It serves as the end cap for floor joists and sits upon the sill plate atop the foundation wall. If you look at the perimeter of your crawl space or the top of your basement wall, you'll see a ...

Construct the interior walls of common 2-by-4 lumber set at 16 inches on center and make sure the bottom (sill) plate is pressure-treated lumber to resist decay and infestation.

Don't count on cheap duct tape to stand the test of time. You also want to lap the vapor barrier up the foundation walls and attach it to the wood sill plate that the floor joists rest on.