How can I insulate an area without an attic above it?
Well, just like in surgeries, you can come in from either side, either take off the top or take off the bottom.  Generally speaking it is easier to take off the bottom.  However, you don't say how much of a gap there is (if you know) between the drywall and the bottom of the roof.  If you don't have enough space to put enough insulation in to make it worth your while,  then you might have to remove the roof and raise it enough to put in adequate insulation/