May 16, 2014 at 12:14 am #1316868
I'm looking at making or having made an inner net with raised side walls 12" high for my Duomid, if I use a breathable DWR nylon will that be less prone to condensation than a silnylon or cuben fabric. Because of my height my sleeping bag sometimes contacts the inner wall so I'd like to minimise any condensation that might accumulate on the inner fabric.
Just to add, I have read some of the articles on condensation but there are aspects I still don't understand. Just in case someone wants to refer me to other previous posts.May 16, 2014 at 8:00 am #2102849
Marko BotsarisBPL Member
@millonasLocale: Santa Cruz Mountains, CA
I think so, to a certain degree, just based on my experience with a Firstlight. But this tent has is a VERY breathable fabric, and suffers from not being %100 waterproof under all conditions. It has been condensation-free when other peoples shelters around me had a bit. Also, and this is not scientific, it certainly "feels" less clammy inside on morninsg when I'm sure a non-breathable shelter would feel a bit misty.
As always, the answer totally depends on your environment. But as people point out, the "R" in the DWR stands for "repellant", not "proof" and may fail you when you need it most. That's why most people opt for more ventilation rather than play with the edge where the water resistance is questionable. It may be the kind of a situation where the breathable shelter works best in conditions where you need to worry about inner condensation least, through there might be some useful overlap.
From the surface physics standpoint (as opposed to lowering the humidity inside the shelter) hydrophobic surfaces where water will bead should encourage condensation since condensation from water vapor occurs by nucleation – the microscopic beads encouraged by the hydrophobic surface can become stable and then grow. Once the surface to volume ratio of the beads becomes small enough you will get stable condensation. Moral of the story – put the DWR finish only on the outside, not the inside. Spray on, not wash in – if you can.
If you are only talking about a tub style floor for your screen, then the breathable aspect should be next to nil for lowering the net humidity. Breathable would be irrelevant, except as an accident. Maybe use something cheaper if its has a less hydrophobic inner surface.May 16, 2014 at 11:58 am #2102963
kevin timmBPL Member
@ktimmLocale: Colorado (SeekOutside)
We use breathable nests in our tents. They could be called 4 season by most standards. In weather below freezing they will condensate a bit, especially from your breath.May 16, 2014 at 1:02 pm #2102988
Given what you are planning on doing, I believe are doing the "right thing" and it will likely help keep some of the condensation off the 12" side inner walls. Of course it ultimately depends on the dew point.
The more "inner wall" you have, the more you create two distinct atmospheres. Of course you then ultimately end up with a double walled tent.May 16, 2014 at 6:37 pm #2103141
The reason I ask this is that I have a friend who uses a Trailstar, his inner net has raised side walls made of cuben and he has problems with condensation forming(in the right conditions) on the inside of the walls. My thought was that a more breathable fabric would be less prone to condensation due to its permeability, am I on the right track here?
The raised walls only have to minimise wind and rain splash back into the inner net, I'm not looking for heavy weather resistance.
If the breathable fabric will still accumulate condensation to the same degree as other options then I may as well go for cuben or silnylon which will offer greater resistance to the elements.May 16, 2014 at 7:18 pm #2103165
David OlsenBPL Member
@bivysack-com-2Locale: Channeled Scablands
Condensation and even frost can form on pretty much anything given the right conditions. However, moisture can evaporate through a breathable material, so in most conditions, things can dry out.May 16, 2014 at 7:33 pm #2103171
If the tent is under a clear sky, the outer material can be 10 degrees colder or more due to radiant heat loss. The inner material is not subject to this radiant heat loss so it will be warmer. Then, water vapor will condense on it less.
Some inner tent materials are like a mesh – ultra-breathable. Not much fabric for the water vapor to condense on. This would be more likely a 3 season tent.
