I want to add some additional insulation to the concrete floors throughout the house. I need to use something which is pretty much incompressible and fairly thin (max 20mm), as I don't want to end up banging my head on the tops of the door frames when I've finished. A quick skim round on the web has come up with products such as Marmox boards and Cellecta Fix boards, which look pricey. Has anyone done anything similar and got any other suggestions that might be cheaper?