How to Mouse-Proof a House: Where Mice Get In and How to Seal It

Mice do not need a hole. They need a gap the size of a dime — roughly a quarter inch — and most older houses have several. Mouse-proofing is the highest-leverage thing you can do to prevent the rodent problems that, in the CDC’s framing, create the exposure scenarios behind hantavirus. This page is the practical sealing guide: where mice actually get in, what materials stop them, and where the routine breaks down for most homeowners.

For the medical framing — why rodent exposure matters and what the CDC’s three-step prevention picture looks like — see Hantavirus Prevention: Seal Up, Trap Up, Clean Up. This page is the deep-dive on the Seal Up half.

Why sealing is the most important step

The CDC’s rodent-control guidance is built on a simple causal chain: rodents in a structure produce droppings and nesting material in places people eventually disturb, and disturbing that material is what can aerosolize the virus. Trapping reduces a population that is already inside. Cleanup handles waste that has already been deposited. Sealing prevents the problem from existing in the first place.

A sealed building does not need as many traps. It does not generate the contaminated environments that make cleanup safety-critical. And it does not turn into a recurring problem every season. If you only do one of the three CDC prevention steps, sealing is the one that compounds.

Where mice actually get in

Mice are athletic. They climb siding, swim through pipes, and squeeze through openings most people would not consider entry points. CDC rodent-control guidance and pest-management research both converge on roughly seven categories of entry, in rough order of how often they fail:

1. Utility penetrations. Anywhere a pipe, wire, cable, vent, or duct passes through an exterior wall or foundation, the original seal around that penetration has probably degraded. Caulk shrinks. Foam rots. Drywall mud cracks. These are the most common entry points in finished homes.
2. Garage and shed doors. The bottom seal on a garage door is consumable hardware that almost no one replaces on schedule. A worn or compressed sweep leaves a gap the entire width of the door. Side doors on outbuildings often have similar problems.
3. Vents and louvers. Dryer vents, attic gable vents, soffit vents, foundation vents, and crawl-space vents are designed to let air through. The screens that keep rodents out are routinely missing, torn, or sized too coarse (rodents go through standard window-screen mesh with no trouble).
4. Foundation cracks and gaps where framing meets concrete. Settling and freeze-thaw cycles open small gaps along the sill plate. From a mouse’s perspective these are highways.
5. Roof line — eaves, fascia gaps, and where roof planes meet. Mice climb. A gap under a fascia board or at the intersection of two roof planes is a viable entry point into an attic.
6. Doors and windows in older structures. Loose-fitting frames, missing weatherstripping, gaps under thresholds. Wood doors that have warped over a few winters often have visible daylight at the corners.
7. Crawl-space access hatches and basement bulkheads. Often closed but not sealed. The hatch closes; the gap around it does not.

The pattern: the older the structure, the more entry points. New construction has fewer gaps; deferred maintenance creates them.

Materials: what actually stops a mouse

Mice chew. That single fact rules out most of what’s intuitively appealing.

What works:

• Stainless steel mesh (the kind sold for rodent exclusion, sometimes branded Stuf-Fit). Stuffed into gaps around pipes and utility penetrations, mice cannot chew through it. This is the workhorse material.
• Galvanized hardware cloth (1/4-inch). For larger openings — vents, foundation gaps, openings around louvers. Cut to size, fastened with screws and washers (not staples, which mice can pull out).
• Concrete or mortar. For permanent repairs to foundation cracks and masonry gaps. Pricier in labor, but it’s not coming undone.
• Rodent-block expanding foam. Foam alone is chewable; the rodent-block formulas combine foam with embedded steel fibers and are appropriate as a backer behind mesh, not a replacement for it.
• Heavy-duty door sweeps. Replacing a worn garage or shed door sweep takes ten minutes and closes one of the most common entry points in any home.
• Sealed food-storage containers. Not a structural seal, but the CDC explicitly names “address attractants” as part of seal-up. Airtight bins for pantry staples and pet food remove the food source rodents are coming for.

