Lightweight fabric for inflatable beams?

Is there anyone here that may know?
Thanks

It sounds like you have a lot of research to do. Cuben that has been folded is not airtight. People (myself included) have attempted to use cuben for inflatables. It gets pinholes from flexion so it is not suitable for inflatables. I've never heard of cuben being welded.

Also, if you're making an arch and you want to avoid lots of wrinkles, you need a fabric that's stretchy. No flat pattern will make an arched tube without wrinkles. The celebrity of cuben is due in part to the fact that it has very low stretch. An arch made from a cuben tube would be a crinkly mess. Nylon fabrics are very stretchy by comparison.

I'm surprised that you thought anyone here would know whether the specific fabrics you identified would be airtight enough to use for inflatables. You need to contact the manufacturer for that.

You need to decide on how you will construct this tube. You didn't say anything about that in your post. If you use a balloon-in-envelope design, you can just sew a tube using ordinary uncoated nylon, then put a long balloon (welded from urethane film by you or prefabricated) inside. Or you can construct a single-wall tube with airtight fabric, by sewing the tube and taping the seams or using heat-sealable fabric and welding the seams. Most inflatable sleeping mattresses are made by welding heat-sealable fabrics. Seattle Fabrics sells a couple of heat sealable fabrics.

Also, remember that the stress on the envelope of an inflatable cylinder is a function of its diameter. For a very large diameter tube like yours, the fabric (and seams in the fabric) will experience large tensile forces (hoop stress) at even very low pressures. You could easily pop the seams of a tube that size without much pressure at all.

Seattle Fabrics has a 3oz/yd 30d heat sealable nylon that is pretty good stuff. I've used it on a couple sleeping pads and have had no problems.

Ryan