Cuben – The 422 mm hydrostatic head dirty little secret

[Richard Nisley]

Ziff,

Greg correctly answered your question; I will just expand on his answer.

Several instruments have been devised to measure air permeability, but the two most commonly used in government test laboratories are the Gurley Densometer and the Frazier Air Permeameter. The key thing to understand about the Gurley is that it only exerts air pressure equal to 1.26 inches of water and it then measures the amount of air that moves through a specific sized orifice at that pressure. The Frazier values are obtained by noting the vertical manometer readings while the pressure drop across the fabric is maintained at 0.5 inch of water pressure, as indicated by the inclined gage.

With many materials there will be a point above the air permeability test pressure at which the HH water pressure will force water through the material’s pores. With the tested MLD Cuben, that force was 16.61 inches of water pressure versus the standard 1.26 inches used to measure air permeability. I routinely test up to a maximum of 3,515 mm H2O. This relatively low level insures that I won't damage the fabric.

Your point about the size differences between air and water molecules do raise some related questions that may not be totally answered solely by the increase in water pressure.

The same phenomenon of zero air porosity and low hydrostatic head was previously observed and documented by me in an earlier post relative to silnylon. I SUSPECT, 1) that the increased hydrostatic water pressure opens micro-fractures in the silicone or Mylar substrates that are closed at the low air permeability pressure test pressure. 2) Water molecules have a large dipole, and this is has a positive energy of attraction for any other molecule, including plastic, whereas there is little attraction for the nonpolar oxygen or nitrogen. Given that a water molecule is also lighter than say nitrogen or oxygen molecule, it moves faster. So water goes 5-10 x faster through most plastics than air due to the combination of the two effects. 3) The interconnected pores in the membranes may be less than the mean free path of the air molecules which may dramatically slow their movement.

[Steven McAllister]

I wonder how the materials I used as a kid would compare? The typical Coleman canvas and the ultralight silk materials come to mind.

[William Johnsen]

I was talking to some people that have worked with cuben to make sails trying to figure out how to best bond it and one thing I was told that was concerning was: "Prior to starting your project you might also want to take into consideration that cuben fiber is extremely expensive. And, no sail cloth is waterproof, so your tent won’t be either."
I figured since there was so much sold and I haven't heard consistent complaints it was waterproof enough, or like others have eluded to the thicker mylar varieties would be suitable. Thank you for your tests Richard the more info that is out there the better. Hopefully there will be some follow up tests to see if it's universal.

[BPLwiia]

Can someone direct us to complaints voiced about cuben fiber? Certainly, enough tents and tarps have been sold to provide an ample sample from which I'd expect we'd be able to find a growing number of complaints.

I don't recall seeing them but maybe someone can show us otherwise.

[Richard Nisley]

W,

In summary most people just call the inability of any shelter to keep a user dry, "condensation". That is true whether it gets knocked off or is brushed against. Do a forum search on condensation and you will find 4,132 hits. None of the 4,132 hits say that condensation keeps them dry; it is always the opposite.

When you exceed the hydrostatic head of Cuben or silnylon it generally doesn't squirt, you get a little micro-droplet of water on the underside of your shelter. A user in the field has no way to determine which micro-droplets are from condensation versus which micro-droplets are from a low hydrostatic head. None the less, the amount of micro-droplets can go up significantly from this phenomenon. They can then get knocked off in a spray from subsequent wind gusts or large raindrops on the outer skin. Condensation creates heat when it forms; in contrast, low hydrostatic head leakage absorbs heat as they spray. This thread originally discussed this concept: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=43902

The relevant post is entitled: Your shelter’s new and current HH and why it is important to know these values

[Chris Kopp]

I'm not versed in material science. However, I would remind everyone that this discussion is based on findings from a sample size of one. Sound conclusions can not be made when the sample size is one. I would like to see a sample size that includes samples from at least 5 randomly selected lots. Until that data is presented I'm withholding judgement.

[Richard Nisley]

Chris,

You have a valid point about the importance of a statistically significant sample size. There are a lot of DIY people on this forum. Please send both a virgin and a conditioned (via machine washing 8 times with no soap) scrap piece of your .7 oz/yd2 Cuben fabric to Roger Caffin for HH testing. I am also very interested to see how his test results compare to mine.

I have two (one virgin and one conditioned) double layer Mylar version Cuben sample being sent to me by Ben to test. My first test was on a lightly conditioned .7 oz/yd2 single Mylar layer of Cuben.