What doesn’t work:

• Steel wool alone. It rusts in damp locations, and once it rusts, it crumbles. Mice can also pull loose tufts free. Use stainless mesh instead.
• Standard expanding foam. Mice chew through unmodified polyurethane foam in days. It looks like a seal; it isn’t one.
• Cloth, paper, or fiberglass insulation stuffed into gaps. Nesting material, basically.
• Sonic / ultrasonic deterrents. Research is mixed at best, and the FTC has taken action against several manufacturers for unsupported claims. Treat as decorative.
• Peppermint oil, mothballs, dryer sheets, etc. Folk remedies. No serious evidence they exclude mice from a structure with viable entry points.

Room-by-room inspection

Plan a single inspection pass with good light. A bright LED flashlight pays for itself the first time you spot a gap you’d been walking past for years.

Basement / mechanical room. Start where utilities enter the house. Water service, gas line, electrical conduit, sewer, HVAC penetrations. Look at the seal around each one from inside. If you can see daylight, see flexible old caulk, or see a gap big enough to fit a pencil into, the seal is gone. Also check the sill plate where the framing sits on the foundation.

Crawl space. Same checks, plus the access hatch itself. Vents on the crawl-space perimeter should have intact 1/4-inch hardware cloth screens.

Garage. The bottom seal on the main door — both ends and the middle. Side doors. Where the garage meets the house (the shared wall is a common gap). The ceiling-to-attic connection if there is one.

Kitchen and laundry. Behind appliances. Behind and under the refrigerator. The dryer vent on the exterior wall (and the corresponding penetration inside). Sink cabinets where water lines and drains pass through the cabinet floor or back.

Attic. Gable vents and soffit vents — screens intact and sized to 1/4-inch. Where the roof meets the eaves. Where two roof planes meet. Look for daylight from inside the attic.

Around the perimeter, outside. Walk the foundation. Look for gaps where siding meets foundation, where the chimney passes through the roof, where the dryer vent and bathroom vents exit. Check the seal around the AC line set entering the house.

This is not a one-day project for most homes. It’s a planned afternoon, a list, and a second trip to the hardware store after you find the things you didn’t expect.

Common mouse-proofing mistakes

• Sealing visible entry points but not utility penetrations. Most rodent entry happens at the boring places — where pipes and wires come into the house — not at dramatic obvious holes.
• Using the wrong mesh size. Standard window screen lets mice through. Use 1/4-inch hardware cloth or stainless rodent mesh.
• Trapping before sealing. Trapping reduces the population that’s already inside; if entry points remain open, you’ve signed up for indefinite trapping. CDC guidance pairs the two for a reason.
• Skipping the garage. It’s a high-traffic exposure zone and a frequent entry point. The attached-garage door to the house is often the only thing between rodents and the kitchen.
• Treating it as one-and-done. Seals degrade. Plan to walk the perimeter once a year and re-inspect the obvious failure points.

When DIY isn’t enough

For larger infestations, persistent re-entry after sealing, or structural problems (deteriorated siding, severe foundation cracks, attic infestations with extensive nesting material), professional pest control or a contractor is the right call. CDC guidance does not assume every homeowner is the right person to solve every problem. Knowing where DIY stops is part of doing it well.

After you seal: handle existing droppings safely

If rodents have already been inside, sealing is only half the job. The droppings and nesting material they left behind still need to be cleaned without creating dust — that’s the half of the CDC framework that’s actually safety-critical. Do not sweep or vacuum dry rodent waste with a standard household vacuum.

For the dust-aware cleaning method, see Rodent droppings cleanup. For closed-up cabins, sheds, and stored RVs, see Seasonal cabins, storage, and RVs.

Related guides

• Hantavirus Prevention: Seal Up, Trap Up, Clean Up — the full three-step CDC framework this page is the deep-dive on.
• Rodent droppings cleanup — the safe, dust-aware cleanup walkthrough for existing waste.
• Seasonal cabins, storage, and RVs — opening-day checklist for closed-up structures.
• Transmission — the exposure routes sealing is designed to break.
• Hantavirus in the United States — where U.S. risk concentrates and which rodents matter.
• FAQ — short answers to common cleanup and exposure questions.