Most homeowners picture mice coming in through an obvious hole. In practice they come in through dozens of small, ordinary gaps: around the dryer vent, along the garage door, under the sink where a pipe goes through the wall, at the top of the brick where it meets the soffit. Finding those gaps and sealing them with the right material is the single most effective thing you can do about mice, and some of the most popular materials don't hold up.
Mice get in through gaps as small as 1/4 inch, about the width of a pencil. The National Park Service notes that if a mouse's head fits through a gap, its slender, flexible body usually follows. They climb brick, siding and pipes, so entry points can be anywhere from the foundation to the roofline.
Seal small gaps with copper mesh or steel wool packed in tight and caulked over, and cover larger openings with 1/4-inch hardware cloth or sheet metal. Spray foam on its own is not enough. In a month-long test with real mice and rats, it was the only material they chewed through.
How Small a Gap Can a Mouse Use?
- Mice: 1/4 inch. The National Park Service's rodent exclusion manual puts it at 1/4 inch (6 mm), about the diameter of a #2 wooden pencil. The University of California adds that house mice can squeeze under a gap 1/4 inch tall and through openings 3/8 inch wide.
- Rats: 1/2 inch. The same manual notes rats can get through holes only 1/2 inch across, smaller than a quarter.
- A simple test: the NPS manual suggests using a pencil or ballpoint pen as a gauge. If it fits in the gap, a mouse can use it.
Can Mice Climb Walls and Siding?
Yes, which is why entry points aren't only at ground level. The NPS manual describes mice climbing any slightly rough vertical surface, such as wood, plaster, brick, metal pipes, wire mesh and cables. They can also run along horizontal wires and ropes and jump down from 8 feet without injury. Brick and textured siding are easy climbs, and a gap at the soffit or roof edge is reachable. In one long DIYChatroom thread, a homeowner sealed gap after gap at ground level before finding the real entry on the roof, where the attached garage roof met the house.
The Entry Points to Check
The CDC's "Seal Up" checklist is a good place to start. We've grouped it by where to look, with the Southern-home details that matter here.
| Where | What to look for | How to seal |
|---|---|---|
| Pipes, wires and A/C lines where they enter the house | Gaps around the line where it passes through siding, brick or the foundation, and where pipes come through cabinet walls under sinks | Copper mesh or steel wool packed tight, then caulk over it |
| Dryer vent | A missing or broken flap, and gaps around the pipe at the wall | A vent cover with a working flap or guard. Seal around the pipe |
| Garage door | A worn bottom seal, and gaps at the corners and along the side tracks | Replace the seal. The NPS notes rodent damage to garage door gaskets is usually at the corners, which can be protected with metal flashing. Brush-style kits for garage doors also work well |
| Exterior doors | Daylight under the door or at the bottom corners of the frame | Heavy-duty door sweeps. NPS calls brush sweeps very effective |
| Weep holes in brick veneer | Open joints along the bottom course of brick and above windows and doors | Mesh or plastic weep screens. Never plug them solid (see below) |
| Foundation and crawl space vents | Torn, rusted or missing screens, and a loose access door | Replace screens with 1/4-inch galvanized hardware cloth |
| Roofline: soffits, fascia, gable vents, roof returns | Gaps where the soffit meets the fascia or brick, and broken gable vent screens | Hardware cloth over vents. Sheet metal or trim coil over gaps |
| Inside | Under and behind kitchen cabinets, closet floor corners, around the fireplace, behind appliances, around the water heater and furnace, floor drains, where walls meet floors | Copper mesh or steel wool plus caulk for small gaps |
A note on weep holes
Brick veneer is everywhere in Carolina and Virginia neighborhoods, and it is built with weep holes on purpose. The International Residential Code requires them in brick veneer walls, spaced no more than 33 inches apart and at least 3/16 inch across, just above the flashing, so water that gets behind the brick can drain out. The most common kind is simply a vertical mortar joint left empty, which is wide enough for a mouse.
