Perhaps even more important than insulating the trapdoor is to make sure there are no drafts in between the ceiling and the trapdoor. Moist hot air finding the way into the attic during the winter season will condense. This will be a threat to wood beams and roof wall boards as they might rot. I have used a P-profile seal strip (http://www.clasohlson.no/Product/Category.aspx?id=251254#34-237) to make sure the trapdoor is properly sealed when closed.
Then I removed the ladder. For insulation I had some leftover Litex insulation that is self supporting and can be glued together. On top of this I added a 6mm plywood sheet before screwing the ladder back on top of this. I would think that the insulation now is at least twice as good as before, by judging the thickness of the insulation.