How Mice Get Into Montreal Homes in Winter — Every Entry Point Explained

Every October in Montreal, the same invisible migration happens. As nighttime temperatures drop below 10°C, the city's mouse population — estimated in the millions — begins searching for warm places to spend the winter. Your home is exactly what they are looking for. And the frustrating truth is that most of the entry points they use are gaps you have walked past hundreds of times without ever noticing.

A house mouse (Mus musculus) can compress its body to fit through a gap as small as 6 mm — roughly the diameter of a pencil. A Norway rat (Rattus norvegicus) needs only 12 mm. These are not dramatic holes or obvious damage. They are the gap where a pipe enters your foundation. The space under your back door where the sweep has worn down. The crack in the mortar joint between two bricks that has been there since 1962.

This guide is the most comprehensive resource available on mouse entry points specific to Montreal housing. We cover every single pathway mice use — organized by location, with exact measurements, identification tips, and the right sealing method for each. By the end, you will know your home's vulnerabilities better than the mice do.

Why Mice Enter Homes in Winter: The Biology

Understanding why mice enter homes in winter helps you understand the urgency of the problem. Mice are not opportunistic wanderers — they are driven by powerful biological imperatives that make your home irresistible from September through April.

Thermoregulation: The Cold Kills Them

House mice are warm-blooded mammals with a body temperature of approximately 37°C. Unlike squirrels or groundhogs, they do not hibernate. They must maintain their body temperature year-round, which requires constant food intake and shelter from the cold. At outdoor temperatures below -10°C — common in Montreal from December through February — an unprotected mouse will die of hypothermia within hours.

Your home, heated to 20°C, represents a 30-degree temperature differential from the outdoor environment. From a mouse's perspective, finding a way into your home is not a preference — it is survival.

Food Scarcity: The Hunger Drives Them

Mice are omnivores that eat seeds, grains, insects, fruit, and virtually anything organic. In summer and fall, Montreal's outdoor environment provides abundant food. By November, snow covers seed sources, insects have died off, and fruit has rotted. The food scarcity that accompanies cold weather is the second major driver pushing mice toward human habitation.

Your kitchen, pantry, and garbage area represent an almost unlimited food source by comparison. A single crumb under the stove, a bag of rice with a small hole, or a pet food bowl left out overnight is enough to sustain a mouse colony through the winter.

Pheromone Trails: One Mouse Invites Dozens

This is the detail that makes mouse infestations escalate so quickly. When a mouse finds a viable entry point into a warm, food-rich environment, it deposits pheromone markers — chemical signals that communicate to other mice: "this is a safe, productive route." These pheromone trails persist for months, even after the original mouse is gone.

A single entry point discovered by one mouse in October can guide dozens of mice into your home by December. This is why sealing entry points is not just about stopping the mice you currently have — it is about eliminating the chemical invitation that will keep attracting new ones.

Foundation Entry Points: The Most Common and Most Overlooked

The foundation is where the vast majority of mouse invasions begin. Montreal's extreme freeze-thaw cycles — the city experiences an average of 40+ freeze-thaw cycles per year — cause concrete and mortar to crack, shift, and deteriorate in ways that create perfect mouse-sized gaps. Most homeowners never inspect their foundation closely enough to find these vulnerabilities.

1. Mortar Joint Deterioration in Brick Foundations

Montreal has an enormous stock of pre-1960 brick construction — duplexes, triplexes, and single-family homes with brick foundations that are now 60 to 100 years old. The mortar between these bricks has been subjected to decades of freeze-thaw cycling, and in many cases it has deteriorated to the point where gaps of 6–15 mm exist between individual bricks.

These gaps are particularly dangerous because they are often at or near ground level, hidden by soil, mulch, or snow for much of the year. A mouse can enlarge a soft mortar joint with its teeth in minutes — their incisors can exert 500 kg/cm² of pressure and never stop growing, making them perfectly adapted for gnawing through deteriorated masonry.

• How to identify: Run your finger along mortar joints at the base of your foundation. Soft, crumbling, or missing mortar is a vulnerability. Use a screwdriver to probe — if it penetrates more than 6 mm, the joint needs repointing.
• Where to look: Focus on the bottom two courses of brick, especially on the north and east faces where freeze-thaw damage is most severe.
• How to seal: Repoint deteriorated joints with hydraulic cement or mortar mix. For gaps larger than 12 mm, pack with steel wool first, then apply mortar over top. Do not use caulk alone — mice can chew through it.
• Cost: DIY repointing costs $20–$50 in materials. Professional repointing runs $15–$25 per linear foot.

