinets, behind appliances, closet corners, and around pipes under sinks and at the water heater. House mice rarely go more than 30 feet from their nest, so droppings tell you which area to search.
- Look for daylight
At dusk, turn off the lights in the basement or garage and watch for light around pipes, doors and the top of the foundation.
- Walk the outside
Check where utility lines enter, the gap between foundation and ground, doors without weatherstripping, eaves, soffits, and attic and crawl space vents.
- Plug small holes
Pack steel wool in tightly and hold it there with caulk or spray foam. Treat steel wool as temporary, since it can rust.
- Cover larger holes
Use hardware cloth, metal sheeting or cement. Cut the metal to fit closely around pipes.
- Cut off the roof route
Trim branches touching the roof and check the roof edge and gable vents, which mice can reach from overhanging trees.
- Trap what's already inside
Set traps close to walls and in dark corners, about 10 feet apart in active areas. Keep them where children and pets can't reach.
Signs of mice and how to clean up safely
Gnaw marks usually turn up near droppings. If droppings return after you clean, mice are still active.
Take these seriously:
- Droppings in cabinets, drawers or closet corners
- Fresh tracks in a light dusting of flour along a wall
- Chewed insulation, food packaging or wires. Have a licensed electrician check any damaged wiring.
- Scratching in walls or ceilings at night
- Nests in stored boxes
Clean with care, because CDC warns that disease-carrying particles can get into the air when droppings and urine are disturbed. Open doors and windows and leave for 30 minutes. Don't sweep or vacuum. Wearing gloves, soak the area with disinfectant or a mix of 1.5 cups of bleach per gallon of water. Wait 5 minutes, wipe up with paper towels and put them in a covered trash can. A heavy infestation calls for extra protective gear, or a professional.
How to seal mouse entry points yourself, or call a pest pro
Most sealing is DIY. Call a pest control pro if fresh droppings keep appearing after you've sealed and trapped, if you can't reach the entry points, or if you find a heavy infestation. Some rodent poisons are available only to licensed applicators.
Leave gaps around gas lines, flues and furnace vents to a licensed plumber or HVAC technician. Never block an attic or crawl space vent. Screen it with metal mesh so it can still breathe.
Questions about how mice get in
How small a hole can a mouse get through?
CDC says about 1/4 inch, roughly the width of a pencil. That's why small gaps around pipes and cables matter.
Does spray foam keep mice out?
Not by itself. Mice can chew through it. Use it to hold steel wool in place, or use metal or concrete for a lasting repair.
Can I vacuum mouse droppings?
No. CDC warns that sweeping or vacuuming can send virus particles into the air. Air out the space, soak the droppings with disinfectant, wait 5 minutes and wipe them up with paper towels while wearing gloves.
Sources: CDC, ipm.ucanr.edu
General guidance for U.S. homes, not professional advice. For gas, electrical and structural work, call a licensed pro.