"Nige Danton" wrote
If you place the new insulation on top of the boards and then store anything on top of it. It will squash the insulation and render it essentially useless. In the areas where you want to retain storage, it would be better to fasten new joists across the existing ones, place insulation between and then put boards on the top.
-- Nige Danton
Alternatively, decide which areas you want to board - put standard insulation between the joists - put proprietory dense board-type insulation across the joists (this is thermally far more effective than standard fibre stuff) - lay boards on top of this and use long screws to secure to existing joists. Use standard fibre stuff in non-boarded areas. Consider using batons and boards to create elvated storage for lighter stuff (Xmas decorations etc). This gives more storage but allows standard insulation to be installed beneath.
Phil