You don't say what type of floor you want to insulate. But in the past Ive used Styrofoam's blue-coloured expanded poystyrene. It's closed cell, vapour-proof, and high density. You can walk on it without leaving a mark. I've since lost the datasheets, but it's thermal value is pretty high - something like 1" thickness = 6" of rockwool. I lined the plasterboard on the stone walls of my kitchen with it, and put it under the floorboards. I found that the body heat of two persons can actually raise the room temperature. As a builder, you probably know the stuff I'm talking about