[Dan @ Durston Gear]

Why stick to 0.7oz? There's gotta be lots of people on here that have scraps of all sorts of cuben. I personally have some new scraps of 0.33oz,0.51oz,0.7oz and 1.26oz cuben that I wouldn't mind mailing away. The problem is that I'm in Canada and cheap postage to Australia usually takes 3 months to arrive. It would be more feasible to send them to San Fran.

[James holden]

maybe some of those "condensation" problems in the rain are unsolvable …. if the results hold …

i wonder if the "misting" can aggravate condensation problems by introducing more moisture in the tent …

hmmmmm

[Roger Caffin]

> The problem is that I'm in Canada and cheap postage to Australia usually takes 3 months to arrive.
Send them to me care of Backpacking Light in Bozeman.

Cheers

[Roger Caffin]

> maybe some of those "condensation" problems in the rain are unsolvable
Bingo. Give the man the prize.

But note that a double-wall tent usually does handle the problem very well.

> i wonder if the "misting" can aggravate condensation problems by introducing more moisture in the tent .
Likely, very likely.
Wet clothing, wet ground under the fly, cooking, breathing …

Cheers

[Steven McAllister]

> i wonder if the "misting" can aggravate condensation problems
> by introducing more moisture in the tent .

The biggest cause is usually the fact that when it rains, the air is near 100% humidity. This wet air is everywhere and condensing on any cool solid surface.
Leakage can add to this, but from my experience, not a leading cause.

[James Marco]

"maybe some of those "condensation" problems in the rain are unsolvable …. if the results hold …

i wonder if the "misting" can aggravate condensation problems by introducing more moisture in the tent …"

Yes, at times it is nearly impossible to eliminate condensation in a smaller tent. Even a tarp with much better ventilation has problems.

Generally, most people are taught that condensation happens at the interface where a cold surface meets saturated air. In actuality, it is not quite that simple. It happens at an equilibrium at that interface. Air does not have to be 100% saturated to produce condensation in a tent. The condensation occurs anyway. It evaporates anyway. These processes do not stop just because of condensation. Example: In a desert, under low humidity, a 10' tarp can give you enough water to drink under bright sunlight relying on modifying local conditions to drive condensation.

As water condenses, it releases heat, under many conditions, enough to drive re-evaporation. If the tent material was a real good insulator, condensation would NOT occur on it's surface.
Hence, (using water proof material in this hypothetical example) no interface between hot/cold and no real condensation…it would evaporate as quickly as it forms.

Another example: The temperature of a molecule is defined as it's movement. If it is moving very fast, then it is hot. So, the temperature of a molecule of water in space is usually very high. It is usually moving very fast, at what we would call super-heated steam heats. But, there are not enough of them. Space is very thin. So, the temperature we see is quite cold. Temperature and pressure are related.

Both examples make a strong case for a double walled tent, since rain will usually drop the temperature of the fabric, and act as a heat sink not allowing re-evaporation of a droplet when it does happen. A cool rain on a tent at nearly 100% humidity is about the worst conditions, ignoring misting and other things.

The two major factors effecting condensation in a tent are temperature and humidity. The higher temps inside a tent allow more moisture to be absorbed into the air. Adding to this is bad. Breathing alone will cause condensation. Hence the concern with floors, wet items in the tent, and any other sources of water vapour, besides your metabolism. Roger is absolutely correct. But, you can do nothing about your metabolism. You WILL get some. Sometimes it is a slight wetness when rolling up the tent. Sometimes the tent looks like it is raining inside.

Raw condensation can be annoying. But can usually be tolerated. In rain, any wetness is to be avoided. Condensation can be knocked off tent and tarp fabric as mist. This will cause a micro climate inside your tent. Cooling the warm/humid air can cause these seed particles to condense more rapidly than they evaporate. So, some of them drop on the sleeper. Others are re-evaporated…depends on their initial size. Everything in the tent gets damp. A couple days of this and they get out and out wet, AND, they always contribute to the internal humidity. Condensation begets condensation, if you will allow this generalization.

Because water is a bi-polar molecule, it tends to stick to other water molecules. Water surface tension is a pretty well known example. So, during a rainstorm, it usually happens that the air humidity, often at about 100% just before a rain, drops. During a rain storm, the air humidity is somewhat less because the drops will attract water molecules out of the air. This fact is not important to small tents, except, that improved or forced ventilation will allow less condensation to occur inside. A candle in the peak vent will help drive this
in the absence of a wind.

