Basements are typically concrete perimeter walls and wood rim joist at the top. They present 2 thermal challenges for the home.
Basements being the lowest position in the home makes them vulnerable to stack, or chimney effect. Air leaving the homes upper floor into the attic pulls in air at the bottom Rim joist of the basement. The wood boards that hold your floor joist together are called rim joist, these typically have 1 foot square insulation squares in them showing evidence of the stack effect with dirt marks on the backside of the fiberglass insulation.