Best Water-Resistant, Breathable Bivy Top Material?

[Mitchell Ebbott]

I've discovered that the Tyvek 1443R on my MYOG bivy isn't breathable enough. I've ended up with significant condensation the past few times I've used it. So I bit the bullet and got a Borah eVent bivy for standalone use, and now I'm looking to retrofit my MYOG creation with something better suited for tarping. M90 seems to be a popular choice, but looking at Richard Nisley's research it seems the non-ripstop version (which isn't make anymore) had significantly better water resistance (HH over 900, vs the ripstop's ~600). Unfortunately the M90 Taffeta isn't available anymore. There aren't a lot of lightweight, non-ripstop DWR nylons around. Ripstop By The Roll has their 10D Membrane, but no published hydrostatic head for it. Does anybody have any suggestions?

[John Vance]

You might check out Dutchware's Argon CS90. Luke's Ultralight uses this for their windshirt that tested very favorably by Richard.

[Adam Kilpatrick]

Yes it seems that taffeta versions of otherwise same-spec fabrics are generally better for HH. 900 is very good for such a breathable fabric…Did Richard have aged HH specs for it? I think I remember Richard posting that the fabric of the Montbell Tachyon's was around 900, that's a ripstop, so quite impressive, but its lower on the breathability (I think a mark of around 9 but don't quote me…) than the gold standards of windshirt materials. But I find it quite breathable enough, mostly, and on a bivy you don't need the same breathability as a windshirt. As an example, Event is close to zero on the breathability scale (I forget the units) but much of the time for most people in a Bivy they don't have condensation with it. So you only need to do a little better than that to garantee no issues. Other options could be doing a mixed material upper. Ie pick Cuben event for most of it, but say have a DWR based fabric for the hood. Then use a very small tarp or something to give you some protection over the top, leave zip open, etc. I've got five yards of black Membrane (0.66oz/yd) from RBTR at home, haven't used it yet, nor tested. But it "feels" like it has a good DWR coating, for sure, while still having quite a nice hand. It certainly feels like it would have a similar or better DWR than what I remember Momentum 90 Ripstop feeling like, if that's anything to go by. Given that it tests very high for downproofness probably means that its weave is quite tight and even, which can only help with aged HH. I'm sure Richard would test a sample if you send him one…I'd send one from my piece but that would be from Australia. Note, there is also a black taffeta version of Dutchware Argon 67. I have some ripstop at home somewhere I think, but I don't reckon it feels as DWR as the RBTR Membrane. Very subjective my thoughts! http://www.dutchwaregear.com/argon-67.html

[R]

I'm currently working with black Argon67 ripstop. As a test, I spread some over the top of an empty water glass, held it in place with a rubber band, pushed it down in to the glass a bit to make a fabric bowl and poured in a few ounces of water. After a half hour, there was a very thin film of moisture on the underside of the fabric. After 8 hours, there was a slightly heavier film of moisture, but still very light. I also did a 2 – 3 hour test in the middle of which I reached down in to the fabric bowl and rubbed the bottom to see if the fibers would wet out and cause more liquid to penetrate to the underside – no apparent effect. The fabric never leaked any drops in to the bottom of the glass. As a control I repeated this with a plastic sandwich bag for a half hour and for 2 hours – no moisture on the bottom, which rules out condensation as the source on the fabric. I don't know how this fares in comparison to other breathable DWR fabrics, or how breathable this stuff is relative to others, but I was impressed with these results.

[Richard Nisley]

