Start at the rim joist
The rim joist is the band of wood sitting on top of the foundation. In most older homes it is bare, and outdoor air leaks straight past it into the basement and up through the floor.
Then the foundation walls
Uninsulated concrete or stone walls keep the basement cold, and the floor above sits on top of that cold space.
The fix
Spray foam or rigid foam on the rim joist, insulation on the foundation walls, and a sealed crawlspace if part of the house sits over one.
We can help: Basement sealing & encapsulation — Rim joist, foundation walls, dirt-floor areas and humidity — a dry, warmer basement.