Solid Floors

Unfortunately it is very difficult to upgrade solid floors. Insulation usually has to be incorporated in the floor slab during construction. This is now required in new homes.

Suspended Floors

Although not impossible it's certainly difficult to upgrade most suspended floors. 

A common suspended floor design is wooden boards on joists over a ventilated cavity. This type of floor can be a significant source of loss through ventilation. In extreme cases the air pressure below can be seen to lift the carpet above!