Mice in the Walls: Signs, Noises and How to Get Them Out
It is 2 a.m. and something is scratching behind the bedroom wall. A few quick footsteps, a pause, then a steady gnawing. It is one of the most common fall scenarios in Québec homes, and it almost always means the same thing: mice in the walls. The good news is that it is a very solvable problem, as long as you avoid the classic mistakes, like putting poison inside the walls or sealing the holes too early. Here is how to confirm you are dealing with mice, understand how they get around, and get them out for good.
What mice in the walls sound like
Mice are mostly active at night, with peaks around dusk and just before dawn. Typical noises include:
- light, quick scratching, like fingernails on cardboard;
- scurrying, rapid little footsteps going back and forth, sometimes vertically;
- gnawing, a dry, steady sound as they chew on wood, wiring or packaging;
- occasional squeaking, especially when there is a nest with young.
The sounds often come from low on the wall near the floor, but they can also come from the ceiling between floors, or from kitchen and bathroom walls where the plumbing runs. Health Canada lists scampering and scratching sounds in the walls at night among the signs of a rodent problem.
Mice, squirrels, rats, bats or insects?
The time of day and how heavy the noise sounds are your best clues.
| What you hear | When | Likely culprit |
|---|---|---|
| Light scratching, quick scurrying, gnawing | At night, especially at dusk and before dawn | Mice |
| Heavier thumps, running, something being rolled | During the day, especially morning and late afternoon | Squirrel (attic, roof space, upper walls) |
| Louder scratching and gnawing, heavier movement | At night, often in the basement and near drains | Rat |
| Rustling and high-pitched chirps, mostly high on walls or in the attic | At dusk when they head out, and at dawn | Bat |
| Faint, steady crackling, sawdust near woodwork | Day and night, mostly in spring | Carpenter ants |
Grey and red squirrels are active during the day: if the noises are heavy and happen in broad daylight, especially near the roof, a squirrel is the prime suspect, and our article on how to manage a squirrel nest in your home will be more useful. Flying squirrels are the exception: they are nocturnal, but noisier than mice and mostly found in the attic. If you can’t decide between a rat and a mouse, our guide on how to tell a rat from a mouse will help.
Other signs of mice in walls
Noise is rarely the only clue. Also look for:
- droppings 3 to 6 mm long with pointed ends under the sink, behind appliances, along baseboards or in drawers. Our guide to mouse droppings explains how to identify them and, above all, how to clean them up safely;
- chewed packaging in the pantry;
- dark, greasy rub marks along walls or around a small hole, left by mice that always take the same path;
- an ammonia smell (urine) in a closed cabinet or closet;
- shredded material such as paper, fabric or insulation used for nesting;
- a pet that stares at a wall, sniffs it or keeps pawing at the same spot.
Why mice move into walls in the fall
As soon as nights turn cold, usually from September and October on, mice look for a warm place to spend the winter. The walls of a house are ideal: insulated, dark, safe from predators and close to the food in your kitchen. Wall cavities also act as highways that let mice move between floors without being seen. That is why noises in the walls are most common from late fall through spring.
How mice get into walls
A mouse can squeeze through a gap of only about 6 mm. Health Canada puts it this way: a crack the size of a dime is enough. The most common entry points are:
- the joint between the foundation and the wood framing (the sill plate), often poorly sealed;
- cracks in the foundation and crumbling mortar joints;
- openings around pipes, wires and cables: water and gas lines, air conditioning lines, cable and internet;
- the dryer vent and unscreened vents;
- the garage door, when its weatherstripping is worn or chewed, and the shared wall between the garage and the house;
- soffits, roof edges and the bottom of the siding.
Once inside the building envelope, mice follow plumbing and wiring runs between floors and come out into the house through gaps around the pipes under the sink, behind the stove or dishwasher, or through a gap under a baseboard.