Sometimes the outer material will have condensation on it, but when you touch the inner material, it's like a warning not to move any closer to the outer material and the condensation.May 17, 2014 at 4:02 am #2103256
I have read a few articles regarding Dew Point, but I still haven't had that "Ahhh, I get it!" moment where it's reasonable to my mind. Could someone please try to explain it in simple language, I really would appreciate it? Or if that's too difficult perhaps point me to some articles that are easily accessible.
DannyMay 17, 2014 at 6:54 am #2103280
The colder air is, the less water vapor it can hold. The humidity is the percent of water that the air can hold – e.g. 100% means the air has all the water it can hold
If air contains some water vapor, and it's cooled, the humidity will increase – since the amount of water vapor is the same, and the amount of water vapor the air can hold decreases, the percent of maximum increases.
When the air is cooled until the humidity is 100%, that's the dewpoint.
If you cool the air further, then there's more water vapor than the air can hold, so the excess water vapor condenses out of the air as dew or rainMay 17, 2014 at 7:34 am #2103300
Jerry that's great, I understand what you mean. Now I need to apply this to exposed surfaces and how they will effect the dew point, I can see this is going to be pandoras box. Already I have more questions, I need to think a little to formulate my questions clearly.
Thanks you.May 17, 2014 at 7:45 am #2103302
First off, just know you are not alone, friend. I believe many of us don't quite "get it" when it comes to dew point, and the whole vapor permeable "breathable" thing.
(I know it took me a while before I "sort of got it".) For example, In the history of building construction, it has probably only been discussed for about 50 years.
Since I work around the building science world, thats where I usually find my sources to describe the mechanics of dew point & vapor drive.
Here's a PowerPoint that might help:
Anyway, an analogy I use about "dew point" is that of holding a sponge over running water. There is a "point" where you have filled the sponge up with so much water that it cant hold any more and starts dripping out.
That's essentially what is happening in our air, but it is being "crouded" with water vapor (the gas state of water). And our air's "sponginess" is all determined by the temperatue, humidity, and pressure at a given point of time. In a nutshell, the colder and more humid the air is, the sooner that dew point will occur.
I realize that understanding dew point means understanding other aspects of vapor transmission as well. I can ramble on, but I hope this helps a little.
MattMay 17, 2014 at 6:04 pm #2103476
Thanks for the link Matt, so is the reason a breathable fabric is less prone to condensation build up due to the air permeability of the fabric. For example, if you had a low air permeable fabric such as silnylon, as the air permeates through the fabric it's molecules will be required to compress causing the vapour molecules to be condensed into a more liquid state, whereas a more permeable fabric such as a standard DWR nylon would allow the air to permeate through the fabric more readily requiring less compression and so the vapour is less likely to form into a liquid.
I have a lot of questions but I want to go slowly and make sure I'm getting this.
Cheers.May 17, 2014 at 6:29 pm #2103482
Not a physicist, but I'll try my best here.
To answer your most basic question, does the breathability of a material change the rate of condensation, the answer is yes and no.
If you look at something called a phase diagram, you'll notice that the state of a particular matter (in this case water) depend on two things: pressure and temperature. As pressure rises, given a constant volume, the matter turns from gas to liquid to solid (think of propane in a tank). Similarly, an increase in temperature causes a matter to go through the same states (ice cube on a hot surface eventually evaporates). Condensation happens when a mixture of gas containing a certain percent gaseous water comes into contact with a cool surface. The surface causes a decrease in temperature at that interface, causing a change in matter state. In this sense, no, the permeability of a material does not change the condensation. The only way to change this would be changing either the temperature (such as defrosters on your car or by a double wall tent serving similar to a double wall thermos) or the pressure.
