I have just 70mm joists on sleeper walls in a victorian house, I can just about add at least 50mm celotex once they have been battened. This leaves me with the following solutions - Expensive- re joist the floor to accept spreader plates (irregular spacings), then 21mm structural plywood/chipboard and 11mm Chevron (lady is set on them) - tongue and grooved board with 12mm pipe, this requires 6mm ply glued on top for rigidity then chevron on top   and my light bulb idea -