Abstract: The invention relates to a method and an installation for controlling the pressure in a working chamber (10) of an insulator (1), which is shielded from the outer environment (U), during opening or closing of the insulator with respect to the outer environment (U), with the aim of avoiding critical pressure bursts in the working chamber (10). The working chamber (10) is provided with an access for introduction or removal of the product to be treated. Purified air is supplied to the working chamber (10) from a circulating air Zone (11), and air arrives from the working chamber (10) at the circulating air Zone (11) via a returning air channel (14), and the working chamber and the circulating air Zone form an interior space (18). A supply air unit (a3) is provided on the insulator (1) and comprises a supply air ventilator (B) and a blockable first supply air adjusting element (21), mounted between the supply air ventilator (B) and the interior space (18).