This post is based on a post in the MYOG section which deals with various fabrics. I put it out here to get feedback from those who have far more real-world experience and those who have relevant backgrounds or those who have actually studied this stuff.

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I think a lot of the condensation issues (conjecture here) are due to fabrics being TOO breathable.

When you heat up the inside of the bivy, your heated air will try to rise out through the fabric. While we've been so focused on "outgoing air" we're forgetting that incoming air plays a factor too. Once we create the "convection cycle" of our heated our escaping and thus being replaced by outside air, in a cold environment we're actually helping to lower the dewpoint inside the system, and thus placing that point closer to our bag.

By using less-breathable WPB fabrics, perhaps we can use the "high pressure system" of heated air inside of our bags/bivvies to help force the moisture out. If the fabric is too breathable it cools the air (by allowing too much exterior air to permeate) before it escapes the exterior shell.

As warm air is less dense and tries to rise away from us, the further our system's exterior fabric is from our bodies the more risk we run of condensation as that air cools before hitting the dew point.

Proposal: Smaller, more form-fitting systems using WPB shells *may* be better than more spacious/airy systems of more permeable fabrics because we can force a heated high-pressure microclimate inside with the less-breathable fabrics. This leads to to the less-dense, moisture laden air (because heated moisture-laden air is less dense than heated dry air) to rise up (and out) without cooling the system down to the point where the dew point is inside the bag. Some transfer will still happen, but with a less-breathable system we'll transfer less air and the interior air which IS transferred ends up being the less-dense-because-it's-got-moisture layer.

SoTP conjecture, feedback solicited!

Thanks!

-mox

Note that this is in direct contrast to the "more breathability is better because we move more air through the system for drying" and seeks to explore the "quality of the air we're moving" instead of just the quantity.

-mox

No one has any thoughts on this, good or bad as it relates to sleep systems?

Edited by moxford on 11/30/2012 15:07:41 MST.

> I think a lot of the condensation issues ... are due to fabrics being TOO breathable.

No takers, probably because no-one agrees with the idea. Sorry.

cheers

Fair enough.

Interesting, at least to me, the old reports of all-eVent bivvies not having the same condensation problems as other bivvies in the group had. Or the HomeWrap Tyvek bivvies reportedly doing so well in comparison to other materials on the same trip.

Thanks for at least saying something. :)

-mox

Breathability has nothing to do with air movement; it's about diffusion of water vapor through the material(s). It's a bit of a misnomer. I think what you're referring to would be called air permeability.

Most of the (waterproof) breathable fabrics I've ever used seem to have quite low air permeability. At any rate, I rather doubt the buoyancy effect is all that great; we're only talking about a six-inch stack.

Edited by texasbb on 12/03/2012 21:38:42 MST.

I agree with Tod, there isn't enough air pressure generated by the person in a sleeping bag to effect condensation. Generally fabrics are pretty good at blocking air. Pertex, eVent, Gortex, etc form a membrane to prevent air displacemet. Down or synthetics are designed to stop air exchanges. Not so much to prevent condensation, but to keep you warm. Conduction and convection are the big heat losses inside of a bag. IR is only about 10-15% of your heat loss. You are saying that a permiable fabric will cause condensation. This depends on conditions. Yes I agree it can since it depends on a differential (usually just heat and humidity.)

IFF our fabrics were perfect insulators, there would be no condensation. You need a heat differential to form a dew point. Water would evaporate to 100% humidity but not evaporate any further given a constant temp. You would still sweat (insensible perspiration) and you get clammy, damp and sticky... generally a VB can simulate these conditions by stopping water and vapour transfer. You change the micro-climate next to you.

IFF you add any permiability for air escape, the normal condition, then water vapour and heat will escape with it. Now there is a problem with condensation.

Since temperature is not a constant, nor is humidity, nor wind pressure (providing far more air pressure than your body heat,) it doesn't really help determining how to make a bag to utilize this. We have vapour barriers with their problems. We have all the rest. How do you propose to solve this problem?