2. Poured Concrete Foundation Cracks

Homes built from the 1950s onward often have poured concrete foundations rather than brick. These develop a different type of crack: vertical cracks from settling, horizontal cracks from soil pressure, and diagonal cracks from differential settling. Any crack wider than 6 mm is a potential mouse entry point.

Poured concrete cracks are particularly common in Montreal because of the clay-heavy soil in many neighbourhoods. Clay soil expands when wet and contracts when dry, exerting lateral pressure on foundation walls that causes cracking over time. The freeze-thaw cycle accelerates this process dramatically.

• How to identify: Inspect the interior and exterior of your foundation with a flashlight. Use a credit card to probe cracks — if it fits, a mouse can too.
• Types of cracks: Hairline cracks (under 3 mm) are cosmetic. Cracks 3–6 mm need monitoring. Cracks over 6 mm need immediate sealing.
• How to seal: For cracks under 12 mm, use hydraulic cement or polyurethane caulk rated for masonry. For larger cracks, pack with steel wool, then apply hydraulic cement. For structural cracks (horizontal or stair-step), consult a structural engineer.
• Important: Seal from the interior as well as the exterior — mice can enter from inside the wall cavity even if the exterior crack is sealed.

3. Utility Penetrations: Pipes, Conduits, and Gas Lines

Every pipe, electrical conduit, gas line, and cable that enters your home through the foundation creates a potential entry point. The gap between the pipe and the surrounding concrete or masonry is almost never perfectly sealed — and even if it was sealed at installation, decades of thermal expansion and contraction have likely opened it up.

This is one of the most commonly missed entry points because homeowners assume that because a pipe is there, the gap around it must be sealed. It almost never is. In older Montreal homes, you may find water supply lines, drain pipes, gas lines, electrical conduits, and cable TV lines all entering through separate holes in the foundation — each with its own gap.

• Water supply lines: The gap around copper or PVC supply lines entering through the foundation is typically 10–25 mm — more than enough for a mouse.
• Drain pipes: Larger diameter drain pipes (50–100 mm) often have gaps of 20–50 mm around them where they exit through the foundation.
• Gas lines: The sleeve around gas lines entering the foundation often has a gap at both the interior and exterior face.
• Electrical conduits: Metal conduit entering through the foundation often has a gap at the entry point, especially in older homes where the conduit was installed after the foundation was poured.
• How to seal: Use steel wool packed tightly around the pipe, then cover with hydraulic cement or expanding foam rated for pest exclusion. Do not use standard expanding foam — mice can chew through it. Use foam specifically formulated with pest deterrents, or cover foam with metal flashing.

4. The Sill Plate Gap

The sill plate is the horizontal wooden beam that sits on top of the foundation wall and supports the floor structure above. In older Montreal homes, the connection between the sill plate and the foundation is often imperfect — there are gaps where the wood has shrunk, where the foundation has settled unevenly, or where the original construction was simply not precise.

This gap runs the entire perimeter of the house and is typically located just above ground level, often hidden by siding or exterior cladding. It is one of the most productive entry points for mice because it provides direct access to the wall cavity and floor structure — the ideal nesting environment.

• How to identify: From the exterior, look for the joint between the foundation and the siding or cladding. In older homes, you may see daylight through this gap when inspecting from the basement interior.
• How to seal: From the exterior, apply a bead of polyurethane caulk or expanding foam along the sill plate-foundation joint. From the interior, pack any visible gaps with steel wool and cover with caulk or foam. In severe cases, install metal flashing over the joint.
• Priority: This is a high-priority entry point because it provides access to the entire wall cavity system.

Door and Window Entry Points

Doors and windows are the most visible potential entry points, yet they are often overlooked because homeowners assume that a closed door or window is a sealed door or window. It is not. The gaps around door frames, under doors, and around window frames are among the most productive mouse entry points in Montreal homes.

