A method for assembling insulating panels, particularly for multipane glazing units formed by a pair of facing glass panes (V', V") and a spacer element (D) interposed between the panes, comprises the steps of: a) placing a first pane (V') in a substantially vertical position and providing a second pane (V") with a spacer element (D) having at least one side (L, L') in correspondence of each peripheral edge (B, B', B") of the second pane (V"); b) positioning the panes (V', V") in a mutually facing relationship with a predetermined horizontal spacing (R) to define an interior space (3) between the panes (V', V"); c) introducing a substantially inert gas in the interior space (3) by using appropriate supply means (4); d) moving closer and pressing together the first (V') and the second panes (V") with the spacer element (D) interposed therebetween, to close the interior space (3) and form an assembled panel (P); e) withdrawing the assembled panel (P). The gas is introduced with the first (V') and the second panes (V") reciprocally offset in a vertical direction to define a lateral gas supply port (5) for gas filling and allow rapid and uniform filling of the interior space (3).