Companion forum thread to:

Condensation in Single-walled Shelters: Contributing Factors and Tips for Reduction

I can't believe Big Sky is used for comparison.

Excellent analysis of the condensation problem. I love the graphics!

One of the tricks that Ive found is to allow for ventilation as much as possible, cook in the vestibule so steam isn't vented into the tent, and to keep your head near the slightly open door, so most of your exhalations don't have to travel through the tent on their (hopefully) way out. -Migs

Will,
Another great article; BPL is very fortunate to have you on staff.
You mentioned with some surprise that the tent wall cools below ambient(inside tent?) temperature; but did not mention a significant process contributing to that.. As moisture wicks through to the outer surface of the tent, it evaporates in the drier air outside the tent and takes with it the latent heat(enthalpy) of vaporization; this is a huge amount, about 2200KJ/kg of H2O. As the water vapor takes the heat away, this process cools the tent surface and its boundary layer.
Even in the hottest conditions, we can make use of this evaporative refrigeration by putting a wet cloth around a canteen, ourselves, etc..
Now, how to use this effect to best use; if you have a WP/B tent, CLOSE the vents incrementally until condensation stops and you have a minium of airflow. My epic BD tent breathes so well with two occupants I close the vents completely in the winter. I do not want any of the heat to escape (since I'm in a 1 lb UL bag!), and in fact any cool air entering the tent would only lower the dew point and cause MORE internal condensation. Closing the vents as much as possible with an epic tent increases the vapor pressure, and forces moisture to transport through the fabric and evaporate on the outside. ( If you were getting condensation on your BigSky; I suspect you might have had a vent open? I don't know that tent, can you close it up? Of course if you are in a 3 sided tarp tent you can't close it up, nothing you can do but wipe occasionally..
And of course, if conditions at the fabric surface are not near the dew point, of course open up the vents and enjoy the breeze, surface condensation won't occur.
This explanation is pretty much at the limit of my chemistry knowledge; I haven't figured out how the latent enthalpy of fusion on the inside tent surface works into this process.. anyone else?

Edited by Brett1234 on 06/20/2007 09:45:57 MDT.

Terrific article. Site selection and venting are SO key. His glossary of terms is invaluable to future discussion of this issue.

I have had better luck with Epic shelters than Will apparently has----never had the complete wet-out he describes, therefore I've not experienced the fabric not "breathing".

The photo of a Big Sky Epic tent instead of one of the BD or ID Tents to represent Breathable Fabric Tents was somewhat distasteful and unrepresentative of the genre.

I'd be interested to know the reference for the quoted respiratory water loss. Every human physiology textbook I see quotes total body water losses (from kidneys, skin, lungs, GI tract) at 2,500 ml/day with respiratory losses at about 300 ml/day. Losing 2 liters of water only from the lungs per day seems massive and certainly not the average.

I know some of the outdoor texts must quote this, but why is beyond me. Of course with exercise those numbers go up a bit, but the massive numbers quoted by outdoor organizations seem out of reality.

>Losing 2 liters of water only from the lungs per day seems massive

Good point. Your urine would be nuclear orange come morning if you lost 2L every night.

Edited by tlbj6142 on 06/20/2007 14:33:28 MDT.

Not to mention weight lose. A person would be 4lbs lighter from just breathing.

Edited by SmokiesHanger on 06/20/2007 14:53:56 MDT.

Will,

Great artcle! Condensation has put me at risk from cold before in unexpected snow where my down bag loft was greatly reduced. It is good to understand it better.

Like Brett asked, when you say, for instance, "the shell of a single-walled tent actually cools below the ambient temperature", do you mean the temperature inside or outside of the tent?

Sorry for the confusion, and thanks for setting me straight.

I always use the term "ambient temperature" to be the current air temperature, and in this case I mean outside the tent.

On the water loss question, I believe the 2.2 to 2.5 liters is the total water loss per day, so breathing is just part of that amount.

Will

In my little experience there are some situations where condensation will just happen. I slept at a tentshow once in early spring. During the day it was nice and sunny and warm (high rel hum) at night it was near freezing (low rel hum) and all that daytime moisture had to go somewhere and it condensed everywhere, even on my tarp.

Now I have a question for more experienced UL-ers. It sometimes happens that on getting up my poncho tarp is wet on the outside as well as on the inside. If it's raining I need to wear the poncho as raingear, but since it's already wet you'll get wet inside.

Is there a good tip on something to do about that or should I just learn to live with it?

Eins (EDIT: great article BTW)

Edited by EinsteinX on 06/21/2007 07:46:11 MDT.

Personally, it's been one of the reasons I use dedicated raingear and a dedicated tarp. But, people can and do make do, wiping down the interior of the poncho-tarp and perhaps wearing over a windshell, generate some body heat to dry that sucker out. Clothing had better be well selected to meet such a scenario.

