How do mice find their way into homes in cold weather?

The single most useful fact in this whole question is the size threshold: a house mouse can pass through any opening its skull fits through, which in practice means roughly a quarter inch, about the width of a standard pencil. That's smaller than most people picture when they imagine a mouse-sized gap, and it's exactly why homeowners are often surprised that a house they consider well-sealed still has an active mouse problem.

This isn't a soft guideline — it reflects a mouse's actual skeletal structure, since the skull is rigid while the rest of the body can compress. CDC frames the same fact in consumer terms: seal any hole the width of a pencil or larger. That's the number to actually measure against, not a general sense of whether a gap "looks small."

Mice enter structures low, near ground level, because that's where the relevant openings are: foundation cracks, the seam where a sill plate meets the foundation, gaps around utility penetrations (water, gas, electrical, cable lines passing through an exterior wall), weep holes in brick veneer, and worn or gapped door sweeps. These are also, not coincidentally, the same spots where warm interior air is most likely to leak outward in a New Hampshire winter — utility penetrations and sill-plate seams are common points of air leakage in typical home construction.

That overlap matters mechanically: a mouse exploring a foundation perimeter in cold weather is more likely to investigate a gap where it can detect warm air moving outward, because that air current also carries food and structure scent from inside. The gap isn't chosen at random — it's found because it's already leaking exactly the cues a cold, hungry mouse is oriented to follow.

Finding the right gaps is only half the job; sealing them with the wrong material undoes the work. Mice are capable of gnawing through foam sealant and caulk, both of which are soft enough that a determined mouse simply reopens the gap over time. Effective exclusion specifically requires gnaw-resistant materials — quarter-inch hardware cloth, sheet metal flashing, or mortar/masonry patching — because these are materials rodents genuinely cannot chew through in a reasonable amount of effort.

This is homeowner-doable work: a foundation walk-around with a flashlight, checking every utility penetration and sill-plate seam, and closing anything a pencil could pass through using metal or masonry rather than foam alone. It's the same exclusion principle covered elsewhere in this hub for rats, just applied at the smaller quarter-inch scale that mice specifically require.