My double-wall assembly, with an outer and an inner 2x4 wall separated by a 3 ½-inch space. Staggering butt joints from course to course, I placed the first and third layers vertically and the middle layer horizontally.
There are eight 3 ½-inch batts to a bag, which covers 60 square feet, and because each bag of Roxul was wrapped in a heavy plastic film, I could leave it outside without worrying about rain or snow damage.
A serrated bread knife works well, but what worked beautifully for me was a $15 electric carving knife.
This knife also allowed me to shave off narrow strips easily. This was helpful for a narrow bay or when my first cuts proved to be too long.
I created a cutting table by stretching a couple of plywood rips over sawhorses and simply ran the electric knife down the middle. For narrow pieces, I hung one edge of the batt beyond the plywood, and carved off what I needed.
Squeezing any fibrous insulation behind electrical boxes is not a good idea, so we cut around the boxes. For wires, we scored the Roxul and inserted the wires into the grooves. For plumbing pipes, we made a V-groove to accommodate each one ...
... always keeping the pipes to the warm side—the inner wythe of my double-stud wall framing served as a utility wall.