I know, I know. I'll never be able to achieve a sub-4# base weight. :-)>

Edited by kdesign on 06/21/2007 08:04:47 MDT.

Eins,

I haven't had the wet inside AND out experience on my poncho-tarp yet, but I have given that some consideration. I plan to just wipe down the inside with a bandanna like I have alway done with single wall tents.

In my experience, even a worn poncho can occasionally get a little wet on the inside from perspiration when it's reasonably warm and/or humid, so starting damp might not be that much different. It is still bound to be dryer than most rain jackets when it's warm out.

This article was very useful in its application for single and double walled shelters. I was hoping to have some of this info applied to the tarp-bivy combo that is so common amongst the editorial staff. This article has raised several questions, and I would appreciate comments.

1) Does a tarp-bivy combo create a double-wall shelter effect? I've often thought it does, but the article states that the important factor in a double-wall shelter is the "semi-dead" air space between the layers. With a tarp and bivy, this space is considerably larger and more drafty. Does this defeat the double-wall effect? In other words, is a tarp-bivy combo just two single-wall shelters, both prone to condensation, or can a tarp, even several feet from a bivy, create a warmer layer of air, enough to keep the surface of the bivy above the dew point?

2) Would using a more enclosed tarp help or hurt the situation? There are contradictory ideas here: on the one hand, a more enclosed tarp (i.e. one in which all sides come near the ground, such as a Gatewood Cape or GG Spinnshelter), would create a better insulating layer between tarp and bivy. This should create "semi-dead" air space similar to a double-walled shelter, thus preventing the bivy from reaching the dew point. For WP/B bivys, this should also help maintain the more effective warm-weather breathability characteristics. On the other hand, the relative lack of air flow would promote vapor buildup, thus increasing the overall chance of condensation. So would it be better to use a bivy with a breezy tarp to encourage evaporation, or would it be better to use a more enclosed tarp to prevent dew-point condensation and promote fabric breathability on the bivy? Or, is it a totally different environment, since one would typically not be breathing into a bivy in this scenario, the way one would in a double-wall tent?

3) This article claims that eVENT is the only fabric effective at a wide range of temperatures, including near freezing, thus keeping the fabric drier. Can this be applied to bivies? I mean, if it has been pouring rain for hours, can an eVENT bivy without an overhead tarp stand a chance of maintaining breathability characteristics?

I personally use a Nemo GoGo bivy that I have modified to bring it's weight down to 25oz (hint: use the long pole from a BD Lightsabre instead of the Airbeam). Though it serves me well as a stand-alone shelter, this article has made me wonder if I should use a small tarp above it to help with condensation (I currently use a small custom awning, but it does not cover the body of the bivy).

And while we are on the fabrics topic, as far as I can tell, the Nemo fabric feels identical to the 2-layer eVENT found in Mountain Laurel Designs products. I would like to see this fabric properly tested against both 2-layer and 3-layer eVENT. I would also like to see 2-layer eVENT reviewed. Because the inner layer is glossy, I wonder if it is more prone to condensation buildup (the inner textured layer of 3-layer eVENT seems like it would hold more moisture, and it's rough surface might reduce condensation buildup). Some apples-to-apples comparisons might prove fruitful =).

Edited by jcarter1 on 06/22/2007 02:58:30 MDT.

John,
I'm looking to replace the airbeam on my Nemo as well. How long is the Lightsabre pole & how close is the fit? Did you cover the ends of the pole to keep them from rubbing holes in the tent floor?

Henry,

Nice to see I'm not the only GoGo user in these forums! What I did is a little complicated to explain, but very easy to do. It involved rearranging the order of the poles, and using some of the pole segments from the smaller half-arc. I ended with a perfect fit with 3.8oz of pole. This removes the weight and clutter of the airbeam, the foot pump, the spare bladder, and the bladder repair kit. Here goes:

The Lightsabre comes with two sets of poles: the full arc that goes from shoulder to shoulder and the half arc that goes behind your head. The full arc pole comes in 7 pieces; 4 curved segments (2 on each side), 2 straight segments (the lowest 2), and a small v-shaped segment in the center top. The half arc has 2 curved segments, 1 straight segment, and a V-shaped segment. Here's the trick: the V-shaped segment on the half-arc is less angled than the V-shaped segment on the full arc. If you use the V from the half arc, you get the proper curve for the GoGo.

Start by removing one of the metal end pieces (they easily pull out) and untie the bungee cord knot on both poles. This will separate all the parts. Then arrange the poles as follows:

Connect 3 of the curved poles, then add the less-angle V-shaped segment (from the original half arc). Then add another 3 curved poles. Insert the bungee through the arc. Unfortunately you will not be able to use the metal end piece, as it will be too large for the end of the curved piece (remember, the original full arc ended in a straight piece, which has a larger diameter at one end to accommodate the metal end piece). You will notice when you disassemble the half arc that the same problem occurs, and BD's solution was to tie off a small washer onto the cord, which fits into the pole. Use this washer to tie off the cord at the end of the last pole.

