The underfloor panel is a rigid panel used to provide a uniform enclosure of the bottom floor decking, particularly when a structure is using a raised floor deck over pier foundations. It is a 1020mm square panel that is inset into the floor deck frame as it is assembled and cannot normally be removed except by partial floor disassembly. It is usually made of a simple 10mm thick unfinished but weather-resistant panel material such as Masonite, plywood, fiber-cement panel, recycled HDPE, cardboard with a polymer laminate, or a thin sheet alloy with a folded edge. However, it can also be fashioned as a composite 30mm panel that includes a layer of integral insulation bringing its outer -bottom- surface flush to the bottom of the floor grid. For floor grids that have two different frame member depths, a longer 1050mm or 2100mm panel may be used without end flanges.