I started another thread about a planned bivy mod I am about to do, which you can read here:
The short of it is that I want to put a vapor/condensation vent on the foot box. I have decided on how I want to do this (a strip of breathable fabric running across the foot end), but now I can't decide on what fabric to use, cotton or nylon.
After cutting a strip off of the bivy, sewing the vent on, and then sewing the bivy fabric partially back on as a flap to protect against rain and wind, this should (or I hope, at least) fix or at least very much help with condensation. Condensation is really the only drawback for me when it comes to using a bivy, so once I solve this, I will be good to go. If this whole experiment does not work, might have to switch to a bug bivy, or come up with a better mod.
But for now, cotton or nylon? Weight is not really an issue, either choice should only add about 10g I think (just a guess). Cotton is more breathable, and can absorb 25 times its weight in water, I just learned while doing some cursory research on the fabric: http://tlc.howstuffworks.com/home/question547.htm
So I am leaning more toward cotton. But maybe one of you fabric gurus can offer some perspective and/or feedback?