I'm fixing up my workshop in the basement. The floor is a concrete
slab, and I plan to cover it with vinyl/ rubber tiles. Moisture is not
a problem.

There is a gap between the slab and the cinder block walls. This gap
is only about 1/8" wide at two of the walls, but along the long wall
the gap widens in a few places to about 1". Along the edge of the slab


Whatever you put in should be squishable. Tarred felt is usual,
isn't it?