Bivvy bag base Lightest but most waterproof WPB??

Why do you want WPB for the floor?

ripstopbytheroll.com offers a 1.4osy WPB rated to a HH of 9000mm, which is adequate for a floor.  However, at 10D, it’s on the delicate side and I wouldn’t recommended it for that application.

RSBTR’s ~1osy waterproof fabrics are popular for tent and bivy floors.  They’re both lighter and cheaper.

“Is there a LW WPB fabric that has a sufficiently high waterproof rating to use as a base that also has reasonable breathability? Would Robic work?”

WPB? Nope.

Robric XL? Looks like it wouldn’t be recommended.

Regarding wpb’s, there’s enough controversy out there to indicate that most WPB’s either don’t really work as marketed, or they wet out after a few extended day trips, so trying to employ them to the bottom of a bivy seems like it would be a genuine waste of money for something that would most likely fail at the worst possible time.

As Rene indicated, RbR has a nice, light wpb which is rather water resistant and has a descent MTVR, but that’s probably under lab conditions, and certainly not a measurement of moisture transfer between you and the ground when sleeping. I’d wager that the MTVR measurement between a person in a bivy and the ground will be zero, especially since most folks sleep on a totally non-breathable sleeping pad.

The underside of a bivy should be both as waterproof (AND DURABLE) as practical, without sacrificing weight. Of course in general, the more waterproof and durable it gets, the heavier it will get. I’d recommend looking at our cottage friends and determine what they are using for their bivy’s (EE, Zpacks, MLD, etc…).

Good luck!

Check out the Borah Event bivy.

http://borahgear.com/eventbivy.html

There seems to be no advantage and a possible downside to a wpb fabric on the bottom of bivvy. It is unlikely that water vapour will pass from inside the bivvy to the ground unless the ground is very dry especially as there is no air flow to disperse the vapour. More likely is that when used on damp ground body heat will create water vapour at the soil surface which may be able to pass into the bivvy. This most probably will not have a big effect as the air in the bivvy will already be at a high humidity.

Save money and just use a properly waterproof fabric.

Main use ot this bivvy is to protect the SBs shell from damp while on top of the mattress system in below freezing conditions.

Think of it as more of a WPB sleeping bag shell that is removable

Influenced here by comments from hard man Andy Kirkpatrick, despite my climbing days being well behind me I still tend to think like a climber, in this bivvy bag I am getting some condensation but only on the urethane proofed bottom section

Rene I never saw that WPB at rsbtr but that would work, I’m happy to replace it as it wears out or alternatively simply buy enough to make a new SB cover.