A blocking device for mounting within a space defined within a hollow structure includes a holder and an expandable member. The holder is adapted to be mounted within the space. The expandable member is supported on the holder and is expandable by heat to form a foam member in order to block or substantially close the space. The holder includes a holder plate extending substantially perpendicular to the longitudinal axis of the space and supporting at least one side of the expandable member. The expandable member includes a constriction absorbing portion that is deformed so as to absorb potential constriction of the expandable member at the beginning of the expansion process of the expandable member.