So, whatever the head pressure of cuben is, it appears to be water proof. Temperature inside a small tent will help drive ventilation and increase water absorption by the air (not exactly linear, though.) All of the above makes a good case for a double walled cuben tent under wet conditions regardless of whether the fabric is highly waterproof or not. The area between the two layers should be sealed to prevent heat loss and condensation. The venting should be to the inside of the tent, only.

So, two layers of .38oz/yd cuben should allow better wet weather performance. Does everyone need this? Does the additional weight of a two layer tent make sense? These are only a couple questions that only the market can answer. Maybe I am all wet with the condensation and cuben fabrics are not the magic bullet we are seeking. A better insulated fabric can help as much as much as a fully waterproof one with condensation. Just a thought…

[tyler marlow]

I think I'll just start bringing a dehumidifier into my shelters at night… :)

[William Johnsen]

I was trying to think about how much misting actually affects the temperature inside a tent. I wasn't sure about how realistic some of the values are but here are my assumptions: The rate of misting is 0.1mL/minute, the temperature of the rain is 10°C, there is one person in a 2.5m^3 tent producing 70W of heat of which 95% is absorbed by the sleeping bag/air molecules tent fabric etc. (essentially 5% of the heat is transferred to the water), the storm lasts 2 hours, there is no ventilation, but the water vapor can transfer through the fabric @ 100% efficiency and the heat transferred to the water is 100% efficient.

Basically over the two hours not enough heat is produced to completely heat and vaporize (⌂H of vaporization is too large) the water entering the tent so the temperature depression is about 8°C. Obviously the assumptions were not real world and made to make it easier to think about/calculate (esp. considering getting some of those values would be pretty difficult). So it does seem significant, it would be interesting to see the affect of a double wall vs a single wall.

[David Olsen]

The 1973 MSR mountain tent was a two layer tunnel tent with foam spacers to keep the two fabrics
separate. The out layer was coated the inner breathable. This was supposed to keep condensation to a minimum in a variety of weather conditions.

===

I had a foam laminated rain parka that was also supposed to reduce condensation.
It didn't work nearly as well as Goretex in my experiance.

—

Remember too that some high altitude tents incorporate a third inner frost liner to
further reduce condensation from reaching the occupants.

===

Where will this all lead?

[Daryl and Daryl]

Do you know if anyone still makes the foam laminated rain gear?

I also remember MSRs tent with the foam spacers. I think Larry Penberthy strapped it to the top of a vehicle and drove down the freeway to test wind resistant. It weighed about 9 lbs if I recall correctly.

[Daryl and Daryl]

I experience very little condensation within the inner tent when I use a small all fabric tent under a rainfly. I think it is because I can heat up the inner tent so easily. While awake I can usually get it up to the 60-70 F level, even if it is near freezing outside, by simply moving around inside. At night I usually maintain a 10-15 degree differential.

I'm guessing here and don't have the expertise of some of the great responses above but I think two things are happening. (1) The heated air within the inner tent is able to pass through the inner tent fabric without loosing its moisture (it then condesnses on the fly). (2) The heat within the inner tent drys some of the inner tent fabric that does get damp.

I do experience condensation on the bathtub floor walls and, for this reason, stopped including them on my homemade tents. I do experience a little condensation near the peak of the inner tent after I go to sleep and things cool down.

Daryl

[jim jessop]

'All of the above makes a good case for a double walled cuben tent under wet conditions regardless of whether the fabric is highly waterproof or not. The area between the two layers should be sealed to prevent heat loss and condensation. The venting should be to the inside of the tent, only.'

This is what the Stephenon's Warmlite system does, though in sil rather than cuben.

I'm awaiting a mesh inner with a cuben bathtub floor. It will be ironic if I have to add a layer of polycro for it to be waterproof, though in other respects, still worthwhile.

[Richard Nisley]

Dan,

Send me a forum email and I will give you my information so you can send the scraps (at least 6" on the shortest axis) to me. There are two questions that we need to answer, 1) how does each material in its virgin state test and 2) how does it test after it is conditioned 8 times? You only need to let the sample dry after the 8 cycles. It is standard industry practice to just use a washing machine's normal wash cycle, without soap, to condition a sample. It crudely simulates a series of thunderstorms, the stuffing, and then un-stuffing of the shelter. Label each sample with a permanent marker as to what type it is and if it is conditioned or not.

We also need to get samples from some source to Roger. Hopefully with BPL's clout, Cubic Tech will send a complete set of every possible sample to him. It is important for testing credibility to have at least two independent set of tests.

