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Are breathable fabrics less prone to condensation.
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Danny Sharp
(Dannyj) - M

Locale: Australia
Are breathable fabrics less prone to condensation. on 05/16/2014 01:14:54 MDT Print View

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.
Thanks!

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.

Edited by Dannyj on 05/16/2014 08:28:04 MDT.

Marko Botsaris
(millonas) - F - MLife

Locale: Santa Cruz Mountains, CA
Re: Are breathable fabrics less prone to condensation. on 05/16/2014 09:00:22 MDT Print View

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.

Edited by millonas on 05/16/2014 09:54:24 MDT.

kevin timm
(ktimm) - M

Locale: Colorado (SeekOutside)
Breathable and Condensation on 05/16/2014 12:58:16 MDT Print View

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.

Matt Dirksen
(NamelessWay) - MLife

Locale: Mid Atlantic
Re: Are breathable fabrics less prone to condensation. on 05/16/2014 14:02:55 MDT Print View

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.

Danny Sharp
(Dannyj) - M

Locale: Australia
Re: Re: Are breathable fabrics less prone to condensation. on 05/16/2014 19:37:06 MDT Print View

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.

David Olsen
(bivysack.com) - F - M

Locale: Channeled Scablands
condensation on 05/16/2014 20:18:02 MDT Print View

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.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: condensation on 05/16/2014 20:33:36 MDT Print View

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.

Danny Sharp
(Dannyj) - M

Locale: Australia
Dew Point on 05/17/2014 05:02:23 MDT Print View

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.

Cheers

Danny

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Dew Point on 05/17/2014 07:54:54 MDT Print View

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 rain

Danny Sharp
(Dannyj) - M

Locale: Australia
Re: Re: Dew Point on 05/17/2014 08:34:20 MDT Print View

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.

Matt Dirksen
(NamelessWay) - MLife

Locale: Mid Atlantic
Re: Dew Point on 05/17/2014 08:45:48 MDT Print View

Hi Danny,

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:
https://www.aiha.org/aihce06/handouts/po109crawford.pdf

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.

Matt

Danny Sharp
(Dannyj) - M

Locale: Australia
Re: Re: Dew Point on 05/17/2014 19:04:37 MDT Print View

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.

Cameron Habib
(camhabib) - F
Re: Are breathable fabrics less prone to condensation on 05/17/2014 19:29:55 MDT Print View

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.

Edited by camhabib on 05/17/2014 19:31:11 MDT.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Dew Point on 05/17/2014 20:17:56 MDT Print View

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.

steven franchuk
(Surf) - M
Re: Re: Are breathable fabrics less prone to condensation on 05/17/2014 21:00:26 MDT Print View

"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.

Edited by Surf on 05/17/2014 21:06:09 MDT.

Danny Sharp
(Dannyj) - M

Locale: Australia
Re: Re: Re: Are breathable fabrics less prone to condensation on 05/17/2014 21:15:14 MDT Print View

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.

Cameron Habib
(camhabib) - F
Re: on 05/17/2014 21:27:41 MDT Print View

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).

Edited by camhabib on 05/17/2014 21:29:02 MDT.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Re: Are breathable fabrics less prone to condensation on 05/17/2014 22:25:13 MDT Print View

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.

Danny Sharp
(Dannyj) - M

Locale: Australia
Re: Re: Re: Re: Re: Are breathable fabrics less prone to condensation on 05/18/2014 00:11:18 MDT Print View

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.

Franco Darioli
(Franco) - M

Locale: Melbourne
Are breathable fabrics less prone to condensation. on 05/18/2014 00:38:43 MDT Print View

Danny wants to have an inner of this type for his Duaomid :

PyraNet 2
(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 ?

Edited by Franco on 05/18/2014 00:40:41 MDT.