The ground-sheet element has a consistent best solution Taffeta is a plain, smooth and flat weave which offers superior abrasion resistance and is thus the preferred choice for ground-sheets. PU (polyurethane) is the best waterproof coating for ground-sheet fabrics. Whereas silicone is a superior coating for fly-sheet applications, PU-coated ground-sheets stand up better to compressive and abrasive forces. The best current option is the RBTR 1.03 oz. PU 2000 polyester taffeta. The top-sheet solution depends on if you have a tarp or tent to block the force of rain If not, you need at minimum 1,500 mm HH. If yes, only splash needs to be protected from and ~ 300mm HH is the minimum. Your MET rate when sleeping is about .8 MET versus 7 MET when backpacking. It only has to pass about 1/9 the amount of water vapor as does an active wear jacket. A LUL 2 layer eVent or Zpacks WPB Cuben top-sheet for no tarp or tent is the safest. For the lightest possible weight, the LUL Pertex Endurance (1.04 CFM & 1476mm HH) should also be considered. For splash protection, many >300 mm HH quality DWR (H6) air permeable fabric will work since they have more air permeability than eVent. Examples include: .9 oz. LUL Argon, .7 oz. Zpacks Ventum, .9 oz. Dutchware Argon 90, and 1.1 oz Thru-hiker MR90. I have previously posted detailed test reports for all of the fabrics I mentioned.

[Mitchell Ebbott]

Thank you all for your responses! Richard especially, your contributions to this forum are invaluable. My use case is the second one you mentioned—I plan on using this bivy with a tarp (a poncho-tarp, mostly). Your response brings up a question that has dogged me for a while now. The outer material of my quilt is Argon 90, which is DWR treated. Assuming it has a hydrostatic head greater than 300 mm (a safe assumption, I think), what is the advantage of adding a bivy made out of identical fabric? That's largely why I'm seeking out a high-HH DWR fabric. If my quilt has a HH of 300 mm and I could find a bivy top with a HH of 900 mm, then the bivy is offering a clear benefit. Yet if I'm simply covering my Argon quilt with another layer of Argon (or equivalent), I don't see the advantage, beyond blocking the wind. Am I missing something?

[Jerry Adams]

I use M50, a little lighter than M90. I'd rather have taffeta but ripstop isn't that much worse. I made a rain jacket from it. Like Richard says, when rain falls on it it doesn't have enough HH so it gets wet underneath. Good if there's only a little drizzle or whatever. Something about when a drop of rain hits the fabric, it has a lot of momentum which puts a lot of hydrostatic pressure on the fabric pushing the water through. I have a bivy made with it. Works great for that. I did a test. Pool some water on top with paper underneath to see if it leaks. I tested M50 and some other lightweight DWR fabric not argon. Both fabrics held the water for a while, but after maybe 15 minutes, the other fabric all of a sudden let all the water through. The M50 kept the water out for a day when I got tired of the test and stopped. My conclusion, M50 much better DWR. If there's a little rain splash or dew it should keep you dry. If you use it in rain, it won't keep out the rain. Works good to keep wind out. I haven't noticed condensation on the inside, but I think there are more breathable fabrics. M50 had a fairly srong chemically smell. Must be because of the DWR. But it faded after a while and doesn't bother me.

[IVO K]

"Am I missing something?" @ Mitchell: Three benefits from adding a 300 mm HH bivy: 1. In the case of using a baffled quilt/bag with 300 mm HH outer fabric, the exposed stitches will let water in earlier than the non-stitched areas. The bivy's outer, being monolithic, would protect the stitches. 2. It is much easier to restore (partially at least) the DWR on bivy's outer than on quilt/bag, especially if the insulation material is down. Less labor-intensive. 3. When you utilize the quilt's ability to vent by partially uncovering your body, what is the HH of the uncovered body (clothing) parts? A 300 mm HH bivy covers and protects these.

[Anonymous]

"I've discovered that the Tyvek 1443R on my MYOG bivy isn't breathable enough. I've ended up with significant condensation the past few times I've used it. So I bit the bullet and got a Borah eVent bivy for standalone use…" Hi Mitchell, My understanding is that Tyvek 1443R tyvek is significantly more breathable than the best eVent materials. Not sure how that will play out in real world conditions or not though. It's too bad they don't make lighter and more breathable EPIC or EPIC like treated fabrics, because that is what i would go with being a so much more durable DWR. But honestly, unless you are using an undersized tarp or are caught in some truly horrendous conditions, you don't really need a super high hydrostatic head for the upper of a bivy since you're basically combining the HH of the bivy with the HH of the quilt material. And, it's actually more complex than that, because if you compare a single fabric of high HH, with two layers of fabric of similar combined HH–the latter is probably going to test better than the former. Because the first layer breaks the initial force of the rain and the other just needs to sit pretty and look good. So i would look more at breathablity than HH, again, unless you're using an undersized tarp or plan to use it in some really epic, wind driven huge/all night downpours.