And that's where the answer turns into a bit of a yes. Pressure is a function of volume, but in this case, not of an enclosed area, but of concentration. Imagine if you will, you have 10 molecules of water vapor in two containers, one is 10mL, the other is 1000mL. While both are at atmospheric pressure (i.e. the typical sense of the word in mmHg), they are at different concentration pressures – i.e. there is 10 part million in the smaller jar and 0.01 parts in the larger. Despite the equal temperatures, the larger jar will form less condensation per given area, as there is less water vapor per area. Matter though favors a state of equilibrium (which is why things like smells eventually disperse into an entire room). Increasing the permeability of the container (in this case fabric) can either help or hurt you. If the exterior humidity (measure of water vapor in air) is greater than the interior, excess moisture will move inside, and condense on the interior wall. If the humidity inside is greater, than visa versa.
I would imagine if there was a hard summer rain, a breathable fabric may not help that much (though I have not tried this out for myself). In something like winter, where the air is typically rather dry, it should help more.May 17, 2014 at 7:17 pm #2103490
I don't know that breathable fabric would have less condensation
Breathable fabric is good for clothing, where you're sweating, and there's a temperature/humidity gradient from the inside to the outside, so your sweat will go through the fabric and exit
In a tent, if there was air flow through the fabric because of wind, that would carry some water vapor away. You exhale a lot more water vapor than you sweat. Or if you have a tarp with raised edges or an open door, there will be air currents that carry water vapor away so it will condense on the inside of the tent less. Even a breathable fabric will impede air flow a lot. I have a double wall tent with waterproof outer and a netting inner – that would let a lot of air flow and there's a big gap under the waterproof outer material.May 17, 2014 at 8:00 pm #2103496
"Thanks for the link Matt, so is the reason a breathable fabric is less prone to condensation build up due to the air permeability of the fabric. For example, if you had a low air permeable fabric such as silnylon, as the air permeates through the fabric it's molecules will be required to compress causing the vapour molecules to be condensed into a more liquid state,"
No, pressure inside the fabric cannot exceed the pressure outside of the fabric. Yes very large pressure changes can cause condensation. but hikers never experience anything close to the pressures needed to cause condensation.
for starters silnylon and cuben are not impermeable fabric. The space between the fibers of nylon are filled with silicon rubber. Air and water cannot pass through. Cuben has plastic fibers sandwiched between to layers of plastic. Cuben has no holes for air or water to pass through.
Think of a glass of ice water on a hot humid day (dew point 60F. the surface of the glass is 32F so when the humid air makes contact with the glass it is cooled to less than 60F and water will condense on the glass. The same thing happens inside a tent. The tent gets fabric gets cold. Also as person breaths inside the humidity increases with each breath. The cold fabric with humidity inside can cool the air below dew point causing moisture to condense on the fabric.
There are basically two ways to deal with condensation:
1. Heat the air inside the tent so that it is always warmer than the fabric.
2. Vent the tent so that the moisture level inside the tent are the same as outside.
For item 2 a air permeable fabric will allow more moist air to escape that silnylone or cuben. Most people just open vents in the tent. This works as long as the air outside the tent stays above the dew point and you have air flow (wind). If the air temperature outside drops to below the dew point condensation will occur outside and inside the tent and you will get wet.
For item 1 most peaple don't carry air heaters in their backpacks for good reasons.May 17, 2014 at 8:15 pm #2103503
Ok so now I'm a little confused again. How about this example, if I have a raised side wall on my inner net under my duomid, lets say 12"-15", why would condensation form on the inside of the raised area of the inner net walls. The atmosphere underneath the shelter and within the net is the same as outside the shelter, there is air moving freely under the shelter walls. The temperature of the raised walls of the inner net couldn't be significantly different could they?
I don't know why I'm finding this so difficult to grasp.May 17, 2014 at 8:27 pm #2103507
If you're asking why it forms on only the raised area, could be due to a few things:
1) Differences in specific heat among fabrics
2) Density of water vapor relative to air
3) Permeability and static surface of solid vs mesh structure
If you're asking why it forms at all, it's due to differences in temperature between air and surface. The only way to eliminate this is eliminate the temperature difference or the substance pressure (aka buildup).May 17, 2014 at 9:25 pm #2103522
When you say raised sidewall, do you mean it's a waterproof material?
