Mice move indoors from late August in Quebec, how to seal the entry points before October

Why late summer is the right moment to seal entry points

Because a hole closed before the mice arrive beats a trap set afterwards. From late summer until the first frost, food gets scarce outdoors and the temperature gap between inside and outside widens. A heated house becomes the most attractive address on the street, and rodents use the same openings that sat there unnoticed all summer.

There is a financial argument too. Rive Nord Extermination, in the price list it published on March 20, 2026, advertises mouse treatment starting at $250. Exterminateurs Associés, on a page updated September 22, 2025, puts mouse treatment between $200 and $400 and notes that sealing entry points, sold as an add on service, can exceed $650. Those figures belong to those two companies on those dates, and are not a market rate. They still make one thing obvious. Acting while there is nothing to treat means paying for one step instead of three.

The outdoor inspection walk, from the roof down to the foundation

Walk the full perimeter twice, once in daylight and once with a flashlight held flat against the wall, working from the roof down to the ground. Raking light reveals gaps the eye misses head on. Budget an hour for a single family home, and write every defect on a list instead of trying to fix each one as you go.

This is the order that works best.

• Roof and eaves. Unscreened roof vents, chimney outlet, the junction between roof and wall, a loose fascia board, a gutter pulling away and leaving a void behind it.
• Walls. Electrical and internet cable pass throughs, mini split refrigerant lines, a dryer vent whose flap no longer closes, the range hood outlet, a poorly caulked outdoor faucet.
• Openings. Door bottoms with no weatherstripping, a worn garage door threshold that no longer meets the concrete, a torn window well screen, a basement window whose frame has shifted.
• Foundation. Cracks, the joint between concrete and siding, the crawl space hatch, water and gas service entries, a French drain outlet with no grate.
• Ground level. Soil dug out along the wall, a settled patio slab, a concrete step that has pulled away from the house.

Then walk the same route from the inside, in the basement and the garage, this time with the lights off. A thin line of daylight showing at the bottom of a wall, around a pipe or under a connecting door gives you the exact position of the opening you spotted outside. It is the fastest way to match both sides of the same defect.

One practical tip. Photograph each defect with a visible reference next to it, a brick or a window corner. By November you will no longer remember the exact spot.

The entry points almost everyone forgets

The worst defects are rarely obvious holes, they are junctions nobody inspects because they look finished. The area around a pipe passing through the wall, for instance, is often sealed with a single bead of silicone applied fifteen years ago and cracked ever since.

The list of usual suspects is short and repeats from one house to the next.

• The bottom of the garage door, especially at both corners, where the rubber curls up and leaves a permanent gap.
• The area around pipes and conduits crossing the foundation wall, on the outside and on the inside.
• The dryer vent, whose flap stays open once it is clogged with lint.
• Window wells and their screens, often torn by a snow shovel or a lawnmower.
• Cracked foundation joints hidden behind a cedar hedge or a woodpile, invisible until you clear the area.
• The crawl space hatch, closed again but never caulked.
• The attached garage, which acts as an airlock. A mouse gets into the garage, then into the house through the connecting door threshold or the wiring pass through.

One important note. You will see a specific minimum opening size quoted everywhere as the gap a mouse can squeeze through. We do not repeat it, because no official Canadian source confirms it. The reliable test stays visual. If daylight shows through a gap, or a screwdriver tip slides in without force, treat that spot as an entry point.

Sealing for good, materials and the order of the work

A seal holds when it combines a metal barrier and a filler, in that order. The metal blocks the passage, the filler stops air, water and insects. The reverse does not hold. Expanding foam on its own insulates, it blocks nothing.

The order that works

• Clear and clean. Remove old foam, cracked silicone and packed soil. Sealant does not bond to a dirty or damp surface.
• Pack it with metal. Stainless steel wool or metal mesh, pushed to the back of the opening with no play left along the sides.
• Cover it. Polyurethane based sealant for joints that move, mortar or repair cement for concrete, low expansion foam only on top of the metal.
• Replace rather than patch. Door weatherstripping, garage thresholds, window well screens. These parts cost little and an improvised repair will not survive the winter.
• Screen the vents. Roof, range hood, dryer, basement air intake, using metal mesh that lets air through without blocking the exhaust.

