in my garage conversion, the studs in the two inner walls -- i.e. walls
between the outside two bays of the garage -- are about 24" apart. i
was planning on using r13 insulation batts but come to find out that in
my area literally no one has r13 in that size, nor can they get it.
it's 15" or nothing for the 2x4s. so:

how much insulating power am i giving up by going to r11, which i can
get in the right width? remember: these are inner walls; but the space
on either side is unheated.

or maybe i could get the 15" stuff and use 1 1/2 batts per opening?

or i could add more studs.

what do you think?