How do I keep mice and rats out of my house?

Seal every exterior gap 1/4 inch or larger — sill plates, foundation cracks, utility penetrations, and the base of exterior and garage doors — with 1/4-inch galvanized hardware cloth, copper/stainless mesh, or sheet metal, not foam alone. Pair that with sanitation (sealed food storage, no accessible pet food or garbage) and an 18-inch clear perimeter along interior walls so rodents have nowhere to hide once they're locked out.

How small a hole can a mouse actually fit through?

About 1/4 inch — roughly a pencil's width or a dime. A mouse's skull, not its body, is the limiting factor, and mice can compress it surprisingly well. Rats need a slightly bigger opening, about 1/2 inch, or a quarter-sized hole. Any gap that size or larger around your foundation, utility lines, or door thresholds is a viable entry point and should be treated as one.

When should I seal my house against rodents in New Hampshire?

Late summer through early fall, finished before mid-October. That's ahead of Manchester's first hard frost window (about October 19 at 50% probability, October 29 at 80%, per NOAA) and Concord's earlier frost, when falling temperatures reliably push mice and rats to seek indoor shelter. Sealing after the cold snap just traps whatever already got in — check for existing activity and trap it out first.

Does sealing gaps actually prevent an infestation, or is it just a myth?

It's genuinely effective and considered the foundation of rodent control by the CDC and wildlife-damage extension programs — but only if paired with removing any rodents already present. Sealing an occupied structure doesn't evict anyone; it locks them in with your food supply. Exclude first (or trap concurrently), then finish the seal once you've confirmed activity has stopped.

Can I just use expanding spray foam to seal gaps?

Not by itself. Expanding foam is easy to apply but rodents chew straight through it, so the CDC specifically warns against relying on it alone. Use it as a filler behind a metal barrier — 1/4-inch hardware cloth, copper or stainless steel mesh, or sheet metal — which is the part that actually stops teeth. Steel wool packed tightly and caulked over works well for smaller gaps.

What attracts rodents to a home in the first place?

Accessible food, water, and clutter that offers shelter. That includes exposed garbage, pet food left out overnight, birdseed, spilled grain, leaky faucets or damp basements, and woodpiles or dense vegetation stacked against the foundation. Removing these — sealed containers, lidded cans, an 18-inch clear zone along walls, and woodpiles moved away from the house — removes the draw even before you finish sealing entry points.

Why keep an 18-inch clear perimeter along interior walls?

Boxes, furniture, and stored items pushed against a wall create instant harborage and hide the droppings and gnaw marks that would otherwise tip you off early. San Diego County's rodent-exclusion guidance recommends an 18-inch gap between stored goods and walls specifically so rodent activity stays visible and rodents have fewer places to nest undisturbed once they're inside a garage or basement.

What mesh size actually keeps rodents out?

1/4-inch galvanized hardware cloth excludes both mice and rats and is the standard recommended by wildlife-damage management extension programs for vents, larger openings, and screened gaps. For tight cracks and crevices, tightly packed steel wool or copper/stainless mesh products (sold as "Stuff-Fit" or similar) work into irregular spaces that sheet mesh can't cover.

Are rodents actually a health risk, or just a nuisance?

A real health risk, not just a nuisance. The CDC links rodents to more than 35 diseases, transmitted directly through bites, droppings, urine, and saliva, and indirectly through the ticks, mites, and fleas that ride on them. Deer mice in particular carry hantavirus, which is why cleanup after any rodent activity should use wet-cleaning methods and protective gear rather than sweeping or vacuuming.

I sealed my house — do I still need traps?

Yes, if there's any chance rodents were already inside. Sealing without first removing an existing population traps rodents indoors rather than keeping them out, which is worse. Set snap traps along walls where you see droppings before you finish the final seal, confirm you're getting zero catches for a week or more, then complete the exclusion and keep a couple of monitor traps in known runways.

PreventionRodents

Rodent Prevention: Seal, Sanitize, and Time It Right in NH

TL;DR

Exclusion is the single most durable rodent control there is: seal every gap 1/4 inch or larger with metal, not foam, and you stop most infestations before they start. Rated HIGH-VALUE — but it only works if you trap any rodents already inside first, and in New Hampshire the sealing work has to be finished before the fall cold-weather push, roughly mid-October.

