a suspended timber floor
most of the heat loss from a floor is by draughts through the gaps, between the boards but mostly the gaps at the edges of the room under the skirting.
Consider.
There is no loss by convection because convection does not go downwards
there is imperceptible loss by radiation because the temperature difference from the underside of the floorboards into the void is very slight
there is very little loss by conduction from the underside of the floorboards to the air layer in contact with it.
Mineral wool can easily be stuffed into the gaps to block the draughts. No precision cutting is required.