According to flipper :
in my garage conversion, i'm laying insulation on the unused attic
floor; ie, right on the flat plywood flooring, not in b/ joists. i
can't get insulation blanket around here in r30, so i'm limited to 15"
and 24" width batts x 48" long. the cost of 15" suits my total-sq-foot
needs better, so i'd like to get it; but will more seam connections
with 15" result in less insulating power?

If you're doing unfaced batts, the seams are irrelevant if you've
installed it properly.

also: around here, neither lowes nor HD stock r13 in the 24" width.
they don't stock it and say they can't get it. i thought 24" was as
standard a width as 15". Is that not so?

R13 is for 2x4 stud walls, and R20 is for 2x6 walls. Building codes
generally do not permit the use of 2x4s on 24" centers, hence, 24"
R13 is a pretty rare size.

You need to base your insulation selection on desired R-value, not
batt width. With unfaced batts, the determining factor in cost
per square foot is R value (thickness) not batt width.

There's probably a reason that insulation manufacturers don't suggest
using building wrap on top of ceiling insulation - it's a bad idea.
Dust in the insulation will not make much of a difference, and you
want to minimize as much as possible any blockage of moisture transfer
_out_ of the insulation.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.