Top hinged panels are the most commonly specified system. If they open, they will come to a near closed position once the internal and external pressure has equalized. In this near closed position, the canopy effect offers the facility protection from wind, rain, and snow. Whereas a bottom hinged panel will remain open at approximately 60 degrees from vertical, thus allowing easy entry of the elements.
Bottom hinged vents would most commonly be specified when panels are located at grade or at roof level where snow drifting might be a concern. It is much easier for a bottom-hinged panel to open downward on top of the drift than have a top hinged unit power through the snow.
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