I made a sock for my WBBB from 1.1 ripstop and gain 10 - 15 degrees in trapped heat. My ripstop is breathable and I have not had any condensation problems worth mentioning hammocking here in Indiana winters down to 10 degrees.
OldGringo here on the forums once said he made a sock from Tyvec - you may want to contact him with some questions since Tyvec is a vapor barrier and you will have condensation without lots of ventilation. I have seen several hammockers use Tyvec for a bottom protector and a breathable material for the top. Any kind of fabric thats not coated will work - even an old/new bed sheet. You would be surprised at how much heat a thin cotton sheet can trap.