When you exhale, the air is fairly warm and contains water vapor. If this air drifted to a cold surface, it would cool below the dew point, and water would condense out of the air onto the cold surface.
Sort of like if you breath onto a mirror, water will condense on it.May 17, 2014 at 11:11 pm #2103536
I have a friend who uses a Trailstar, underneath that he has a inner net with 12" raised tub floor made of Cuben. I was going to make or have made the same style of inner net, he told me that it may be better to have a breathable fabric instead of the cuben as he experiences a lot of condensation on the inner surface of the cuben material. This is why I wish to understand condensation and how it comes about in a little more depth, I would like to make informed decisions and choices.May 17, 2014 at 11:38 pm #2103539
Franco DarioliBPL Member
Danny wants to have an inner of this type for his Duaomid :
(waterproof floor ,high wall with net top)
His question is : should the "high wall" bit (not the floor/not the mesh top) be in Cuben (as in this PyraNet2 from Bearpaw (http://bearpawwd.com/shelters_floors/pyraNet2.html)or in silnylon or should that wall be made with breathable fabric ?May 18, 2014 at 6:48 am #2103573
Is there more condensation closer to your mouth? That would be consistent with water vapor from your breath. But your humid exhalation air can drift all around. Having a bathtub sort of contains the humid air.
I don't think breathable fabric would make much difference, not breathable enough.
Maybe don't make the bathtub so high? The mesh upper is more breathable, and it's mostly air/little fabric for water to condense on. Why do you make your bathtub so high? Is it splash from the sides? If you're worried about blocking a stream or puddle, maybe it's better to chose a site better.
The basic principal of humidity increasing as it gets colder is straightforward, but from there it's not obvious and you have to do experiments.
I have noticed that for my sleeping bag within a foot of my mouth, regular slippery nylon condenses more, supplex nylon less. Supplex has sort of a cloth like feel, something about the micro texture. But it's sort of heavy.May 18, 2014 at 6:56 am #2103577
"There are basically two ways to deal with condensation:
1. Heat the air inside the tent so that it is always warmer than the fabric.
2. Vent the tent so that the moisture level inside the tent are the same as outside. "
There is a third:
Use matereials which better "manage" vapor transmission and condensation.
Years ago we were camping at a site somewhere close to Big Sur, and the Pacific Ocean up on the top of a hill. My wife and I were in a Bibler Eldorado (I admit-overkill), and our friends were in a typical double walled tent. Although we went to bed on a dry, clear night, we woke up to siginifcant fog in the morning. Our friends tent had significant condensationon on both surfaces of the tent fly. (They even had some on the inner wall.) The Eldorado was dry to the touch on the outside, and a very tiny bit slight dampness on the inside. The Bibler Toddtex fabric is unique due to the fact that it is designed with both a decent perm rating to allow for reasonable vapor transmission, and provide for a measureable amount of condensation management, should it happen to begin to form. This is achieved through the use of a thin "fuzzy" yet hydrophilic inner fabric. In my experience, the fuzzy stuff works brilliantly and I have had very few issues of interior condensation in the tent (except for when I was on those cold/humid trips.)
Ok, to the point.
As I suggested early on, I believe you are on the right track is because of the following:
1) the typical "inner wall" ripstop nylon I think you are suggesting to use will likely have properties which not only allow for actual CFM (unlike Syl & Cuben) but will also allow for better condensation distribution on the fabric itself, due to its constitution (again, like a sponge.) Although mosquito netting has an insanely high cfm, it has very low absorption properties so it actually performs poorly at condensation management, imho. Personally, when I use the term double "walled" tent, I don't normally think of tents where most of the inner wall material made of netting, because the dominate use of netting will negate the primary advantage of the inner wall system.
