The internal ribs must run vertically.
The top of each panel should be sealed (foil-faced tape is often used; I covered that with a plastic U-channel for added protection). The “breathable” bottom tape is to allow condensation to drain.
My PC-covered greenhouse had a metal frame. All sides of the panels were covered in U- or H-channels, and the self-drilling gasketed screws went through the central gap in the H-channel or through a metal strip over the U-channels on the perimeter, and not through the panels themselves, giving them some expansion/contraction room.
The manufacturer supplied butyl caulk for between the channels and the frame, but bathroom silicone (with mold prevention) is a better alternative, in my mind.
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