How can I keep pests from entering through chimneys in winter?

An uncapped chimney reads to wildlife the way a hollow tree does — dark, sheltered, vertical, and offering a warm updraft from the house below. Raccoons climb flues specifically to den, often in a chimney attached to an active fireplace; birds, notably chimney swifts, nest inside flues as a matter of course; and squirrels and bats will use an open flue opportunistically. None of this is unique to winter — it happens year-round — but winter raises the stakes, because an animal that gets down the flue and can't get back out is now trapped inside a wall of your house.

A correctly sized chimney cap with mesh sides — not a solid cap, which would block the chimney's function entirely — excludes animals while letting smoke and gases vent normally. This is the standard, extension-endorsed answer, and it's durable once installed correctly.

The part worth being careful about: some guidance suggests covering an open flue with ordinary hardware cloth as a stopgap. Wildlife-damage authorities warn against this specifically, because hardware cloth over a flue opening can rust, ice over in a New Hampshire winter, and block the venting of carbon monoxide and other combustion byproducts — a genuine safety hazard, not a hypothetical one, particularly on gas-fired appliances that vent through the same flue. A purpose-built cap, sized and installed correctly, avoids this entirely. This is not a place to improvise with materials meant for a foundation vent.