[Anonymous]

I didn't read the other responses before i posted–probably should have since i'm somewhat repeating what other have said.

[Richard Nisley]

The HH for Tyvek 1443 (based on specs) averages 850mm. The air permeability for Tyvek 1443 (based on specs) averages 2.01 CFM. So, if specific environments yielded condensation with a Tyvek 1443R bivy top-sheet, then they probably also would with an eVent bivy top-sheet. Most probably, the water vapor was condensing do to the temperature of the bivy surface rather than the bivy top-sheet material not being generally appropriate for the application. Another possible explanation is that Tyvek 1443R has a very broad range of HH values (unlike eVent). Its CFM varies in an inverse relationship to its HH. So, if a specific bivy top-sheet is using material with a HH of 115cm or 1150mm HH, versus the average of 85cm or 850mm, then the corresponding CFM would be significantly reduced and would cause condensation when the average Tyvek 1443R would not. It is obvious from the Tyvek 1443R's HH that it is only appropriate for splash protection. In contrast, eVent's HH is adequate any conceivable rain/wind force you would typically encounter. With the variability of Tyvek 1143R characteristics, we should also expect that some users of this material will correctly claim that it worked adequately as a tarp or tent. The problem is the next person to build a tent or tarp may get the 550mm HH material from a different lot.

[Opogobalus]

To bump an old thread… doing research on investing in a bivy and this interested me:

Your MET rate when sleeping is about .8 MET versus 7 MET when backpacking. It only has to pass about 1/9 the amount of water vapor as does an active wear jacket.

And

So, if specific environments yielded condensation with a Tyvek 1443R bivy top-sheet, then they probably also would with an eVent bivy top-sheet. Most probably, the water vapor was condensing do to the temperature of the bivy surface rather than the bivy top-sheet material not being generally appropriate for the application.

I was considering a Borah bivy with argon 67 or argon 90 top, or a local maker using a different top fabric with a reported CFM of 12.6 ASTM D737.

The argon fabrics are much more breathable.

Splash resistance is of no real concern to me, my tarp is large and shaped. The bivvy is of interest for cowboy camping on clear nights and cutting drafts when I’m getting close to my quilts 30f rating.

From Richards comments, it seems like the cfm of any of these materials will be sufficient to minimise condensation inside?

[Robert Alexander]

I think what Richard Nisley was saying is that there’s a point where breathability will not aid in preventing temperature related (dew point) condensation. Mesh would prevent it but it would not create an air gap and move the dew point like many traditional bivy fabrics would. Maybe Nisley could confirm my understanding?

[Alex H]

Robert has it right.  It has to do with the dew point and condensation on the inside of the fabric no matter the breathability.  The clear nights cowboy camping are the worst conditions for that kind of radiative cooling.

[Elliott Wolin]

I’ve found that the mesh top on my 2P bivy (no-see-um netting) is fine enough that it does create somewhat of an air barrier, as long as the wind doesn’t blow straight onto it.  Under a tarp or in a lean-to, where the wind tends to only blow across the fabric, there does seem to be a temperature difference inside vs out.

And it seems to breathe really well as far as moisture is concerned.

These two observations seem somewhat contradictory:  little air flow across the mesh resulting in a temperature difference vs good moisture transport, but it does seem to be partially true.  Not sure why…

[Jerry Adams]

I think bivy top material should resist splashes or sprays of water, and dew.  Some sort of DWR coating.

Even in a tent trying to keep it dry, it sometimes gets some water on it.

[Elliott Wolin]

Note that my 2P bivy top has a border of uncoated ripstop and a center section of mesh, so the border mostly takes care of splashes.

[Martin D]

I’m happy with condensation inside my bivy. Because if conditions make so, if you don’t have a bivy where do you think it will condensate? I’d rather have water on the outside of the shell fabric than tale the risk of having it on the inside.

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