2) There is usually both a wet bulb and dry bulb temperature difference between the top inside surface of the tent fly, and the ground, as well as at the center point where you sleep and the ground around the edges of your outer fly. All these temperature differences will create the opportunity for vapor to move from both warm & moist to cooler and drier. This is in-part why air circulation occurs, as was mentioned before. But because the nylon walls will allow for a (slightly) warmer climate in the space where you are sleeping, the vapor will also drive out to the coldest areas around your shelter: typically down at the ground surface outside of your inner walls. This will occurr until equalibrium/saturation is reached. If this material were less permeable, like your friend's Syl / Cuben, the vapor will quickly condense on the surface. I'd wager that the Syl might absorb some of the condensation (due to its properties), but not the Cuben.
3) Don't forget that, although we are usually the hottest/most humid things around when camping, when it is very hot/humid or cold/humid, all this vapor drive is not going to help too much, since there ain't any more room in the air for your vapor to take a ride on. And that's why dew point is reached faster in these conditions.May 18, 2014 at 7:01 am #2103578
"This is achieved through the use of a thin "fuzzy" yet hydrophilic inner fabric"
there's some other property of fabric than breathability that effects how water condenses on it
and people don't advertise it or test for it or anythingMay 18, 2014 at 7:19 am #2103581
Matt that's cool, thank you. I think I'm starting to get my head around this now. Thanks for taking the time on this, much appreciated.
Jerry, I wanted to raise the side walls of the inner net to reduce splash back, where I am there are mainly hard surfaces to camp on. Also to provide a wind barrier.May 18, 2014 at 7:42 am #2103583
Marko BotsarisBPL Member
@millonasLocale: Santa Cruz Mountains, CA
"If you look at something called a phase diagram, you'll notice that the state of a particular matter (in this case water) depend on two things: pressure and temperature."
I think the discussion in this thread is very interesting. I will just point out here something to keep in mind my 2 cents as a physicist. When you mention or otherwise use arguments concepts related to equilibrium physics – phase diagrams, equations of state, and so on you may be neglecting something very important here which is the non-equilibrium aspect. You can get a ton of information by considering equilibrium physics, and yet one (if not the main) "problem" here is the case where there is an overnight dip in temperatures, with perhaps already humid air in the environment, and made worse from the added water vapor coming off your body all the time. The water can condenses on the inside of the shelter, but of course it does not do so all at once.
If you are talking about subtle difference between fabrics (the breathablility of a "tub" floor for the (totally permeable) inner net probably is not going to greatly effect the humidity near that surface, then you are also talking about the speed at which the water vapor condenses, and not just whether water in bulk is stable at the current temperature and pressure. So a non-equilibrium issue. For example, some mornings go out and look closely at the dew distribution on different surfaces. Some will be dew free, and others not, in spite of the fact that the temperature and pressure is uniform.
The problem of condensation on the inner surfaces of shelters (at least once it start to happen) is largely one not of stable phases, but of the non-equilibrium process of changing phases – the way water condenses, and where, and the speed at which is does so on certain surfaces, with prevailing (non-quilibrum) air flow, and so on. Once you have neutralized as much as possible the increased humidity inside the tent (by ventilation) it is all about the PROCESS of condensation, not phase stability. In this case before all the water has condensed out of the air the temperature and pressure will change again when the sun comes up.
Just some food for thought.
Edit: For example I'll give you a more specific example. A droplet in (quasi) equilibrium with its environment will have at its surface a humidity gradient. Right near the surface of the drop the humidity will be very high, and then decay off and an some characteristic distance it will approach the humidity of the ambient air. It there is a breeze this "protective" distribution of humidity can be convected away so that the humidity near the drop is effectually lowered, increasing the rate at which water molecules leave the surface. This is a non-equilibrium process that we all know intuitively – ventilation make the humidity lower, but further a slight breeze will make it even harder (slower) for water vapor to condense.
A similar type of thing of course holds for heat (quasi) equilibrium – a breeze will make you colder much faster. It strip away your protective "skin" of heat near you body and makes the rate of heat lose greater. Again, something we all know from experience.
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