How can I insulate a finished 1 1/2 story space
You can do it outside, if you're planning any siding work. But, inside you could cut out (so you can reinstall it) a 1-foot or more slit across each wall &amp; literally shove faced fiberglass batt insulation up to the top &amp; down to the bottom.

This is tedious, but once you get a feel for it &amp; in using a push stick, then it's not so bad. You can also do this with Rigid Foam Board insulation that's cut snug to the stud spacing. This is a little cleaner &amp; easier but you'll need to put in 2 layers of 1.5" thick or 2" &amp; 1" thick layers. You need that 1/2" dead air space for maximum insulation value with foam.