5. Under Exterior Doors: The Gap You Can See Daylight Through

Stand inside your home at night with the lights off and look at the bottom of your exterior doors. If you can see light coming through, a mouse can come in. A gap of just 6 mm under a door — less than the thickness of a pencil — is sufficient for a house mouse to enter.

In Montreal's older housing stock, door frame settling is extremely common. The wooden frames of duplexes and triplexes built in the 1920s–1960s have shifted, settled, and warped over decades, creating gaps under doors that were not there when the building was new. Door sweeps wear out and are rarely replaced. Thresholds crack and deteriorate.

• Back doors and side doors: These are higher risk than front doors because they are used less frequently and inspected less often. The gap under a back door that is only opened twice a day is a permanent mouse highway.
• Garage doors: The rubber seal at the bottom of garage doors deteriorates and cracks, leaving gaps of 10–25 mm. This is one of the most common entry points for mice into attached garages and from there into the home.
• Basement doors: Exterior basement doors are often the worst offenders — old, rarely used, with deteriorated frames and no door sweep.
• How to fix: Install a heavy-duty door sweep on all exterior doors. For garage doors, replace the bottom seal. For severely warped frames, the door may need to be rehung or the frame replaced.
• Door sweep specification: Use a brush-style or rubber-blade door sweep rated for pest exclusion. Standard aluminum door sweeps with thin rubber blades are often insufficient — mice can push through them.

6. Door Frame Gaps: Where the Frame Meets the Foundation

The gap between the exterior door frame and the surrounding masonry or siding is another common entry point. In brick construction, the door frame is set into the brick opening with mortar, and over time that mortar deteriorates and pulls away from the frame, leaving a gap around the entire perimeter of the door.

• How to identify: Run your finger around the exterior perimeter of the door frame where it meets the brick or siding. Any gap you can feel is a potential entry point.
• How to seal: Apply a bead of exterior-grade polyurethane caulk around the entire perimeter of the door frame. For gaps larger than 6 mm, pack with backer rod first, then caulk over top.
• Maintenance: Exterior caulk around door frames should be inspected and replaced every 3–5 years in Montreal's climate.

7. Basement Window Frames

Basement windows in older Montreal homes are a significant vulnerability. The wooden frames have often warped and pulled away from the surrounding masonry, the glass may be cracked or missing entirely, and the window wells outside may be filled with debris that provides cover for mice approaching the window.

Even a basement window that appears intact may have gaps at the corners of the frame, around the latch mechanism, or where the frame meets the sill. These gaps are often at or below ground level, making them easy for mice to access from the exterior.

• How to identify: Inspect basement windows from both inside and outside. Look for gaps at the corners of the frame, around the perimeter where the frame meets the masonry, and at the sill.
• Window wells: Clean out window wells and install covers. Debris-filled window wells provide shelter for mice approaching the window.
• How to seal: Caulk around the perimeter of the frame. Replace cracked or missing glass. For windows that are never opened, consider permanently sealing them with a rigid foam insert covered with a metal plate.
• Screen condition: If basement windows are used for ventilation, ensure screens are intact and fit tightly. Standard window screens have openings of 1.5 mm — too small for mice but not for insects. For pest exclusion, use hardware cloth (6 mm mesh) over the screen.

Roof and Attic Entry Points

While most people think of mice as ground-level invaders, they are excellent climbers. House mice can scale rough brick walls, climb along pipes and wires, and use tree branches as bridges to access roof-level entry points. Once in the attic, they have access to the entire wall cavity system of the house.

8. Roof Vents and Attic Vents

Roof vents — the mushroom-shaped or louvered vents that allow attic ventilation — are a common mouse entry point that most homeowners never think to check. The standard plastic or aluminum louvers on these vents have openings of 10–20 mm, and the louvers themselves can be pushed aside by a determined mouse.

Gable vents — the triangular or rectangular vents at the peak of the gable ends of the house — are even more vulnerable. These are often covered with a simple screen that deteriorates over time, and the gap between the vent frame and the surrounding wood is often unsealed.

• Roof vents: Inspect from the attic interior. Look for daylight around the vent perimeter and check whether the louvers are intact. Cover the interior of roof vents with 6 mm hardware cloth secured with screws.
• Gable vents: Inspect from the attic. Replace deteriorated screens with 6 mm hardware cloth. Caulk around the perimeter of the vent frame.
• Soffit vents: The perforated soffit panels that run along the underside of the roof overhang are a common entry point where they have been damaged, pulled away from the fascia, or where the perforations are large enough for mice.
• Ridge vents: The continuous ridge vent that runs along the peak of the roof can be an entry point if the mesh insert has deteriorated. Inspect from the attic.

9. Gaps in Soffits and Fascia

The soffit (the underside of the roof overhang) and fascia (the vertical board at the edge of the roof) are common entry points in older Montreal homes. Wood rot, storm damage, and deterioration of the connection between the soffit and the exterior wall all create gaps that mice can exploit.

• Where to look: Inspect the soffit-wall junction from the exterior. Look for gaps where the soffit panel meets the exterior wall, where soffit panels have pulled away from each other, and where the fascia board has pulled away from the rafter ends.
• How to seal: Replace damaged soffit panels. Caulk gaps at the soffit-wall junction. Install metal drip edge flashing where the fascia meets the roof deck.
• Tree access: Mice use overhanging tree branches to access soffit and roof-level entry points. Trim all branches to at least 2 metres from the roofline.

10. Chimney and Fireplace Openings

Chimneys are a less common but real mouse entry point. Mice can descend a chimney and enter the home through the fireplace opening, or they can access the attic through gaps around the chimney where it passes through the roof.

• Chimney cap: Install a chimney cap with mesh sides (6 mm hardware cloth) to prevent mice from entering the flue.
• Chimney-roof junction: The flashing around the chimney where it passes through the roof often develops gaps over time. Inspect and reseal with roofing caulk.
• Fireplace damper: Ensure the fireplace damper closes completely when not in use. A partially open damper is an open invitation.

Plumbing and HVAC Entry Points

The mechanical systems of your home — plumbing, heating, ventilation, and air conditioning — create a network of penetrations through walls, floors, and ceilings that are among the most productive mouse entry points in any home.

11. Dryer Vents and Exhaust Fan Ducts

The dryer vent is one of the most commonly used mouse entry points in Montreal homes. The standard plastic flap damper on dryer vents deteriorates quickly in Montreal's climate — the plastic cracks, the flap warps, and the gap around the vent where it exits through the wall is often unsealed.

Bathroom exhaust fans and kitchen range hood vents have the same vulnerability. The duct exits through the wall or roof, and the gap around the duct is rarely properly sealed.

• Dryer vent: Replace plastic flap dampers with a metal louvered vent cover. Cover the exterior vent with 6 mm hardware cloth, leaving enough clearance for the flap to open during operation. Seal the gap around the vent where it exits through the wall with caulk or expanding foam.
• Bathroom exhaust fans: Inspect the exterior vent cap. Replace deteriorated plastic caps with metal ones. Seal the gap around the duct.
• Range hood vents: Same as bathroom exhaust fans. Ensure the backdraft damper inside the duct is functioning properly.
• Important: Never permanently seal a dryer vent or exhaust fan duct — these must remain functional for safety and moisture control.

12. Plumbing Penetrations Through Floors and Walls

Every pipe that passes through a floor or wall creates a potential entry point. In older Montreal homes, these penetrations were often made with a hole saw that left a gap of 10–25 mm around the pipe — more than enough for a mouse. These gaps are typically hidden under sinks, behind toilets, and inside kitchen cabinets.

• Under kitchen sink: Open the cabinet under your kitchen sink and look at where the supply lines and drain pipe pass through the cabinet floor and back wall. These gaps are almost always present in older homes.
• Under bathroom sink: Same as kitchen sink. Also check where the toilet supply line enters through the floor.
• Behind the stove: The gas line or electrical conduit for the stove often passes through the wall with a gap around it.
• Basement ceiling: Where pipes pass through the basement ceiling into the floor above, there are often gaps that allow mice to move between floors.
• How to seal: Use steel wool packed tightly around the pipe, then cover with hydraulic cement (for floor penetrations) or caulk (for wall penetrations). Decorative pipe escutcheons (the metal collars around pipes) are cosmetic only — they do not seal the gap.

13. HVAC Ducts and Cold Air Returns

Forced-air heating systems have ductwork that runs through walls, floors, and ceilings throughout the house. Where this ductwork connects to registers and grilles, there are often gaps between the duct and the surrounding drywall or flooring. Mice can enter through these gaps and travel through the duct system to access any room in the house.

