This is entirely un-exciting, but a simple fact of building a house – under your basement slab the Ontario Building Code requires the installation of a vapour barrier.

Typically, and in our case, this is 6mil plastic sheeting.  It comes in rolls of 20’x100′ and is a bit unwieldy to maneuver, but once cut to size (with a generous overlap for the seams),  it lays down quick: