Just finished one of my many insulating jobs in my 1930's solid wall home.
I've already insulated all my external walls internally to save the look of the house (Plus its cheaper).
Apart from my kitchen extension the whole of my downstairs floors are timber. Very cold and drafty so I thought I'd insulate them.
I decided not to do the whole room in one go but in two sections, that way I only had to move the carpet to the other side of the room. I insulated all my CH pipes and treated the joists with anti woodworm/wet rot solution.
I knew I wasn't going to replace my old flooring but did want to keep most of it for other things, mainly to make a large toy box for my children.
I used foil bubble wrap insulation as the means to keep the earth wool in place and stapled it down and over each joist.
Once I did one side of the room I'd make sure all was airtight, I'd use expanding foam and sealant on any gap I could see.
I went for 18mm OSB board for the flooring instead of the usual chipboard as I've heard its alot more durable and better at coping with liquid spillages. Also cheaper by about £50.
The whole job cost me approx £250. So far its not really been cold enough to tell a huge difference but you can tell once you walk on an un insulated part of the downstairs floor.