MYOG Waterproof Bivy questions.

I have 20 inch wide, 1 inch thick mattress. I don't like the bottom more than 22 inches wide.

If it goes up the sides, I get condensation there.

Thanks Jerry. So you're saying, bring the breathable fabric down the sides, and only use the pure Cuben for the base? The breathable Cuben comes in 54" rolls, so I was hoping to make the strip 27" wide, minus seam allowance, so I could get enough for the two bivys out of each width. If I am to come down the sides as well, I would have to rethink this.

Yeah, that's my experience.

When it get's cold, like below 32 F.

To me, a 54" circumference is a little small anyway. 60" is better. You don't want to compress sleeping bag loft. I also like a little extra room to wear something inside sleeping bag.

If bottom waterproof part is 22", then top should be 38". Plus seam allowance. That uses up your 54" pretty good. Maybe even make it a couple inches wider? But making it too big isn't good either.

Maybe take some cheap plastic, make a tube that's 60 inches around, and put your mattress, sleeping bag, and you inside and see if it's a good size.

That breathable cuben isn't very breathable unless the latest versions have improved. Far below eVent numbers and many folks have condensation even with eVent. They are both listed somewhere here on BPL. That may be OK with you, just didn't want you to be surprised after paying for it.

P.S- IIRC breathable cuben cannot be bonded, or at the least can only be bonded on one side.
Ryan

j@erry adams I didn't mean the overall circumference would be 54". Simply that the roll comes in 54" width, and I hoped to split that between my, and my wife's bivy. But if I have to use the full width per Bivy, then so be it. Do you think a 60" circumference is about right then? What are the issues with making it too big? I'm 6'1", and a slimish build, but I also sleep cold so normally sleep with clothes on.

@Ryan Smith Yeah, I can't find many real life reports bout how the breathable cuben fibre performs in the field. However, John B. Abela put up a post on his website recently regarding the new generation of the breathable cuben fabric, and I quote;

"I do not have solid numbers on the material at this point, but the numbers I have been provided from an independant source indicate to me that this 3rd generation material is in the 40,000-45,000 g/m2/24hrs range

"Up first is of course that this new information being provided by Cuben Tech about their latest generation of WPBCF is a quantum leap forward for them and their material. This is a doubling of breathability (MVTR) from gen2 to gen3."

Which if true, is certainly good news, and why I am seriously considering using it to make a bivy. Thanks for the information regarding bonding. I'll have to look into that to see what my options are. :)

I understood what you were saying about using up the 54 inch efficiently, it's expensive. I hate having left over strips that are a foot wide or whatever.

If you make it too big, it weighs more. It might actually be colder because there's a larger surface area to lose heat out of. Maybe it blows around more in the wind. It requires more pack volume.

Condensation, condensation, condensation.
Utterly unavoidable when it is cold.

Cheers

You might do some research here, but I think eVent's MVTR is in the 20,000-25,000 range, so the 3rd Gen CFWPB numbers seem like a stretch. This is all from memory so take it with a grain of salt(like any mfg'ers numbers too I guess). Maybe someone will chime in that knows for sure.

Edit: I read Abela's article. Apparently, Cuben Tech is actually using eVent now so the MVTR is probably in +20k range with Gen3.
Ryan

"Condensation, condensation, condensation.
Utterly unavoidable when it is cold.

Cheers"

Depends on what you call cold. If it is below 40f then I have good experience with VB clothing. Solves the condensation issue at least on a standard bivy. Not sure about a standalone, no tarp bivy. Would have to see where moisture from respiration is going.

@Ryan Smith I sent Joe Valesko from Zpacks a few questions, and regarding the fabric he said:

"The breathability should be somewhere between 35000-45000g/m^2/24hr based on their JIS L 1099 testing.
We only have the new stuff now. The original material was also eVent, but now it has a new surface pattern that improves the breathability."

Obviously these are Cuben Tech's figures, so I guess take them with a pinch of salt, but it sounds promising. Also interesting that the Gen2 fabric also used eVent without anyone knowing.

@Hiking Malto, Do you not get really clammy in VB clothing? I haven't used it before, but just imagine feeling hot and sticky all night.

As for respiration moisture, I am hoping to design the bivy so that all respiration goes straight out through the bug screen and outside.

I have been pleasantly surprised by the VB experience. I normally will wear a super light base layer then the VB followed by normal clothes. The closest feeling that I can describe is that it is like you were back in a womb. I don't find it uncomfortable at all and since the base layer is light it dries almost instantaneous when you strip down in the morning. It is also cool to watch the steam come off. Keep in mind that it creates a high HUMIDITY environment not a sweaty environment unless you over insulate. I would certain encourage folks to experiment with it. I believe it is one of the more game changing UL concepts not just for extreme cold but for the temperature range a few degrees from freezing either way. At some point I want to do a bit more experimentation on the upper limit for vb. my main barrier to it is philosophical. The only way to take advantage of VB at warmer temps is by getting a new lighter high temperature rated bag. It violates my goal of trying to specialize my gear to conditions. Rather i want a single kit that gets added or subtracted not replaced.