Insulation, Radiant and Vapor Barriers - Old House Stud Spacing

I'm in the process of insulating a second floor living space in an old home (circ. 1921). The wall studs are not on typical 16" centers. Instead they vary with most running wide leaving a gap of about 16-17" between studs. Since R13 insulation is sold in 15" widths I'm not quite sure how to fill these spaces properly.

I've considered adding an extra stud to one side of the space to fill the gap but this seems like a lot of work as well as being wasteful. I've also considered slicing a narrow width of insulation to fill in the gap but I'm concerned about getting a good vapor barrier.

Any suggestions would be greatly appreciated!!

They're always different ways to do things.
In this instance, I would make sure that old or new studs hit 4' centers for drywall staggered end purposes, and clearly mark the others at the top and bottom of the wall for drywall "field" purposes for the screws.
I would cut unfaced insulation to fit between the studs, whatever the width, and then hang plastic sheeting as the vapor barrier.
Seems to me that this would be faster and less expensive in the long run.
Just one way.
Good Luck!

I think it would work out better for you if you got what we call friction fit insulation it comes inR 13 cut it to fit in the studs no paper on it. when done put a 2 mil poly over it on all the walls and ceiling and dry wall it;) ED