Insulating a basement bathroom
Consider if you need ceiling insulation at all.  In a bathroom there's lot of extra moisture, and you don't want some to enter the space between floor and the fiberglass moisture barrier, and have trouble drying out.

Floor insulation helps with sound transmission a bit (though an STC rated ceiling is better).  But in your case it might have more downsides than upsides.

This IS a good chance to insulated the joist bays from the outside, usually a weak place in standard home construction.  Post photos.  Research and remember some golden rules of insulation: seal air leaks first, never trap moisture between two vapor barriers, use the right insulation for your climate zone.  Get a local expert, as climate matters a lot.