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NH License #782664Family-owned since 2017Updated Jul 2026

Mouse entry gap
1/4 inch — about the width of a pencil, or a dime
CDC Healthy Pets & Healthy People, rodent seal-up guidance.
Rat entry gap
1/2 inch — about the size of a quarter
CDC seal-up guidance.
Barrier that actually works
1/4-inch galvanized hardware cloth, copper/stainless mesh, or sheet metal
Expanding foam alone will not stop rodents — they chew through it. CDC.
Clear-perimeter rule
Keep an 18-inch gap between stored items and walls
Removes harborage and hides fewer droppings. San Diego County DEH.

The honest verdict

Does this actually work?

HIGH-VALUE and the most durable rodent control available — but honestly slow to execute right, and worthless (even counterproductive) if you seal in an existing population instead of removing it first.

Works — High

Evidence: The CDC and wildlife-damage extension programs both frame exclusion as the foundation of rodent control, not a supplement to trapping or baiting. The physics are well established: mice need only a 1/4-inch gap, rats a 1/2-inch one, and expanding foam alone fails because rodents chew through it — metal barriers are required.

Works when

Every gap 1/4 inch or larger is sealed with hardware cloth, copper/stainless mesh, steel wool, or sheet metal — not foam alone
Sanitation (sealed food, secured garbage, no exposed pet food) removes the attractant before rodents commit to a site
Sealing is finished before mid-October, ahead of NH's fall cold-weather push
Any existing rodent population is trapped out before final sealing, not after

Fails when

Gaps are sealed with expanding foam alone, which rodents gnaw through within days
The garage door gap or another obvious entry point is overlooked because it "doesn't look that big"
Sealing happens after rodents are already inside, trapping them with your food supply instead of keeping them out
Sanitation is skipped — birdseed, pet food, and garbage stay accessible even after exclusion work is done

At a glance

Why sealing beats reacting — and how to time it in New Hampshire

How it works

Moderate DIY

Prevention removes the three things a rodent population needs to establish: a way in, food, and shelter. Physical exclusion — sealing every gap 1/4 inch or larger with metal, not foam — is the primary lever because it stops the problem before it starts. Sanitation and moisture control remove the draw for any rodent still scouting the perimeter, and NH-seasonal timing makes sure the sealing work is finished before the fall cold-weather push sends mice and rats looking for a way indoors.

Vital stats

Time: A weekend for the initial inspection and sealing pass, plus ongoing sanitation habits and a follow-up check in two weeks
Best for: Every NH home, but especially older houses with fieldstone or rubble foundations, unsealed sill plates, and detached garages or sheds near woodpiles or dense vegetation.
Cost: Low to moderate — hardware cloth, steel wool, caulk, and door sweeps are inexpensive; rooflines, crawlspaces, or extensive foundation repair can push into professional-job territory

Every rodent-control method on this hub — snap traps, baits, exclusion hardware — is a response to mice or rats that already got inside. Prevention is the only lever that stops them from getting in at all, and it's genuinely the cheapest, most durable one available. The physics are simple and well documented: a mouse can flatten its skull and squeeze through a gap the width of a pencil (about 1/4 inch); a rat needs a hole about the size of a quarter. Anything that size or larger on your home's exterior is an open door.

The catch is that exclusion only works as a first-and-last step, not a one-time fix. Seal a structure that still has mice inside and you've just built them a much better home. This page walks through the four levers — exclude, sanitize, fix moisture, and time it to New Hampshire's fall calendar — in the order that actually prevents an infestation, plus the checklist to run every year before the cold sets in.

