Why do spiders show up in attics and basements during winter?

Not every spider found indoors got there the same way. Some species truly are outdoor spiders that occasionally wander in seeking shelter as temperatures drop. But the spiders homeowners most commonly report finding in attics and basements — the American house spider and the long-bodied cellar spider, among others — are indoor-established species. They complete their life cycle indoors, are active in heated structures year-round, and don't disappear or hibernate the way an outdoor insect might.

For those species, "why do they show up in winter" is slightly the wrong question. They didn't show up — they were there in September too. What changes in winter is how often you go into the spaces where they concentrate.

Spiders gather where two things overlap: a quiet, undisturbed environment and a reliable food supply. Attics and basements offer both. They're low-traffic year-round, they hold clutter and stored items that create structure for webs, and they typically host the small flies, moths, and other insects spiders feed on — some of which are themselves winter overwinterers, which means spider prey is often concentrated in exactly these spaces during the cold months too.

That overlap of quiet and prey is the actual draw, not temperature. A basement isn't warmer than the rest of the house in any way that would specifically attract a spider, and an attic in a New Hampshire winter is often colder than the living space below it — so "seeking warmth" doesn't hold up as the mechanism for most of what people find there.

The honest driver of "more spiders in winter" is often behavioral, on the homeowner's side rather than the spider's. Winter is when people go into the attic for holiday decorations, check on stored items, or head into the basement more often to deal with the furnace or a leak — all activities that disturb spaces spiders have quietly occupied undisturbed for months. Digging through a box that's sat untouched since spring is exactly when you find the web and the spider that built it.

That reframes the fix: this isn't a seasonal invasion to seal against the way you would rodents or overwintering insects arriving from outside. It's about reducing what makes an indoor space attractive to a resident population — less clutter, less insect prey, fewer undisturbed corners — which you can do at any time of year, not just before a frost date.