A valve includes a film having side edges and an opening to extend across a passage in a housing of a HVAC system. The film has a circular aperture in the film adjacent each rounded corner of each opening for relieving stress. Either or both of the leading edge and the trailing edge of the film are undulated to prevent deformation of the edges. A pair of racks is disposed on the film and sprockets engage the teeth of the racks for moving the racks and the film linearly. Each rack includes a strap and bearing member for retaining the associated sprocket in engagement with the teeth. The bearing member is in rolling engagement with a bearing surface of the associated strap as it rotates with the sprockets.