Thanks!i do see why tyvek is so appealing for myog bivis now! Ive been fiddling around with designs and ideas, and ive come to a design with is similar to the borah snowyside bivy, just with a centre zip and double opposing storm flap and chest zip. Ive seen something similar with tyvek, so ill go that route i think. If im not happy with the properties of tyvek (id like t use it as a standalone shelter, so for aumn/winter it might not be suitable) then ill do the same in a goretex or event if i could get hold of some (the uk has barely any suppliers of goretex, let alone Event).