Going to need more info.
Let me guess, really old house.
Ballon wall constrution.
No idea on what, or if there's any insulation in in the walls.
Never been fire blocked in the basment between the walls or in the attic.
Wood siding attached directly to the studs with no vaper barrer or sheathing.
How much gap is there under the closit door?
What type roof venting do you have?