Now that the mouse nests have been removed, and wall cavities bleached and cleaned up, we are installing styrofoam insulation panels. They're in big sheets, 2" thick. My husband has a system for measuring & cutting the pieces. We explained to our kids that it's like Tetrus, and they didn't know what we were talking about.
It is satisfying though, when the parts fit together, especially in these irregularly shaped spaces in our ancient walls. We put two pieces in each space, so it's 4" of insulation. This is followed by spray foam in the wee gaps and corners that remain. We finished the gable end wall, on the front of the house. We've lots more to do!