The video says not to use a vapor barrier. But why can't I first use the plastic strips that fit between the joists up against the floor and then put up the Roxul batts. The vapor barrier is then on the warm side of the insulation.
what flooring is above where you propose to install the vb? it better be very vapor open, or you are asking for mold and rot. what is the reason you want the vb, anyway? why not air seal and apply insulation as per buildingscience.com? you will have a vb on the ground, right?