What does not hold

Duct tape, a rolled up rag, mineral wool, soft wood and cardboard are not sealing materials. They get chewed through, they soak up water and they hide the problem for a season. Sealant with no metal packing behind it does not last either on a wide opening.

One useful licensing point before you hire anyone. According to CAA-Québec, on a page updated June 16, 2025, an RBQ licence 7 is required for sealing entry points, because that counts as construction work, while it is not required for applying pesticides. In other words, the company that treats and the company that seals do not carry the same obligations. Ask exactly what the quote covers before you sign.

Cutting off food and shelter around the house

A perfectly sealed wall loses much of its value if the yard still offers bed and board. Mice settle outdoors first, right against the house, then look for the way in. Removing the shelters that touch the building pushes the starting point further away.

• Clear a strip along the wall. Trimmed cedars, cut vines, raked leaves, mulch kept low. A clean gravel strip along the foundation is the best permanent inspection tool there is.
• Move the firewood. Away from the wall and up off the ground, never stacked against the siding.
• Close up the food. Québec.ca recommends keeping food and waste in airtight containers so rodents are not drawn in. That includes dog kibble, birdseed and the bag of compost in the garage.
• Rethink bins and compost. Lids that actually close, rinsed containers, a reasonable distance from the building.
• Tidy the shed and garage. A stack of cardboard boxes against a wall is a ready made nest.

This perimeter work pays off twice. It is the same logic described in our article on spiders on the outside of the house, and a single late summer pass settles both problems.

The health risk, what public health actually says

The risk is real, it is rare, and it is managed with simple habits. According to Québec.ca, hantavirus infection is rare in Quebec, with six cases reported since 2005, the year of the first case recorded in the province. Transmission happens by inhaling particles contaminated with the urine, droppings or saliva of infected rodents, particularly while cleaning enclosed or poorly ventilated spaces.

The single most important official recommendation fits in one sentence. Never pick up rodent droppings by sweeping or vacuuming, since both put particles into the air. Québec.ca also advises avoiding all contact with rodents and their droppings, and keeping food and waste in airtight containers.

In a rented home, who handles what

Sealing is building maintenance, so it falls to the landlord in the vast majority of cases. According to the Tribunal administratif du logement, the presence of rats or mice is explicitly listed among unsanitary conditions, and the landlord must keep the dwelling in a good state of habitability.

Tenants are not without obligations either. Éducaloi points out that a tenant can be held responsible for refusing to cooperate, for unsanitary conditions they caused themselves, or for clutter that prevents treatment. In practice, report in writing, keep a record of the dates, and give the technician access.

The full topic, including remedies and timelines, is covered elsewhere in our articles on pest management in rental housing. Every situation is decided on its own facts, and the information above is not legal advice.

When to call an exterminator, and what calling before October changes

Call now if you cannot work out where they are getting in, if the house has a crawl space or an attic that is hard to reach, or if you had mice in a previous winter. A building that has been visited once will be visited again for as long as the opening stays open.

What the calendar changes is simple. Before October, the job is usually limited to an inspection and a sealing pass, with no established population inside. After the first cold spells, you have to treat first, then seal, then clean, which is three steps instead of one.

Axion Extermination has been a certified exterminator since 2011, with technicians certified in categories CD5 and CD6, an MELCC permit for pesticide application and products approved by Health Canada. Every intervention comes with a written guarantee, from $99, often within 24 hours. The details are on our rodent control in Montreal page, the full coverage map is under our 57 service areas, and the form is on the contact page. For a building, a daycare or a restaurant, go through our offer for businesses and property managers instead.

Off the island, the calendar is identical. Suburban homes with an attached garage and a crawl space often stack several of the defects listed above, and one late summer visit clears them in a single pass. Our teams cover the north shore and the rest of the Greater Montreal service area.

Frequently asked questions