Mice in the Walls: What That Scratching at Night Means (and How to Get Them Out)

Quick answer: Light scratching, scurrying or faint gnawing inside a wall — almost always after dark, usually between 9 p.m. and 3 a.m., and usually low down near your baseboards or along a ceiling line — is mice. Heavy thumping, rolling or dragging sounds during the daytime, higher up toward the attic, is more likely squirrels. You cannot trap a mouse inside a sealed wall cavity. You find where it exits the wall to feed, trap at that point, and seal the entry holes it used to get into the building in the first place. In Ontario, late September is when most homeowners first hear it. Sani IQ inspects for a $95 fee, credited in full when you book the treatment — call (705) 302-1887.

What You’re Actually Hearing

The first cold nights of the year in Ontario do two things at once. They push mice that spent the summer in your garden beds, under your deck and along your foundation to look for somewhere warm. And they make you close the windows, shut off the air conditioner and go to bed in a quiet house. That is why so many people believe mice “showed up” in late September — the mice were already on the property. What changed is that they moved inside the building envelope, and the house got quiet enough to hear them.

A wall cavity is close to ideal habitat from a mouse’s point of view: dark, insulated, warm, and running the full height and width of your home like a highway system. Mice travel inside walls because it gets them from a foundation crack to a kitchen cabinet without crossing open floor. What you hear through the drywall is a mouse doing ordinary things — dragging insulation fibre to build a nest, scratching for grip on a stud, or gnawing, which mice do constantly because their incisors never stop growing.

Sound travels along framing, so the noise is often not where the mouse is. Homeowners in older Toronto semis and 1970s Markham backsplits hear it in a bedroom and assume that’s the problem room when the activity is two joists over. Our Toronto pest control and Markham pest control teams run this call constantly through October, and the room people report is wrong about half the time.

Mice or Squirrels? Diagnose It by Sound and Clock

Before anyone opens a wall, you want to know what is in it. The two variables that matter most are when you hear it and how heavy it sounds.

Clue	Mice	Squirrels
Time of day	Night — roughly 9 p.m. to 3 a.m.	Daytime, heaviest at dawn and just before dusk
Weight of sound	Light, fast scratching and scurrying	Thumping, rolling, dragging; you can hear body weight
Location	Wall cavities, under cabinets, along baseboards, behind appliances	Attic, soffit, roofline, top of the wall
Extra tells	Droppings the size of a rice grain, gnawed food packaging, greasy rub marks	Nuts or cones in the attic, chewed soffit corners, visible roof traffic
Pattern	Steady nightly activity, multiple animals	Bursts of activity, often one or two animals

There’s a third answer people miss: no animal at all. A loose duct plate, water hammer in a pipe or a vine tapping siding can imitate scratching well enough to fool anyone — one reason a proper inspection starts with droppings, rub marks and entry holes rather than with cutting drywall.

If the sound is heavy, daytime and near the roof, you’re looking at wildlife rather than rodents — different legislation, different exclusion work, different equipment. Our pest and wildlife service overview lays out which is which; the house mouse profile in our pest library has the rodent identification details.

Why You Can’t Trap Them Where You Hear Them

This is the most common wasted weekend in Ontario pest control: someone hears scratching in a wall, buys traps, and sets them on the floor of the room where the noise is loudest. Mice inside a wall cavity are not going to come out through drywall to reach a trap — a sealed cavity has no bait access, no scent pathway and no reason for the mouse to leave its runway.

What works is following the animal, not the sound:

1. Find the exits. Mice leave the wall to feed and drink through the same handful of openings every night: behind the stove and fridge, under the sink where supply lines come through, inside the toe-kick of base cabinets, behind the dishwasher, at floor registers, and where pipes enter from the basement.
2. Read the traffic. Mice run along edges, not across open floor. Grease and dirt from their fur leaves dark rub marks on baseboards and pipe runs, and droppings concentrate where they feed and rest. That trail tells you which exits are live.
3. Trap at the exit, tight to the wall, perpendicular to it, trigger end against the baseboard. A trap out in the middle of a floor gets ignored for days.
4. Seal, in the right order. Sealing is what ends the problem — but seal while mice are still inside the walls and you trap them in there to die in place. Reduce first, then close the building.

Opening a wall is occasionally warranted, mostly for a dead animal or a nest against a heat source, but it’s a last step. Every hole cut in drywall is a repair bill, and it doesn’t address how the mice got in.

Where Ontario Mice Get In — and What Actually Seals It

A house mouse can pass through an opening around 6 mm across, about the width of a pencil. That figure surprises people, and it’s why “I don’t have any holes” is almost never true. The openings that account for most Ontario ingress:

• The sill plate gap where the wood frame meets the concrete foundation — the single most common entry point in GTA housing stock.
• Utility penetrations: gas line, hose bib, AC line set, electrical service, cable and internet drops. The foam or caulk left by the original trades is rarely rodent-proof, and it degrades.
• Weep holes and brick veneer gaps, including behind the veneer where the mouse is never visible from outside.
• Garage door corners — even a good seal leaves a triangular gap at each bottom corner, and from an attached garage a mouse is already inside the envelope.
• Dryer vents and bath exhaust terminations with broken or missing flapper louvres, and basement window wells where grade has settled.

