Why is it colder inside the cottage than outside?

Why is it colder inside the cottage than outside?

Why does it feel colder inside the cottage than outside when I first get there in the winter?
–Andrew Kiss, Halifax, N.S.

“There’s no mysterious physics going on,” says Patrick Tevlin of the Ontario Science Centre. “A cottage has what’s called ‘thermal inertia,’ which means once it cools off it takes a while to heat up and once it heats up it takes a while to cool off.”

Overnight, the temperature inside your unheated cottage will drop to meet the outside temperature. During the day, as the sun heats the exterior of the building, conduction will slowly radiate warmth inside. Cottagers who arrive early in the afternoon are walking into a cottage that hasn’t had time to warm up.