Cottage Pest Prevention: Keep Mice Out When Closing for Winter

How Mice Get Into Your Cottage

A mouse can squeeze through a gap the size of a dime. That is about 6 millimetres, roughly the width of a pencil. When you understand that fact, you start seeing your cottage differently. Every pipe penetration, every gap where siding meets foundation, every spot where a cable enters the building is a potential entry point.

Deer mice and house mice are the two species most common in the Collingwood and Blue Mountains area. Deer mice are the bigger concern because they carry hantavirus, which can be transmitted through droppings and dried urine. Both species are looking for the same thing in October: shelter from the cold, a food source, and nesting material. Your closed-up cottage provides all three.

The most common entry points we find during cottage inspections:

• Gaps where plumbing pipes enter through the foundation or floor (often 10 to 15 mm of clearance around the pipe)
• Where the sill plate meets the foundation wall, especially in older cottages where the wood has shrunk over decades
• Dryer vents, range hood exhausts, and bathroom fan vents with missing or damaged flaps
• Door sweeps that are worn, missing, or do not make full contact with the threshold
• Utility line entries for electrical, cable, and gas, where sealant has cracked or was never applied
• Gaps in soffit and fascia, especially where roof lines meet walls at inside corners

What Mice Actually Damage

The droppings and the smell are unpleasant, but the real costs are structural and sometimes dangerous.

Electrical Wiring: A Genuine Fire Risk

Mice chew on wiring constantly. Their teeth grow continuously throughout their lives, and they gnaw to keep them manageable. Electrical wiring is a favourite target because the insulation material has a texture and flexibility they find satisfying to chew. When a mouse strips the insulation off a wire inside a wall cavity, that exposed copper sits against wood framing. It is one of the leading causes of unexplained cottage fires. Insurance adjusters in Ontario see this regularly, and it is not a theoretical risk. The Electrical Safety Authority of Ontario lists rodent damage to wiring as a known fire hazard.

Insulation

Mice love fibreglass batt insulation as nesting material. They tunnel through it, shred it, and compress it, reducing its R-value significantly. We have opened up cottage crawlspaces in the spring to find entire sections of insulation pulled apart and redistributed into nests. Replacing insulation in a crawlspace runs $2 to $4 per square foot, and a typical cottage crawlspace is 800 to 1,200 square feet. That is $1,600 to $4,800 in damage from a problem that costs $50 in prevention materials.

Food Contamination and Health Risks

Anything left in your cottage is fair game. Mice chew through cardboard boxes, plastic bags, and even thin plastic containers. They contaminate far more than they eat because they leave droppings and urine trails everywhere they travel, which is everywhere. A single mouse produces 50 to 75 droppings per day.

Deer mice are the primary carrier of hantavirus in Ontario. The virus is transmitted through dried droppings and urine that become airborne when disturbed. This is why you should never sweep or vacuum mouse droppings dry. Always wet them with a disinfectant solution first, wear gloves, and ventilate the space for at least 30 minutes before starting cleanup.

Soft Furnishings and Stored Items

Couch cushions, mattresses, linens in closets, paper products, and anything soft become nesting material. We have seen cottage owners open up in spring to find their sofa cushions hollowed out from the inside, with no visible damage from the outside until you sit down and the whole thing collapses. Replacing soft furnishings after a bad mouse year can easily run $2,000 to $5,000.

Exclusion Methods That Actually Work

Traps and poison are reactive. Exclusion is preventive. The goal is to make it physically impossible for mice to enter your cottage. This is more effective and more humane than any other approach.

Steel Wool and Copper Mesh

Stuff steel wool or copper mesh into any gap larger than 6 millimetres. Mice cannot chew through metal. Copper mesh is better for long-term use because it does not rust, but steel wool is cheaper and works well for interior gaps. Pack it tightly. A loose wad of steel wool is something a determined mouse will simply pull out.

Expanding Foam (With a Caveat)

Expanding foam alone does not stop mice. They chew through standard foam easily. However, expanding foam combined with steel wool creates an effective barrier. Fill the gap with steel wool first, then seal over it with foam to hold the steel wool in place and block air flow. Some manufacturers now make pest-resistant foam that contains a bittering agent, but the steel wool layer is still the real barrier.

Hardware Cloth (1/4-Inch Galvanized)

For larger openings like crawlspace vents, dryer exhausts, and foundation gaps, quarter-inch galvanized hardware cloth is the standard. Anything larger than quarter-inch allows mice through. Secure it with screws and washers, not staples, which mice can work loose over time. Cover soffit vents, foundation vents, and any opening larger than about 25 millimetres.

Door Sweeps and Weatherstripping

Replace worn door sweeps on every exterior door. The gap under a cottage door is often 10 to 15 millimetres, more than enough for a mouse. Use a metal or heavy-duty rubber sweep that contacts the threshold firmly across its entire width. Check garage doors especially, where the rubber seal at the bottom degrades quickly and leaves gaps at the corners.

Caulking and Sealant

Seal all pipe penetrations, utility entries, and gaps in siding with exterior-grade silicone caulk. For gaps larger than about 10 millimetres, use backer rod first, then caulk over it. A full exterior caulking pass on a typical cottage takes 2 to 3 hours and costs about $30 to $60 in materials. It is one of the highest-return maintenance tasks you can do.

The Mouse-Proofing Closing Checklist

Add these items to your regular fall closing routine, ideally during the mid-October window outlined in our Georgian Bay cottage closing timeline. Most of this work takes less than a day and uses materials you can pick up at any hardware store in Collingwood or Wasaga Beach.

Signs of Infestation When You Open in Spring

Even with good prevention, mice are persistent. Here is what to look for when you open the cottage in spring and how to respond.

Droppings. Small, dark, rice-shaped pellets. Check drawers, under sinks, behind appliances, and in the pantry. Fresh droppings are dark and soft. Old droppings are grey and crumbly. The volume tells you the severity: a few droppings means a mouse passed through. Droppings in every drawer means an established colony.

Urine odour. A strong, ammonia-like smell, especially concentrated in enclosed spaces like closets and cabinets. This smell does not go away with simple cleaning. Affected surfaces need to be treated with an enzymatic cleaner.

Nesting material. Shredded paper, fabric, or insulation gathered into a ball, usually in a warm, enclosed space. Check behind the fridge, inside the stove insulation, in basement ceiling cavities, and in any stored boxes.

Chew marks. Look at the edges of cabinet doors, baseboard corners, food packaging, and most importantly, any visible wiring. If you see chew marks on wiring, stop and call an electrician before restoring power to those circuits.

Grease marks. Mice travel the same paths repeatedly and leave dark, greasy rub marks along walls and baseboards. These marks show you their travel routes, which is useful for trap placement if the problem is ongoing.

When to Call a Professional

If you find evidence of heavy infestation, droppings in most rooms, nesting material in wall cavities, or chewed wiring, bring in a licensed pest control company. A professional can identify every entry point, do a thorough exclusion, and assess whether the wiring or insulation needs remediation. In the Collingwood area, a full cottage mouse exclusion service typically runs $400 to $800. A full exclusion with insulation replacement in a crawlspace can be $2,000 to $5,000.

For minor issues, a few droppings in the kitchen and one or two entry points you can identify, the DIY approach described above is perfectly adequate. The key is doing the exclusion work before closing, not dealing with the aftermath in spring. And while you are hardening the cottage against pests, consider the human-intrusion side too. Our guide on what actually deters cottage break-ins covers physical reinforcement and monitoring strategies that complement pest exclusion work.

Frequently Asked Questions