Moisture performance of newer non-PFAS WPB fabrics? Montbell Drytec bivvy vs ???

I have the old version as well. I’ve had good experiences with it in the Sierra. Have you seen the new MLD FKT ProVent UL BIVY? I just got one, it is about 20g lighter on my scale than my Breeze drytec long/wide. I’m really impressed with it so far; I don’t have a lot of time in it yet. It packs down close to the Montbell also. The MLD does have a zipper; good for unexpected weather, bad for long term durability if you’re not careful.

Thanks, Paul. Unfortunately, any bag that’s half non-breathable is a no-go for me. There is no way I am going to be still through the night and not wind up with the “floor” up on top (I’ve tried it). I guess it must work for a lot of people. A zipper would be of no use in the way I use the bag.

I would make my own if I had a source for quality WPB fabric. . . .

Discovery Fabrics in Vancouver B.C. has neoshell and powershield pro which are quality WPB.  They are friendly to people from the U.S., no tariff because it originally came from the U.S.

Me, if it’s raining I put my tent up, so I just use breathable nylon in my bivy.  But there can be dew or frost on it which you need a good DWR nylon.

Noting that the huge majority of “breathing”  happens on the top and sides of a bivy for many reasons.  Yes, some people use a bivy in a way that can lede to the bivy roating and some of the floor moves to the sides or even to the top.   Also noting that some  UL bivys have much less WPB fabric on top of the head end, foot end and and sides due to design and that can be a big factor in total area of breathability and thus potentially  even more important than the WPB fabric’s numbers. We can build the bivy with the WPB fabric on the bottom. It’s a wee bit heavier and a bit more $.