How to Keep Mice Out of Your Montreal Home This Winter
Every fall, the same migration happens across Montreal. As temperatures drop, mice move indoors. Not because they want to live with you — because they will die outside.
House mice (Mus musculus) cannot survive sustained cold below -5°C without shelter. In Montreal, that means from roughly mid-October through March, every mouse within range of your house is looking for a way in.
The good news: preventing mice is easier than eliminating them. But it requires action before the first sustained freeze — not after you hear scratching in the walls.
The Critical Window: September to November
Mouse prevention in Montreal has a specific timeline. The most effective window for exclusion work — sealing entry points — is September through November, before sustained freezing temperatures drive mice indoors.
Once temperatures consistently drop below -5°C, mice that have not already found shelter will attempt to enter any available structure with increasing desperation. By December, the mice that got in are breeding.
Act in fall. Not in January when you hear them.
Where Mice Enter Montreal Homes
A mouse can squeeze through any gap larger than 6mm — about the width of a pencil. In Montreal homes, the most common entry points are:
Foundation level:
- Cracks in poured concrete or block foundations (common in pre-1970 buildings)
- Gaps where utility pipes and wires enter the building
- Weep holes in brick facades
- The junction between the foundation and the sill plate
- Basement window frames, especially older wood frames with deteriorating caulk
Ground level:
- Gaps under exterior doors (if you can see daylight, a mouse can get through)
- Dryer vent covers that are damaged or missing
- Gaps around the garage door seal
- Where the siding meets the foundation
Upper level:
- Roof-soffit intersections (mice can climb brick, stucco, and rough siding)
- Gaps around plumbing vents on the roof
- Damaged soffit panels
- Where additions or extensions meet the original structure
The Prevention Checklist
Exterior — Do This Before October
Inspect the foundation perimeter. Walk around the entire building, looking for any gap, crack, or hole larger than 6mm. Mark them with tape. Come back with materials to seal them.Seal with the right materials:
- Steel wool + caulk for small gaps around pipes (stuff steel wool in the gap, then seal over with caulk)
- Galvanized hardware cloth (1/4" mesh) for larger openings like vent covers and weep holes
- Metal flashing for gaps along the roofline
- Concrete patch for foundation cracks
- Door sweeps for exterior doors (replace annually — they wear out)
Do NOT use:
- Expanding foam alone — mice chew through it easily
- Plastic mesh — mice chew through it
- Rubber weatherstripping without a backing — too soft to resist gnawing
Interior — Ongoing
Food storage. All dry goods in glass or heavy plastic containers. No open bags of flour, sugar, rice, pasta, or pet food. Mice can chew through cardboard and thin plastic packaging. Garbage. Secure lids on indoor trash cans. Take garbage out every evening. Never leave bags on the floor overnight. Pet food. Store in sealed metal or heavy plastic bins. Pick up pet bowls at night. Clutter. Reduce ground-level clutter in basements, garages, and storage areas. Mice nest in undisturbed materials — cardboard boxes, old newspapers, stored clothing.For Multi-Unit Buildings
If you live in a Montreal apartment building, individual prevention only goes so far. Mice travel through shared wall cavities, plumbing chases, and utility conduits.
What you can do:
- Seal gaps around pipes under your sinks with steel wool and caulk
- Install door sweeps on your apartment door
- Keep your unit clean and food secured
- Report any signs of mice to your landlord immediately — early detection prevents building-wide infestation
What the building needs:
- Professional exclusion of the entire exterior envelope
- Shared basement and utility room clean-up
- Ongoing bait station program in common areas
- Coordination between the property manager and a licensed exterminator
Frequently Asked Questions
When is the best time to mouse-proof my Montreal home?
September and October — after summer construction settles and before sustained freezing begins. This is the window when exclusion work is most effective. By November, mice are already attempting entry. By December, they are inside.How many mice are in my house if I see one?
The general rule in the pest control industry is that if you see one mouse, there are likely 5-10 you do not see. Mice are nocturnal and cautious — you typically only see them when the population is large enough to force some individuals into less ideal hiding spots or foraging times.Got a pest problem?
Extermination DMP serves Montreal, the South Shore, Laval & the West Island — 24/7.
Call 438-879-5706