In reply to raussmf:
Attics seem like such a horrible place for a training wall but they are huge unused spaces.
Before you go too far with your plans you'll need to work out what length of timber you can get round all the corners in your house (including through the hatch) and what size boards. Try one before you buy a pallet. Also where the underlying support is (the interior walls).
You could stiffen the ceiling joists by gluing and screwing timber to them (adding depth) and making sure any (butt or better if you can, scarf) joins in the new timbers lie above supporting walls but you'll probably still find old plaster lets go eventually if you're jumping about on it.
I'd suspend a new floor above the ceiling and decoupled from it, feed the load as directly as practical into walls below. It doesn't need to be super stiff but you should give some thought to future owners who'll almost certainly use it for storage and to what will happen when any structural screws rust if the floor ends up not bearing directly onto the walls. If for example the ceiling joists are 3" deep you could put 5" joists between them, with 1" of isolation from the joist to the ceiling and only 3" of built up thickness. Plenty of room for insulation in that. Also make sure the air trapped between the new floor and ceiling below has plenty of freedom to escape or they'll end up coupled together by air pressure anyway as you flex the new floor.
jk