Abstract: Underlayment for a floating flooring includes a sandwiched membrane that may be used as a separator between the floating flooring and the rigid foundation thereof. The membrane includes a closed cell low-polyethylene foam whereto two polyethylene films are simultaneously heat-sealed. The films enwrap the foam in its entire width. An overlapping is formed that may overlap and pressure-fix over a self-adhesive strip of the first film of a similar sandwiched membrane adjacently extended and establishing a heading joint between the edges of the foam of both membranes and of the second film, belonging to this overlapping. The overlapping overlaps this heading joint and thus it makes a mechanical tight association between the first thicker film of both membranes, at the same time making a continuous vapor barrier between both membranes, an acoustic buffer and a damping element, which extends along the whole arrangement of sandwiched membranes over the floor.