I've been playing with the idea of making a bivy once I'm done with my overmits. Something for emergency use only, or inside a snowcave. The materials I wanted to use are silnylon for the bottom and something light and breathable for the top. I haven't settled on a pattern yet, but making a bathtub floor seems reasonable. So seams will need to be sealed. Which brings me to my question; if you make the seam between the breathable material and the silnylon a felled seam, should you fold the silnylon over the breathable fabric, or the other way around? I was thinking sealing the seam would be better when done on the silnylon side.
(other tips and advise are appreciated too)