The front wall of my house has 6' of extensions ( think buttresses) of which I used one for my front shop wall. I opened it up last night to find it full of squirrels nests. The original builder had offset the studs so that he could put the Cedar siding directly over those studs without building board. One or two knotholes were all they needed to get in.
The sill plates were covered in 6" of droppings, leaves and insulation. Most of the former insulation was just hanging there, completely trashed.
After clearing that I stood in the shower for an hour to get it off my skin and out of my lungs.
Today I begin restructuring with 2X4's and 2X6's so I can install a hexagon window that opens. I will have to put something behind the siding to block further pioneers from repeating the damage. Not sure how without removing the siding outside. I don't want too.
I'm thinking a metal mesh just stapled between the existing studs will do it.