• Floor registers: Check the gap around floor registers where the duct meets the subfloor. This gap is often 5–15 mm and is a direct pathway from the basement or crawl space into the living area.
• Wall registers: Same issue where wall registers meet the drywall.
• Cold air return grilles: These large grilles, typically located in hallways and living areas, cover openings that connect directly to the duct system. Ensure the grille fits tightly and the duct connection is sealed.
• How to seal: Use sheet metal screws and HVAC foil tape to seal gaps around duct connections. For floor register gaps, use caulk or expanding foam around the perimeter of the register boot.

Exterior Wall Entry Points

The exterior walls of your home have numerous potential entry points beyond the obvious doors and windows. These are often the hardest to find because they require close inspection of areas that are rarely examined.

14. Gaps in Exterior Cladding and Siding

Exterior cladding — whether brick, wood siding, vinyl siding, or stucco — develops gaps over time. Wood siding shrinks and warps. Vinyl siding pulls away from trim pieces. Stucco cracks. Brick mortar deteriorates. Any gap in the exterior cladding that connects to the wall cavity is a potential mouse entry point.

• Wood siding: Look for gaps at the ends of siding boards, at corners, and where siding meets trim. Caulk all gaps.
• Vinyl siding: Check where vinyl siding meets window and door trim, at corners, and at the bottom course where it meets the foundation. Vinyl siding is particularly vulnerable at the J-channel trim pieces.
• Brick veneer: Check the weep holes at the base of brick veneer walls. These small holes are intentional (they allow moisture to drain from the wall cavity) but they are also mouse-sized. Cover weep holes with stainless steel mesh inserts specifically designed for this purpose.
• Stucco: Cracks in stucco should be repaired with elastomeric caulk or stucco patching compound.

15. Electrical and Cable Penetrations Through Exterior Walls

Every electrical outlet, light fixture, and cable entry point on an exterior wall is a potential mouse entry point. The gap between the electrical box and the surrounding drywall or masonry is often 5–15 mm, and the gap between the exterior wall and the conduit or cable is rarely sealed.

• Exterior electrical outlets and fixtures: Inspect the gap around the electrical box where it meets the exterior wall. Use foam gaskets behind outlet covers on exterior walls.
• Cable TV and internet entry points: The coaxial cable or ethernet cable entering through the exterior wall often has a gap of 10–20 mm around it. Seal with caulk.
• Outdoor light fixtures: The gap between the fixture mounting plate and the exterior wall is often unsealed. Apply a bead of caulk around the perimeter of all exterior light fixtures.
• Hose bibs: The gap around the pipe where the outdoor hose bib exits through the wall is a common entry point. Seal with caulk or hydraulic cement.

Montreal-Specific Vulnerabilities: Duplexes and Triplexes

Montreal's iconic duplexes and triplexes — the plex buildings that define the city's residential neighbourhoods — have unique structural features that create mouse entry points not found in single-family homes. If you live in or own a plex, these vulnerabilities deserve special attention.

16. Shared Wall Voids Between Units

In a duplex or triplex, the wall between units is typically a double-stud wall with a void space in the middle. This void runs from the basement to the attic and connects to the floor and ceiling cavities of every unit. A mouse that enters one unit through any of the entry points described above can travel freely through this void to access every other unit in the building.

This is why mouse infestations in Montreal plexes are so difficult to control without treating the entire building. Eliminating mice in one unit while the shared wall void remains accessible simply means the mice relocate to another unit and return when conditions improve.

• What to do: If you are a landlord, treat all units simultaneously and seal the shared wall void at the basement level with fire-rated expanding foam or mineral wool insulation.
• If you are a tenant: Notify your landlord in writing. Under Quebec law, the landlord is responsible for pest control in rental units. Request that all units be inspected and treated simultaneously.
• Temporary mitigation: Seal gaps around pipes and electrical boxes in the shared wall from your unit's side. This reduces (but does not eliminate) mouse movement between units.

17. Exterior Staircases and Balconies

Montreal's famous exterior staircases — the winding iron or wood stairs that climb the facades of plexes — create unique mouse access routes. The staircase structure is attached to the building, and the connection points between the staircase and the building wall often have gaps. Mice use the staircase structure as a climbing route to access upper-floor entry points.