What holds: stainless steel mesh or copper wool packed tight, backed with exterior-grade sealant or mortar. Metal is the operative word — mice gnaw through expanding foam, silicone, plastic and wood. What doesn’t hold: foam alone, kitchen-aisle steel wool (it rusts and crumbles in a season), duct tape, and every repellent on the shelf. Peppermint oil, ultrasonic plug-ins and mothballs do not exclude a mouse that has already chosen your wall as winter habitat.

Timing matters: this work is far easier in October than in January. You can still see the gaps, sealants cure properly, and you’re closing the building before the main ingress wave rather than after. We run the combined exclusion-and-control visit all fall — details on the Etobicoke mice control page and the Markham mice control page. Health Canada’s guidance reaches the same conclusion, leading with exclusion and sanitation before any chemical option — see their summary on rats and mice.

The Smell Problem Nobody Warns You About

Two to five days after a mouse dies inside a wall the smell becomes unmistakable, and there is nothing you can spray on it. It has to run its course — one to three weeks in a heated cavity, depending on where the carcass sits relative to a heat duct.

That’s the main risk of DIY bait in a wall or attic: bait doesn’t send a mouse outside to die. It dies where it is, often somewhere you can’t reach without cutting. Which is why our rodent approach relies on tamper-resistant stations in controlled locations, trapping at active runways, and exclusion — not loose bait in cavities.

One Winter of Sound, or One Year of Protection

Every Sani IQ service starts with a $95 professional inspection, credited in full when you book the treatment. That visit is where the entry points get found — the part that determines whether the problem ends or repeats. Canonical rodent pricing:

• One-Time Mice Treatment — $345. Trapping and station placement, with our 60-day guarantee on one-time services.
• Complete Mice Protection — $495. The one-time treatment plus exclusion work on identified entry points.
• Complete Mice Protection plan — $895 per year. Year-round coverage: scheduled visits, monitoring, re-treatment included.
• Essentials — $1,195 per year for rodents plus general insect pressure under one plan.

No initiation fee, no setup fee, no monthly billing. Full pricing sits on the plans and pricing page.

Now the honest version of the decision. A one-time visit solves the mice in your walls right now. It doesn’t change the fact that your house has the same entry points, the same neighbours’ garages, the same ravine or field behind it, and the same first-cold-night trigger next September. A pair of mice can produce dozens of offspring in a season, and reinfestation the following fall is the norm.

Weigh that against what a mouse population costs when it’s left to run: gnawed wiring (a fire risk, not an inconvenience), contaminated insulation that has to be removed and replaced, chewed HVAC flex duct, ruined pantry stock, drywall opened to retrieve a carcass. Any single one of those items typically exceeds a year of protection.

So: treat the one-time service as the entry point if you need the noise to stop this week, and treat Complete Mice Protection at $895/year as the actual solution — it’s the only version that includes somebody coming back to confirm the seals held. The numbers are laid out in one-time pest control versus an annual plan.

Sani IQ is BBB Accredited, carries $5 million in liability coverage, is licensed under the Ontario Pesticides Act and registered with the MECP, and holds a 5.0 rating across 126 Google reviews. Our technicians are employed by us, licensed and background-checked — never subcontractors. Where treatment is warranted we use reduced-risk products in controlled, tamper-resistant placements.

Call (705) 302-1887, or start with the free quote quiz and we’ll come out for the $95 inspection — credited when you book. Full scope is on our residential pest control page.

And if you’re hoping this sorts itself out once real winter arrives — it doesn’t, for a reason worth understanding: do mice hibernate?

FAQ

How do I know if it’s mice and not rats? Sound weight and dropping size are the fastest tells. Mouse droppings are about the size of a grain of rice with pointed ends; rat droppings are 12–20 mm, roughly a raisin. Rats are also loud enough that you rarely wonder — you hear body weight, not scratching.

Can I just seal the holes and skip the treatment? Only if you’re certain nothing is inside yet. Seal an occupied structure and you get dead mice in wall cavities, with the smell and the repair work that follows. Order matters: reduce first, seal second.

Is it safe to use poison in my walls if I have pets? We don’t place loose bait in wall cavities or attics as standard practice — for that reason plus the carcass problem. Where rodenticide is appropriate it goes in tamper-resistant stations in controlled locations, and we’ll tell you where every one of them is.

Do you service my area? The GTA, York and Durham Regions, Simcoe County, Barrie, Orillia and Muskoka. Call (705) 302-1887 and we’ll confirm scheduling for your address.