[Roman Dial]

The Cuben Mid that Dave Olsen of Oware put together for the Arctic 1000 is still in use. I use it for my super-special trips. My Sil Nylon Go-Lite hex-mid (forgotten the name) is my "beater" shelter for long, abusive trips. We took it on a six month trip around the world, for example, and to the tropics where mold can be an issue.

Michael Brown and I shared the cuben tent (and when I say tent, I mean single walled, un-floored shelter) while we joined Andrew Skurka in the Brooks Range and the Wrangells last summer.

If you asked what tent would I take to a really windy, hard-driving rain turned to snow-type place (Alaska Peninsula, New Zealand west side Southern Alps, Patagonia, Iceland), I'd be equally willing to take either one and be absolutely certain to bring a synthetic bag and a stove I could hold in my lap. Both tents will be wet inside and out from rain and condensation and then packing them wet. Both will be stuffed wet into my pack and then set up wet the next day if the weather stays crappy. But both will protect me far better than being in the rain or huddling under a bush or shivering in a bivy bag.

Both seem to take an inordinate time to dry out during a sunny high latitude morning after a clear night w/lots of condensation — and I dislike drafts so I always pitch my tents close to the ground and block any drafts and have lots of condensation — in contrast to Skurka who pitches high to allow drafts to blow through, but his physiology is not really human, after all, but more arteriodactyloid and he sleeps particularly warm, stationary, and chewing his chocolate cud should he get chilly mid-way through the night

Roger and Richard are amazingly thorough scientists and statistical outdoorsmen, but that Oware cuben fiber mid, now five years old, remains my favorite shelter, and while it may be statistically wetter than when I first used it, it is not practically different in any sample of windy rain-storms than Silnylon, in my experience.

Why not? Because most of the variability in moisture-related misery or discomfort inside shelters for me is due to interactions between condensation, humidity, wind speed, temperature, falling precip, and how well the tent was pitched, not to mention how tired, hungry, thirsty, frightened, and close I sleep to my tent-mate.

The images that Richard shows of water on the fabric in his YouTube video I have seen on my cuben tent in its older age (too busy to have noticed it its youth on the Arctic 1000). But it doesn't drip from the walls. It just rolls down and drips from the bottom edge to the earth. Just like heavy condensation would.

As older and wiser men have said, "It's really no big deal."

However, making dry-bags from it seems like a mistake.

[Steven Paris]

"His physiology is not really human, after all, but more arteriodactyloid and he sleep particularly warm, stationary, and chewing his chocolate cud should he get chilly mid-way through the night."

Change the words to first-person and Andrew Skurka could use this for his Match.com profile; it's a very flattering description!

[dan mchale]

I'm doing some what I would call real world testing of some heavier Cuben today, fabrics in the 4 oz/yd relm. I think in general, we will discover in this current post, the irrelevancy of the hydrostatic testing for these applications. I have even gone to the extent of disassembling the Cuben into it's constituent layers and they don't even leak. I don't think the 'dirty little secret' is going to have a very long life but that's just my humble opinion. Oh, and I have a microscope as well. I don't buy Richard Nisley's blurring of the boundries of leakage and condensation either. Related to that he says, "A user in the field has no way to determine which micro-droplets are from condensation versus which micro-droplets are from a low hydrostatic head." He arrives at this conclusion through a totally irrelevant and inappropriate test that totally ignores Cubens other unique qualities and real life performance. What, are we all supposed to believe that the condensation we get is actually leakage? This is non-sense.

Nisley even says, "Unless Cubic Tech or Mountain Laurel Designs can solve the problem with the poor hydrostatic head performance of my MLD Cuben Grace Duo I will probably never buy another Cuben product."

As somebody that has been working with Cuben for at least 4 years I did not think I would end up having to defend it's waterproofness traits.

[Greg Mihalik]

Dan,
Others have mentioned the observed waterproofness of the heavier (1.26ounce) cuben, for instance, when kneeling on wet ground. And I believe initial testing of lightweight virgin cuben also showed good integrity.

For many on this forum this question is particularly relevant to the .35/.75 ounce materials commonly used in shaped and flat tarps. I have had a personal communication where it was possible to squeeze water through a lightweight cuben tent body that had seen many nights of use. They formed a depression, added water, gathered into a ball, squeeze/twisted, and watched drops emerge.

Have you tested any "worn/used/multiply-stuffed" samples of the 0.75, or less, weights?

Thanks.

[Martin RJ Carpenter]

Seems like some (heavier?) Cuben might do better initially – Terra Nova are claiming a 3000 hydrostatic head for their new ultra stuff which certainly looks somewhat like some sort of cuben relative at least.

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