Abstract: A method of retrofitting ventilation systems with security grilles. A first step involves providing a security grille that is larger in size than an existing air duct to be covered by the security grille. The security grille has a flow area sufficient to permit a desired number of air exchanges per hour within a holding cell. A second step involves forming an enlarged plenum chamber where the existing air duct passes through a wall into the holding cell. The plenum chamber has a flow area compatible with the security grille. A third step involves attaching the security grille over the plenum chamber, such that the security grille is fed by a flow of air through the plenum chamber.