A method for manufacturing an insulation block, preferably an insulation plate, in which method the casting mold is adjusted to a desired size by moving at least one wall of the casting mold, the pre-expanded polystyrene, polypropylene or polyethylene beads are led to the casting mold, the pre-expanded polymer beads are steamed in the casting mold, and the formed finished insulation block is removed from the mold. In a method according to the invention at least one wall of the casting mold is moved during the steaming and/or during the cooling phase after the steaming.