What not to do
Put poison inside the walls
This is the most common mistake, and the one people regret most. A poisoned mouse doesn’t die on the spot: it often returns to its nest inside the wall and dies there. The result is a dead-animal odour that can last several days or even a few weeks, sometimes flies, and a wall to open if the smell becomes unbearable. Health Canada also notes that baits and poisons should not be used as the only method of control, and they are a risk to children and pets. To weigh the options, read our article Mousetrap or rodenticides: which to choose?
Seal the holes too soon
Sealing an opening while mice are still in the wall doesn’t make them go away. They chew a new way out, often into your living space, or die inside the wall. Seal after the mice inside have been dealt with, or as part of a strategy that gets them out, never before.
Drill blindly or wait it out
Drilling holes in the drywall to “take a look” or spraying insecticide inside doesn’t solve anything and just creates more passages. Waiting for mice to leave on their own doesn’t work in the fall either: they are there for the winter, and a female can have several litters.
What works to get mice out of your walls
1. Find where they come out of the wall
Mice live in the wall but come out to eat. Find the spots where droppings and rub marks are concentrated: under the sink, behind the stove, in the pantry, in the basement along the foundation. That is where you catch them, not inside the wall.
2. Set traps along their runways
Place snap traps perpendicular to the wall, with the trigger end against the baseboard, where mice travel. Use several traps at once rather than just one. Health Canada suggests a three- to four-day warm-up period with baited but unset traps, then checking every trap daily. Keep traps out of reach of children and pets, for example inside a trap box. Our guide on how to get rid of mice has more tips on bait.
3. Seal the entry points
Once the captures stop, walk around the outside of the house and seal every opening with materials mice can’t chew through: steel wool or metal mesh covered with sealant, metal flashing, cement. Install a door sweep and replace worn weatherstripping on the garage door. Our article on caulking against rodents covers the materials and the spots to check.
4. Cut off the food supply
Store cereal, flour, birdseed and pet food in airtight containers, and don’t leave the dog or cat bowl full overnight.
If the noises come from the top-floor ceiling instead, also read our tips for eliminating mice in the attic, since the approach is a little different.
When to call an exterminator
Call a mouse exterminator if:
- the noises continue after a week or two of trapping;
- you find droppings in several rooms or on several floors;
- you can’t find how the mice are getting in;
- you live in a multi-unit building, where mice travel between units through shared walls;
- a dead-mouse smell appears in a wall;
- the noises are heavy or happen during the day, which points to a squirrel or other wildlife.
A technician inspects the inside and outside of the building, finds the entry points, chooses methods suited to your home (taking children and pets into account) and can seal the openings. For an idea of pricing, see our article on the cost of a mouse exterminator.
Frequently asked questions
Will mice in the walls leave on their own?
Rarely, especially in fall and winter. The walls give them warmth, shelter and access to food, so they have no reason to leave. Without action, the population grows with each litter.
How can I tell if it’s a mouse or a squirrel in the wall?
Go by the time of day. Mice are active at night and make light, quick sounds. Grey and red squirrels are active during the day, especially in the morning and late afternoon, and make heavier noises, often near the roof.
What should I do if a mouse dies in the wall?
Find where the smell is strongest; flies gathering in one spot are another clue. If the carcass is reachable, remove it following safe cleanup guidelines. Otherwise, air out the room: the smell usually fades within a few days to a few weeks. Opening the wall may be necessary if it persists.
Can I put mouse poison inside the walls?
It is not recommended. Poisoned mice often die inside wall cavities, causing lingering odours, and poison is a risk to children and pets. Traps set along mouse runways, combined with sealing, give better results.
Why do I only hear mice at night?
Because mice are nocturnal. The house is also quieter at night, which makes their movements easier to hear. During the day they sleep in the nest, often inside the wall itself.
Need a mouse exterminator?
Scratching in the walls keeping you up at night? The sooner you act, the less time mice have to settle in for the winter. Our certified technicians work across Greater Montreal, from Laval to Longueuil, to find where mice are getting in, get rid of them and seal the openings. Learn about our mouse extermination service, call us at 514-677-7526 or request a quote.
