insulating a cathedral ceiling

we have an open ceiling in our cottage and also in a new boathouse/loft area and would like to insulate it. Many home shows use Roxul for insulation between floors and outside walls. Some others use styrofoam board or foam core type panels. We have limited space (rafters are 2" by 4" from the old days in the roof structure so we cannot use Roxul. Do you have any suggestions for insulation in this application?

Family cottage (circa 1960) needing insulation &...
New boathouse/loft needing insulation...