Although the emphasis on saving home heating and cooling energy has been on attic insulation, exterior walls have a similar surface area that transmits energy.
Homes built prior to 1960 had no insulation or at best R-5. Between 1960 and 1980’s average R values increased to about R-11.
Insulating wall cavities that were empty or under-insulated will save energy. A 2″ x 4″ stud framed wall filled with expanding foam insulation will provide an insulating value of R-22.
Before insulating side walls you need to find out what is in there. Visual inspection through openings may reveal what is already in place, if anything, or an infrared thermal imaging will reveal heat loss in areas that are hidden or have varying insulating value.