Cuben is waterproof, right. So, it is also airtight. You may not be able to compress the quilt effectively if it is cuben on both sides. Also, I'm not sure about you, but when I use a vapor barrier, I have lots of condensation. Cuben is a water barrier, so you may want to test a space blanket or something cheap before you make it out of expensive fabric.
It does sound cool, though and would most certainly be light weight.
Good luck with your design.
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