As for protecting the corners of the GoGo, I attach the caps from my hiking poles. I have two kinds, the small plastic caps that came with the poles (0.1oz pair) and a pair of Leki rubber walking tips (0.8oz pair, sold at REI). Though the Leki tips are heavier, I much prefer them, as they grip the silnylon floor and stay in place, and provide great protection from the pole ends. To keep them on the ends during assembly, I have wrapped some duct tape to make a larger diameter. And now I have some more emergency duct tape!

I have found that this setup stays in place so well that I do not need to worry about strips of Velcro to attach it to the GoGo Velcro tabs.

Here's where it gets even better: I discovered that two of the curved poles fit perfectly into the slots of my SMD Starlite backpack. They are the exact same length as the curved aluminum rods, and the exact same curvature. And because the V-shaped curve is so small, I am able to fit everything perfectly into my backpack (see photo below). I’m pretty sure this would work in a Mariposa as well. This shaved an additional 5 oz out of my total gear, making a total weight savings of 10oz! Pretty impressive for such a cheap accessory. Here are some pics:

This shows the custom pole and the remaining pieces. You can see that the non-used V-shaped segment is too angled. I've also included the aluminum stays from my Starlite to compare the curve.


This shows the curve of the poles from the back of my backpack.


Here's how I get the whole pole to fit into my backpack. You can see that the lower poles are inside of the Starlite's stay holders, and the upper parts of the pole drape down into my bag, out of the way. This wouldn't be possible without the smaller V-shaped segment. I imagine you could bundle this up into a foam pad and use it in a backpack without the aluminum stay holders, but this particular backpack hold everything together nicely.


Here's the pole in the GoGo. It's loosely setup with no stakes, but you can see the perftectly taut arc.


Finally, Here's the inside, showing the rubber hiking pole tips. I like how the pole fits into the long, narrow black sylnylon segment (where the airbeam normally goes). This keeps the pole away from any seams, which might cause leakage under such tension.


You can order the poles through BD's website for $25. Hope this helps! I'll be happy to answer any questions if none of this makes sense when you attempt it.

Edited by jcarter1 on 06/23/2007 14:26:33 MDT.

John,
Pure genius! I can't wait to try. I have not had great luck with the airbeam staying upright w/o guying out the beam section of the bivy. The beam always wants to fold. This looks like a great solution. One of the left over pole sections looks like it might be long enough to support the swallowtail (foot end) of the bivy as well. Thanks for the detailed instructions.

Will - nice job, very informative article. I don't have nearly the experience you do, and most of it is in drier conditions, but when I have been in more condensation-prone conditions, my experience tracks yours. I appreciate the clear analysis and recommendations for mitigating the effects.

While everyone seems to agree that perimeter venting is essential, there doesn't seem to be unanimity about peak vents.

Will clearly believes they're important, as does Roger Caffin. Ryan Jordan seems more ambivalent, and if I recall correctly Roman Dial finds them unnecessary in pyramid shelters. For the record, Ryan's Alphamid has a peak height of 49", while Roman's pyramid was considerably taller.

Thoughts and opinions??? Has anyone done some controlled testing?

Edited by chris_jackson on 06/25/2007 00:19:09 MDT.

In Mariah Walton's superb 2004 BPL article onNight Time Condensation on Tarp and Tent Fabrics, she first determined that tarp-walls cool below air temperature because they emit infrared radiation. She subsequently followed up this observation by testing several aluminized materials, such as a mylar blanket, that do not emit in the infrared. Theoretically this means that these materials do not cool below air temperature.

In practice, there was no condensation on the mylar blanket during the twelve night testing period!

If one takes Mariah's study to its logical conclusion, the use of aluminized materials in tarps and also in the shells of sleeping bags has great potential for reducing the frequency of condensation. In a sleeping bag, increasing the shell-temperature by about 4F might also result in a warmer bag. (Obviously the aluminized shell would have to be breathable.)

Interestingly, Stephenson Warmlite use aluminized materials in their shelters and sleeping bags, but on the interior rather than the exterior. Mariah's results clearly show that there would be a significant benefit to having aluminum on the exterior.

Mylar (aluminized polyester?) would be particularly easy to incorporate in cuben fiber, since cuben is formed from a laminate of spectra fibers and a thin (polyester?) film.

Will - did you perform any testing on aluminized materials?

Perhaps BPL can bring out a line of condensation-resistant materials and products???

Edited by chris_jackson on 06/25/2007 00:22:05 MDT.