How can I add more insulation to my finished attic walls without trapping moisture
I have done what you asked when I was an insulator for a few years. Unfaced batts were added horizontally. After each run was set and bedded well over the lower insulation, wires were placed at the top of the horizontal run, using screw eyes,    there is a tool that zips these right in that you use on your drill, or staples. Then the next horizontal run was added and wire were added above that run as well. These wires were installed to hold &quot;chicken wire&quot;, a light weight, large spaced mesh to keep all the insulation in place. This kept the insulation tight to its surroundings. Using R-19 will help you keep it in place better than R-11 or R-13.