Don't fill them with mortar, caulk or foam. That traps moisture behind the brick. The Brick Industry Association's guidance is that non-corrosive metal, mesh or plastic screens or vents can be placed in the open joints to keep insects out while letting the wall drain. For mice, choose stainless steel or copper weep vents: UC IPM lists plastic screening among the materials that are ineffective against house mice, because they can chew through it.
Which Sealing Materials Actually Work
This is where most do-it-yourself sealing goes wrong. Two sources tested or field-checked these materials: a month-long trial by trapper Shawn Woods (about 2 million views on YouTube), and the National Park Service's exclusion manual, written for keeping deer mice out of park buildings.
| Material | Month-long rodent test | NPS manual | Verdict |
|---|---|---|---|
| Spray foam alone | Failed. Mice and rats chewed straight through it, and a rat got in | Useful as a filler with mesh behind it. Must be trimmed and painted or sunlight breaks it down | Not on its own |
| Steel wool | Held, both fine and coarse | Deters rodents, but rusts, is flammable, and rodents work their way between it and the edges of the hole. Always caulk over it | Good, if caulked in |
| Copper mesh | Held. Rodents pulled at it a little | Doesn't rust. Caulk over it, since rodents can push it aside | Best small-gap filler |
| Silicone caulk | Held, with tooth marks | Seals fillers in place and shows where rodents have tried | Use over a metal filler |
| Hardware cloth, 1/4 inch | Not tested | The standard for vents, skirting and larger openings | Best for vents and big gaps |
| Lath screen or metal | Not tested | "Highly rodent-proof," doesn't rust, and can be wadded into odd-shaped holes | Excellent filler |
Homeowners report the same thing. In Reddit threads Google surfaces on the subject (read through Google's excerpts, since Reddit blocks automated reading), one renter's mice kept digging around a hole stuffed with steel wool until it was redone with copper mesh and tape. A pest control technician described using copper mesh "because it won't rust like steel wool" with foam over it. The most common answer to "what's a mouse-proof sealant?" was hardware cloth.
The combination almost everyone lands on, and the one Woods suggests after his test, is metal first, then sealant: pack copper mesh or steel wool tightly into the gap, then caulk or foam over it to hold it in place.
For bigger holes, the Internet Center for Wildlife Damage Management's construction guide recommends 24-gauge hardware cloth with 1/4-inch mesh for mice (19-gauge, 1/2-inch mesh for rats), and backing any hole 3 inches or larger with hardware cloth before patching it.
How to Find the Gaps You Can't See
- Look for daylight. On a sunny day, go into the basement, crawl space, garage or attic with the lights off and look for light coming through around the sill plate, pipes and vents.
- Follow the droppings. Fresh droppings and grease smudges usually lead toward the entry point.
- Use the pencil test on every gap you find. If a pencil fits, seal it.
- Dust for tracks. Sprinkle a little talcum powder or flour near a suspected gap and check for footprints the next morning.
- Look up. Check the roofline, especially where a lower roof meets a wall, and anywhere branches touch the house.
When to Seal
Mice move indoors as nights get colder. Using NOAA climate normals, normal overnight lows drop below 50°F around October 17 in Greenville-Spartanburg and Raleigh, October 22 in Columbia, October 29 in Norfolk and November 5 in Charleston. Sealing in the weeks before those dates keeps the fall wave out. See when mouse season starts for the full table.
If Mice Are Already Inside
Seal and trap at the same time. Sealing alone can trap mice inside, where they keep breeding. Set snap traps along walls where you see droppings. Once the traps stop catching, close the last gaps. Droppings larger than a grain of rice, like a raisin, point to rats, which need 1/2-inch protection and different trap placement. See signs of rats in your walls.
When to Call a Professional
- You've sealed what you can find and mice keep coming in
- The likely entry is on the roof, in the soffits or high on the gable
- You have a crawl space or attic you can't safely inspect
- You're finding signs of rats, not just mice
Our rodent control service includes a full exclusion inspection from the foundation to the roofline, sealing with metal-backed materials, and trapping, with follow-up visits to make sure nothing is getting back in.
