Keeping mice out of a Michigan home means closing every opening larger than a quarter inch — about the width of a dime — and doing it before the first cool October nights. A single female house mouse produces 5 to 10 litters of 5 to 6 pups per year, so one mouse in October becomes a population by spring. The fix is exclusion: a methodical exterior survey followed by mesh-and-sealant work on every gap.
The 1/4-Inch Rule and Michigan's 7 Most Common Entry Points
A mouse can squeeze through any gap larger than 1/4 inch — roughly the width of a dime. A young mouse can squeeze through gaps even smaller because their skull is the widest part of the body. In Metro Detroit homes, the same seven entry points show up over and over:
- Garage door corner seals. The seal between the door and the side jamb almost always has a gap. This is BTR's #1 entry point in Wayne and Oakland County homes.
- Dryer vent hoods. Plastic vent flaps that no longer close or are missing entirely.
- Gas / utility line penetrations. Where the gas, electric, cable, or water line enters the foundation — these are usually finished with expanding foam, which mice chew through.
- Foundation settlement cracks. Hairline cracks at the corners and at the brick-to-block transition.
- Weep holes. The drainage holes in brick walls allow water to escape and mice to enter.
- Gaps under siding. Where the bottom course of vinyl or aluminum siding stops short of the foundation.
- AC line chases / dryer vents on second story. Mice climb rough surfaces and brick.
The BTR Exclusion Checklist
Work through this list in order. Materials needed: a quarter (for gap-size reference), stainless-steel mesh, copper wool, exterior-grade sealant or caulk, garage door corner seals, and a flashlight.
- Walk the perimeter. Use the flashlight at dusk. Look for any gap larger than the side of a quarter.
- Replace the dryer vent hood. Install a metal-mesh hood that closes when the dryer is off. Plastic flap-only hoods do not stop mice.
- Seal foundation cracks. For hairline cracks, exterior-grade sealant alone. For anything wider than 1/4 inch, pack stainless-steel mesh into the gap and overlay with sealant.
- Install garage door corner seals. Rodent-proof corner-seal brushes attach to the side jambs and bottom corners.
- Pack every utility penetration. Gas, electric, cable, water, AC line chases. Copper wool first (mice will not chew it), then exterior-grade sealant over the wool.
- Screen weep holes. Stainless-steel mesh inserts that allow drainage but stop mice.
- Seal gaps under siding. Backer rod + sealant where the siding meets the foundation.
- Trim vegetation. Mice use shrubs touching the foundation as a ramp. Trim everything back 18 inches.
- Store dry goods, pet food, and bird seed in sealed containers. Glass or hard plastic; not the original cardboard bag.
- Schedule professional exclusion if you are unsure. BTR provides a free exclusion estimate after inspection — included with any rodent service.
The Michigan deadline is mid-October. Once overnight lows drop below 50 degrees Fahrenheit (usually the first week of October), mice begin moving indoors. See the October entry in the Michigan Seasonal Pest Calendar for the full migration timeline.
What Exclusion Alone Cannot Fix
Sealing the exterior stops new mice from entering. It does not eliminate mice that are already inside. Three situations need rodent service in addition to exclusion:
- An active interior population. If you have seen droppings, heard scratching, or found gnaw marks, mice are already breeding inside. They need bait and monitoring.
- Wall-void nests. Established nests inside walls need professional product penetration that retail products do not match.
- Long-term colonies. If mice have been in the house 6+ months, the population is mature and dispersed. Full rodent service + exclusion is the only complete fix.
When to Call BTR
Call BTR when you see any of: droppings (1/8 to 1/4 inch, pointed both ends — see the house mouse identification page for photos), gnaw marks on food packaging or wood, scratching or scampering sounds at night, greasy rub marks along baseboards, or shredded paper / fabric / insulation. BTR offers free inspection. Mice treatment starts at $235 with a 90-day warranty — the full rodent control cost breakdown covers what is included.
Frequently Asked Questions
How big a gap can a mouse fit through?
A house mouse can squeeze through a 1/4-inch gap, roughly the side-width of a U.S. dime. Young mice can squeeze through even smaller gaps because their skull is the widest part of the body. Any gap larger than 1/4 inch is a potential entry point and must be sealed.
When do mice come into Michigan homes?
Mice begin moving into Michigan homes when overnight low temperatures drop below 50 degrees Fahrenheit, which usually happens the first week of October. Scouting behavior starts in September. Sealing entry points and completing exclusion before mid-October is the most reliable way to keep them out.
Will steel wool keep mice out?
Steel wool deters mice short-term but rusts within months and falls out. Use copper wool instead — it does not rust, holds shape, and mice will not chew through it. Pack copper wool first, then overlay with exterior-grade sealant. For larger gaps, use stainless-steel mesh + sealant.
Does expanding spray foam work for sealing entry points?
Foam alone does not work — mice chew through it within days. Foam is fine as a backer or filler, but every mouse-rated seal needs a metal layer (steel mesh or copper wool) that mice physically cannot get through, with foam or sealant only as the surface finish.
How long does professional rodent exclusion last?
BTR exclusion work uses rodent-rated materials (steel mesh, copper wool, exterior-grade sealant) that last 5+ years in Michigan weather. Building settlement and new construction occasionally open new gaps, so an annual exterior walk-through is good practice. BTR exclusion comes with a written report identifying every gap sealed.
Do I still need exclusion if I have indoor cats?
Yes. Indoor cats deter mice but do not stop them from entering. Mice in walls or attics may never encounter the cat. And mice carry hantavirus, salmonella, and leptospirosis whether a cat catches them or not. Exclusion is the only fix that closes the entry points themselves.
What is the difference between exclusion and extermination?
Extermination kills the mice in the house today — bait, traps, or both. Exclusion seals the entry points so new mice cannot come in. Without exclusion, extermination is recurring. With exclusion, it becomes a one-time job. BTR provides exclusion estimates with every rodent service.
Need an Exclusion Estimate?
Free inspection. Written exclusion estimate. Licensed Michigan applicators. 90-day warranty on most services.