Great, I didn't know this plugin ! [=D>]Actually, there is an easy way: Plugin #5 ComputeWallsSurface will calculate for you (albeit with some caveats mentioned on plugin download page and discussions such as thread 7516: Calculating the surface of the wall).
If you take into account that it measures the inside of the wall, it should do pretty much what you want if you install the plugin, restart sh3d, select the outer walls and run the plugin from the tools menu.
Good luck with your insulation project.
ok
But there is a problem, this plugin calculates surface from the middle of the walls. [:(]
Platax asks us an outside walls surface... it's not the same ! [:p]
So, the lonely solution is manual calculations... [:((]
See below the demo (height of the walls = 1.0 m) :
See lines and distance with the calulated surface...