• Staircase-wall connections: Inspect where the staircase stringers and landing supports attach to the building wall. Seal any gaps with caulk or expanding foam.
• Balcony-wall connections: Where balcony decking meets the building wall, there is often a gap. Seal with caulk.
• Under balconies: The underside of balconies provides shelter for mice and direct access to the building wall. Keep this area clear of debris.

The Complete Sealing Protocol: How to Mouse-Proof Your Montreal Home

Now that you know every entry point, here is the systematic approach to sealing them. This is the same protocol our technicians follow during professional rodent exclusion services.

Step 1: The Exterior Inspection

Start outside. Walk the complete perimeter of your home with a flashlight, a screwdriver, and a pencil. The pencil test: any gap a pencil fits through is large enough for a mouse. The screwdriver test: probe mortar joints and wood — soft or crumbling material needs repair.

Do this inspection at two heights: ground level (foundation and lower wall) and at eye level (upper wall, soffits, and roof edge). Use binoculars to inspect the roofline and chimney from the ground.

Step 2: The Interior Inspection

Move inside. Start in the basement and work your way up. In the basement, inspect every pipe penetration, every window, the sill plate, and the connection between the basement ceiling and the floor above. In the living areas, open every cabinet under sinks and check pipe penetrations. In the attic, inspect every vent, the chimney, and the connection between the attic floor and the wall cavities.

Step 3: Prioritize and Seal

Not all entry points are equal. Prioritize sealing in this order:

1. Foundation utility penetrations (pipes, conduits, gas lines) — highest priority, most commonly used
2. Under exterior doors — install door sweeps immediately
3. Foundation cracks and mortar joint deterioration
4. Sill plate gaps
5. Dryer vents and exhaust fan ducts
6. Plumbing penetrations through floors and walls (under sinks, etc.)
7. Roof vents and attic vents
8. Exterior cladding gaps and weep holes
9. Electrical and cable penetrations
10. Chimney and fireplace

Materials You Need

• Steel wool (grade 00 or 000) — for packing gaps around pipes and in irregular openings
• Hydraulic cement — for sealing foundation cracks and pipe penetrations through concrete
• Polyurethane exterior caulk — for sealing gaps in wood, siding, and around door and window frames
• Pest-exclusion expanding foam (e.g., Great Stuff Pestblock) — for larger gaps in walls and floors
• Hardware cloth (6 mm galvanized mesh) — for covering vents and larger openings
• Metal door sweeps — for all exterior doors
• Sheet metal screws and HVAC foil tape — for sealing duct connections
• Backer rod — for filling large gaps before caulking

When to Call a Professional: Signs You Need Expert Help

DIY exclusion is effective for prevention and for minor infestations caught early. But there are situations where professional help is not just recommended — it is necessary.

• You have found droppings in multiple rooms — this indicates an established colony, not a single mouse
• You hear scratching in walls or ceilings at night — mice are active in the wall cavity system
• You have found a nest — shredded paper, fabric, or insulation in a hidden location
• You have seen a mouse during daylight hours — mice are nocturnal; daytime sightings indicate a large population
• You have sealed obvious entry points but mice keep appearing — there are entry points you have not found
• You live in a duplex or triplex — effective exclusion requires treating the entire building
• You have tried DIY traps for more than 2 weeks without eliminating the problem

At Blackline Pest Control, our rodent exclusion service includes a comprehensive inspection of every entry point described in this guide, a strategic trap network, and professional-grade sealing of all identified entry points using materials that mice cannot chew through. We serve Montreal, Laval, the South Shore, and the Vaudreuil region. Call 514-809-1999 for a free inspection.

The Best Time to Act: September Is Your Window

The single most important thing you can take from this guide is this: the best time to mouse-proof your Montreal home is September — before mice have entered for the winter. Preventive exclusion in September is dramatically less expensive, less disruptive, and more effective than treating an active infestation in December.

Once mice are inside and established, you are dealing with a colony that has already found food sources, established nesting sites, and laid down pheromone trails. Eliminating that colony requires trapping, baiting, and exclusion — a multi-week process. Preventing entry in the first place requires only a few hours of inspection and sealing work.