Step by step

How to do it

Inspect for gaps 1/4 inch or larger
Walk the full exterior and check sill plates, the foundation, door thresholds, utility line penetrations, and the garage door for any opening a pencil could fit into. Check the interior too — basements, crawlspaces, and behind appliances.
Tip: If you can see daylight under an exterior or garage door, that's a rodent-sized gap — a reinforced door sweep fixes it.
Trap any rodents already inside before you seal
Set snap traps along walls wherever you see droppings, gnaw marks, or grease rubs, and keep trapping until you get several consecutive nights with zero catches.
Caution: Sealing a structure with rodents still inside just traps them with your food supply — always confirm activity has stopped first, or trap concurrently with sealing.
Seal small gaps with steel wool or metal mesh, then caulk
Pack copper or stainless mesh, or steel wool, tightly into cracks and crevices, then seal over it with exterior caulk. This combination resists gnawing in a way foam alone cannot.
Caution: Don't rely on expanding foam by itself — the CDC specifically flags it as ineffective against a determined rodent.
Screen larger openings and vents with 1/4-inch hardware cloth
Cover vents, larger foundation openings, and gaps around utility lines with galvanized 1/4-inch hardware cloth, and use sheet metal or flashing on any edge already showing gnaw marks.
Tip: 1/4-inch mesh excludes mice and rats both — no need for a finer or coarser grade.
Install reinforced door sweeps and fix the garage-door gap
Add or replace door sweeps on all exterior and garage doors so there's no visible gap at the threshold. This is one of the most commonly missed entry points.
Tip: Check thresholds again after the first hard freeze — cold-weather settling can reopen a gap that looked sealed in September.
Remove food access and secure garbage
Store pantry food, pet food, and birdseed in thick plastic, metal, or glass containers with tight lids. Keep garbage in lidded cans and don't leave pet bowls out overnight.
Tip: A mouse survives on just a few grams of food a day — even minor spills and crumbs sustain a population.
Keep an 18-inch clear perimeter and manage moisture and woodpiles
Move stored items and boxes 18 inches off interior walls, elevate woodpiles and compost away from the house, and fix any leaks or standing water that give rodents a water source.
Tip: This perimeter also makes any new droppings or gnaw marks easy to spot early, before a population establishes.

What you need

Tools & materials

1/4-inch galvanized hardware cloth
Screen vents and larger foundation openings against mice and rats
The standard mesh size for both species
Copper or stainless steel mesh / steel wool
Pack into small gaps and crevices as a chew-resistant barrier
Sold as pre-formed products (e.g. "Stuff-Fit") or plain steel wool
Exterior-grade caulk
Seal over packed mesh or steel wool to lock the barrier in place
Not a standalone barrier — pair with metal
Sheet metal or flashing
Cover edges already showing gnaw marks
Rodents can't chew through metal the way they chew wood or foam
Reinforced door sweeps
Close the gap under exterior and garage doors
Check for visible daylight under the door as a quick test
Snap traps
Remove any existing rodent population before final sealing
Confirm several nights with zero catches before sealing the last gaps
Metal or thick-plastic/glass storage containers
Deny access to pantry food, pet food, and birdseed
Tight-fitting lids are essential

Stay safe

Safety first

General
Caution
If a gap you're sealing shows existing droppings or gnaw marks, treat it as an active entry point — trap first, confirm the activity has stopped, then seal. Sealing over an active population is a common and costly mistake.
General
High risk
Wear gloves, an N95 respirator, and avoid sweeping or vacuuming when cleaning up droppings or nesting material near a gap you're about to seal — deer mice can carry hantavirus, and dry cleanup can aerosolize it.
Applicator
Caution
Rooflines, chimneys, and crawlspace work involve real fall and confined-space risk — use a stable ladder and stop if a job needs more than basic hand tools.
Children
Low risk
Store and place any snap traps used during the trap-before-you-seal step out of children's reach; a triggered trap can injure small fingers.

Avoid these

Common mistakes

Why this method fails in practice.

Sealing exterior gaps with expanding foam alone, which rodents chew through within days
Sealing before confirming any existing rodent population is gone, trapping mice or rats inside
Overlooking the garage door as an entry point because it doesn't look like a typical "gap"
Finishing exclusion work after mid-October, once the fall cold-weather push has already begun
Skipping sanitation — leaving birdseed, pet food, or garbage accessible even after sealing

New Hampshire context

Why prevention timing matters more in New Hampshire

New Hampshire's older housing stock — fieldstone and rubble foundations, settled sill plates, deteriorating mortar — is riddled with 1/4-inch-plus gaps that widen every winter with the freeze-thaw cycle. Rodent pressure surges hard as outdoor temperatures drop, because mice and rats are looking for the same thing you are: shelter from the cold. That surge lines up almost exactly with the region's first hard frost, so the practical rule is to have exclusion work done before mid-October — comfortably ahead of Manchester's median frost date and Concord's, which arrives even earlier.

