A rotary mill is configured to cut fibrous insulation material into tufts of loosefil insulation. A housing of the mill has a rotary cutter assembly mounted therein having a plurality of radially outwardly extending vanes extending to the housing and being configured to sweep along the housing in a machine direction. A screen having a plurality of screen openings is positioned in the housing, each screen opening having an effective opening size. The screen comprises an inner plate having a plurality of inner openings formed therethrough and an outer plate having a plurality of outer openings formed therethrough. The inner openings correspond with the outer openings to define the screen openings, and the screen openings have an effective cutting edge oriented transversely to the machine direction. The inner and outer plates are mounted for movement relative to each other so the effective opening size of the screen openings can be changed. The length of the effective cutting edges of the screen openings are kept substantially constant regardless of any change in the size of the screen openings.