I am thinking of adding insulation to our attached garage wall, to
increase the insulation between the living space. Is adding Dow
styrofoam board or something similar a good idea, or worth the cost?
(It's more flammable than the 5/8 fireshield)

For example, the Dow 1" stryofoam board would add R5 to my wall, which
is only 3.5" with R-11 fiberglass.

Thanks guys!