Peak season

Late summer through early fall, before the first hard frost

First-frost anchor: Manchester first hard frost ~Oct 19 (50% probability) / Oct 29 (80%) per NOAA 1991-2020; Concord ~Oct 3

Exclusion, sanitation, and sealing are DIY-legal statewide with no pesticide license required — this is non-chemical work under RSA 430/NHDAMF.

UNH Cooperative Extension backs the CDC's exclusion-first approach: keep mice out, then trap what's already inside — chemical control is a last resort indoors.

This method targets

Pests this applies to

Eastern Gray Squirrel

Field Mouse

When to call a pro

Rodent Control with Anchor

Exclusion done well on your own resolves most rodent-prevention needs, but rooflines, extensive foundation gaps, or a property with an active infestation already in the walls often needs more reach and equipment than a DIY weekend can cover. Anchor's licensed technicians combine full-structure inspection, exclusion, and monitored trapping so the seal actually holds.

Call us when…

You're finding new droppings or gnaw marks after completing a full sealing pass
The entry points are on the roofline, chimney, or another spot that's unsafe to access yourself
You suspect rodents are already established inside wall voids or attic spaces
You've confirmed rats, not mice — larger droppings and 1/2-inch-plus gaps mean a different scale of exclusion work
A vulnerable household (infants, elderly, immunocompromised residents) needs the hantavirus and sanitation risk handled professionally

Rodent Control

free inspection; same-day service across southern and central New Hampshire.

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Keep them out

Have exclusion and sealing finished before mid-October, ahead of the fall cold-weather push
Keep an 18-inch clear perimeter along interior walls in garages and basements
Store pantry food, pet food, and birdseed in sealed metal, glass, or thick-plastic containers year-round

Common questions

Frequently asked

Sources & further reading(6)

[1]
CDC — Healthy Pets, Healthy People: Seal Up (Rodent Control)
1/4-inch mouse gap, rat gap size, steel wool + caulk method, foam-alone ineffectiveness
govverified 2026-07-13
[2]
CDC — Rodent Control overview
Rodents linked to 35+ diseases, direct and indirect transmission routes
govverified 2026-07-13
[3]
Internet Center for Wildlife Damage Management — House Mouse Damage Prevention and Control Methods
1/4-inch hardware cloth standard, sill plate/foundation/utility inspection points, exclusion as the foundation of control
extensionverified 2026-07-13
[4]
San Diego County Department of Environmental Health — Rodent Exclusion
18-inch clear perimeter, woodpile/stored-item spacing, sanitation attractants
govverified 2026-07-13
[5]
garden.org — Frost Dates: Manchester, New Hampshire
Manchester first hard frost probability dates (NOAA 1991-2020 Normals)
govverified 2026-07-13
[6]
UNH Cooperative Extension — How can I get rid of mice in my house?
Exclusion-first NH guidance; seal exterior gaps and utility penetrations before trapping/chemical control
extensionverified 2026-07-13

Written & reviewed by

Verified

Daniel Brady

NH-Licensed Pesticide Applicator · License #782664 (NHDAMF, RSA 430)

Daniel Brady is the New Hampshire–licensed applicator behind Anchor Pest Services, a family-owned pest control company serving the Manchester area since 2017.

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Anchor Pest Services

When rodents won't quit, we will.

Same-day service across Southern New Hampshire. NH-licensed #782664. Family-owned since 2017. We diagnose the pest, then treat it the right way — with a satisfaction guarantee.

Get a free inspection Call (603) 785-0118

Keep Exploring

Rodent Prevention: Seal, Sanitize, and Time It Right in NH

Works when

Fails when

Why sealing beats reacting — and how to time it in New Hampshire

How it works

How to do it

Inspect for gaps 1/4 inch or larger

Trap any rodents already inside before you seal

Seal small gaps with steel wool or metal mesh, then caulk

Screen larger openings and vents with 1/4-inch hardware cloth

Install reinforced door sweeps and fix the garage-door gap

Remove food access and secure garbage

Keep an 18-inch clear perimeter and manage moisture and woodpiles

Tools & materials

1/4-inch galvanized hardware cloth

Copper or stainless steel mesh / steel wool

Exterior-grade caulk

Sheet metal or flashing

Reinforced door sweeps

Snap traps

Metal or thick-plastic/glass storage containers