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
