Robic 7D for a lightweight bivy

I’ve been thinking lately of doing a MYOG Bivy, including some sections of this Robic.

1000mm is a fair bit, bearing in mind that ten years ago most silnylons on the market had not much more than that and rarely misted. I figure with a solid reliable 1000mm under any minimalist tarp I’ll be fine. Having said that I’m a little worried about the breathability of this fabric, I’ll counter it with plenty of mesh sections (probably the 0.5oz stuff from RBTR)

I’ve used M50 for bivy which works pretty good, from thru-hiker.com, 0.7 oz/yd2, I’ve found the DWR to be quite effective, likely the Robic would be just as good

I have a jacket w/ 7d Robic as the outer material- pretty decent breathability, but certainly not really breathable

I looked at M50 and it breathes well, but most put the HH in the 300 range- I want something w/ more water resistance

I’ve also seen Pertex Endurance mentioned- ~ 1000 HH, not sure on breathability though

If you want to stay dry when it’s raining, you need higher HH.  Raindrops hit the fabric with a lot of speed, so exert pressure on the fabric, so you need higher HH.

I don’t use bivy for rain, I put my mid up, so that doesn’t matter

Otherwise, M50 is good – rain or splash blowing into the tent, dew or frost when I’m cowboy camping, wind

I’m trying to build a bivy that would withstand some rain if need be.  This would be used in conjunction w/ a Apex hooded jacket and Apex half bag as a short term shelter (3-4 hour doze and then move) or as part of an emergency shelter carried in a day pack.

I have a bomber eVENT/cuben bivy- but it’s a little heavier and higher in volume than I would like for the above scenarios.

rain – Robic is supposed to be HH > 1000 so should work?  (That was your original question : )

that’s what I was hoping :)  was also thinking using a small sheet of polycro underneath if need be

Do they (dont they?) say that 1500 is the beginnings of waterproof?  Is that to account for the pressure from rain falling?  Cause really 300 is waterproof, but it cant take the pressure from the fall from the sky, and maybe 1500 is where is starts to be able to handle the pressure?  Or maybe 1500 is 40 mph wind driven rain or something like that?

(and 1500 is the minimum you would want for a shelter so you could expect to be in a 40mph wind with rain, and assuming your shelter stayed up you would be waterproof?)

*40mph is just an example

Many current main stream tent manufacturers list the fly at 1200mm and are at least inferring it is waterproof.  Gossamer Gear uses a 7d sil/pu nylon (is the Robic coated?) for the roof of “The One” and list >1200mm.  Not a lot of reviews but if it leaked much I would expect to see mention of that on the web.  They also have the Twinn tarp that is constructed of the same 7d material.

” Is that to account for the pressure from rain falling?”

I think that’s correct

I have pooled water on M50 overnight and it never leaked through.  Another DWR fabric had pooled water for a while, then all of a sudden it found a way through and the pool quickly went through.  M50 is fine with water on it.  I assume though it would leak rain because it falls at speed.

Will you have any tree cover? That helps a lower HH fabric withstand the real down pours. Even if not, >1,000mm is going to fend off most rains.

in most places yes- my plan is to duck under tree cover for the “nap”

I would go for it then. What design of bivy are you looking at? I dont think that 7d is breathable enough for a closed bivy.

A small square made from .34 cuben could work well to enhance this?

With a one inch edge seam and corner guys, 52*52inches (54*54 material), that’s 0.9oz, not inc any extra reinforcement patches. The length of the diagonal would be 73 inches long, a diamond pitch would have it almost completely covering you.

At such a small size with such little complexity and actual seams, the durability of the .34 cuben wouldn’t be an issue. As it wears and develops pin holes over time, again not an issue given the HH of your Bivy.

One idea I’ve been thinking about is to make the bottom ~third of my bivy out of the 1.4 WPB from RBTR, then above that its the 0.5 mesh. In addition, where the mesh seams with the WPB around the knees or thighs, sewing in a rectangle of a DWR fabric (could be the 7D robic, or something else), which I can just fold back most of the time for better breathability (not having the WPB footbox too long would allow moisture from that end to make it more easily to the mesh). When temperature is approaching the limits of my insulation, pull up the DWR, or of course if there is rain. Use a couple of camsnaps at the top end to keep it in place, but it would have extra width to deal with splash. The bivy floor would be bathtubbed.

One of my thoughts is an actual WPB footbox means that I have zero stress about what is going on at my feet, I mainly then worry about my torso and prioritise keeping that out of the heaviest rain. Much easier to find or make a dry spot that size than the entire length of my body.

I was thinking a very simple closed bivy- ala MLD’s sleeping bag liner/bivy

^ good ideas- I have a 52×52 sil “dog” tarp that I’ve used before, if the forecast was calling for rain it could be thrown in- low volume, I think it’s ~ 4.5 oz

I was thinking I could wrap the lower foot part w/ the polycro, but a true waterproof foot wold be a good idea- you definitely can get sucked up under a tree relatively easy, but your feet might be hanging out a bit

Are you planning on using  Robic as the shell for the half bag and warm top? If you are then a secondary Robic layer may be superfluous

My limited experience has been that for short recuperative breaks a poncho makes a better short term shelter combined with a plastic rubbish bag over the feet and legs than a full sized bivvy bag

You trap a better bubble of warm air under the cover of a poncho in a sitting rest position

Have you thought about the Hilleberg Bivanorack?

Bivanorack is interesting concept- a little heavier than what I would like- unsure how breathable a PU fabric would be as well

poncho might not be a bad idea; the one drawback would be utilizing if for rainwear- I end up typically doing a fair bit of off trail stuff and negotiating a lot of blowdown- all of which has the potential to shred something like a poncho