Key Takeaways
- Mice use gaps as small as 1/4 inch, about a pencil's width. Rats need 1/2 inch.
- They climb. Check the roofline and soffits, not just the foundation.
- Spray foam alone fails. It was the only material rodents chewed through in a month-long test.
- Use metal, then sealant: copper mesh or steel wool packed tight and caulked over. Use 1/4-inch hardware cloth for vents and larger holes.
- Screen weep holes. Never plug them, since the building code requires them for drainage.
- Seal before the nights cool, and trap at the same time if mice are already inside.
Sources: Centers for Disease Control and Prevention, "How to Seal Up to Prevent Rodents"; National Park Service, "Rodent Exclusion Manual: Mechanical Rodent Proofing Techniques", 2019; R. O. Baker, G. R. Bodman and R. M. Timm, "Rodent-Proof Construction and Exclusion Methods", Internet Center for Wildlife Damage Management; N. M. Quinn and M. Frye, "Pest Notes: House Mouse", University of California IPM; International Code Council, International Residential Code § R703.8.6, weepholes; Brick Industry Association technical notes on weeps; NOAA NCEI U.S. Climate Normals 1991-2020; S. Woods, "The Best Ways To Block Mouse/Rat Holes", YouTube; owner accounts from DIYChatroom and Reddit (via Google search excerpts).
Frequently Asked Questions
Through small gaps, usually around pipes, wires and A/C lines where they enter the wall, dryer vents, worn garage door seals, gaps under exterior doors, open weep holes in brick, torn foundation or crawl space vent screens, and gaps at the roofline and soffits. Any opening about 1/4 inch or larger, roughly the width of a pencil, is big enough.
About 1/4 inch (6 mm), roughly the diameter of a pencil, according to the National Park Service. Mice have slender, flexible bodies, so if the head fits, the body usually follows. Rats need about 1/2 inch.
Yes. Mice climb any slightly rough vertical surface, including brick, wood, plaster, metal pipes, wire mesh and cables, and they can run along wires. That is why entry points can be at the roofline as well as the foundation.
Smooth vinyl is hard for them to grip, but the National Park Service notes mice climb rough surfaces, metal pipes, wire and cables, so corner trim, downspouts and utility lines running up the wall give them a route. Gaps where siding meets the soffit, trim or a utility line are common entry points, so check them as well as the foundation.
Copper mesh or steel wool packed tightly into the gap, with caulk or foam over it to hold it in place. For vents and larger holes, use 1/4-inch galvanized hardware cloth or sheet metal. The CDC, the National Park Service and a month-long test with live rodents all point to metal-based materials.
Not on its own. In Shawn Woods' month-long test with mice and rats, spray foam was the only material they chewed through, and UC IPM also lists expanding foam as ineffective for plugging mouse holes. Foam works as a sealant over copper mesh or steel wool, and outdoor foam should be trimmed and painted so sunlight doesn't break it down.
Both held up in testing. Copper mesh doesn't rust, while steel wool rusts outdoors and in damp spots. The National Park Service recommends caulking over either one, because rodents can work around the filler or push it aside.
Don't plug them solid. Building code requires weep holes in brick veneer so water behind the brick can drain. Use mesh or plastic weep screens made for the purpose. They keep mice and insects out and still let the wall drain.
Replace a worn bottom seal and check the corners, where the National Park Service says rodent damage is most common, and the side tracks. Protecting the corners with metal flashing or adding a brush-style seal along the sides closes the gaps mice use.
Seal and trap at the same time. Set snap traps along walls where you find droppings, and close the last entry points once the traps stop catching. Sealing a house with mice still inside leaves them indoors to keep breeding.
Photo credits: Ahunt, CC0; z22, CC BY-SA 3.0, via Wikimedia Commons.