A flow control device has a duct section with a plurality of damper blocking elements, each having a major plane. The damper blocking elements are pivotably connected to the duct section and movable in a range that is limited to ensure that, when the duct section is mounted in a preferred orientation, the damper blocking element major planes always form an angle of at least 45 degrees from the horizontal throughout the range. The range is such that the plurality of damper blocking elements can selectively close and open the duct. The blocking elements can completely close the duct, for example, to block natural convection.