As a follow-up, I looked at the REI bivy specs and have a couple thoughts: The Elements fabric is used in their affordable rainwear and is undoubtedly PU WPB, which has less breathability than Goretex or especially eVent. Waterproof, yes, but I'd be cautious about condensation in high-humidity situations. I also see nothing about taped seams, so plan on hours of seam-sealing.

Have to say, it's a lot of bivy at less than a pound.

Cheers,