Bivy Condensation ?

hellooo….. anybody out there….. helloooooo…..

Greg,

I've had a better time with the bivy sack itself than going just bag only. Then at least it seems to push the moisture between the bag and the bivy or out completely. Usually I don't have an issue either way in Colorado

There are MANY MANY long threads already on this subject. A search would be very productive.

Cheers

>There are MANY MANY long threads already on this subject. A search would be very productive.

It's also a little tiring to discuss a subject that not a lot of people have a lot of knowledge of. But I've been thinking about the subject for a while, so here's a complete list of my knowledge of bivying:

-Bivying under a tarp is different from bivying under the stars (less exposure to dew, can get away with using a more breathable/less waterproof top fabric, not to mention you have a dry entry/exit in a rainstorm).
-Bivying under the stars is best in a desert environment, whether you're using a Gore-tex bivy or a water-resitant bivy.
-Sometimes the only purpose of a bivy (for instance under a tarp) is to keep the shell of your sleeping bag dry, but that can be a very important function if your sleeping bag is filled with down.
-In certain conditions, you are going to get wet at night, bottom line (camp in a river bottom, for instance, 100% relative humidity can permeate everything).
–In very moist conditions, sometimes the last thing you want is a wet piece of bivy fabric draping on your sleeping bag (this is where tent-style bivies might come in handy, if they're ventilated enough).
-In certain conditions, any bivy will stop being breathable (when the outside temperature dips below freezing and frost builds up on the the bivy fabric).
–If your bivy fabric is close enough (a snug-fitting bivy) to your sleeping bag, the idea is that the fabric itself will be too warm to frost up (what Andrew Skurka might describe as having the dew point outside of your bivy–that's the idea of a close-fitting bivy).
–If you know you will reach these conditions at night, however, you can stop your own perspiration from making the inside of the bivy (and potentially your insulation) excessively moist by using a vapor barrier.

On the other hand, there's a lot to be said for just using a breathable (mesh and nylon) "inner tent", to avoid a wet-out fabric coming in contact with your sleeping bag, and to allow ventilation if it begins to frost up, but then again, that would be a double wall tent, not a tarp and bivy.

I use a BMW bivy under a tarp for most of my hiking.
Here are my experiences.

At around 200 grams, the bivy provides a ground sheet, stops the wind chilling me in my quilt and provides spray protection in heavy rain. These factors add maybe 5 degrees celsius to the warmth of my bag, may be more, I don’t know. Getting into that bivy with a quilt on top of you really does make you feel snug as a bug.

If I have to seal my bivy because of bugs or the cold, I find condensation on the outside of my bag around the head and foot of the quilt quite often. It is not a big deal as it is only surface condensation. I am willing to live with it given the lightness of my overall setup.

I wish there was a commercial bivy that had insect mesh over the entire area of my face. This would help moist air from my breadth escape and I am sure that it would help with condensation. I would still want the Pertex Quantum over my body, and at the back of my head (for rain spray and wind). That to me would be the perfect bivy for tarp camping.

Is the proximity of a bivy cover to a warm sleeping bag enough to drive vapor through?

If it is cold and the air is damp, no, definitely not. If you feel the outside of your sleeping bag/quilt on a coldish night, it just isn’t that warm. Condensation still develops. On the other hand, on a warmish windy night in dry air you wont get any condensation.

Petras,
It sounds like you want the Titanium Goat bivvy with the full head-net option. That seems to be exactly what you are describing. :)

Art,
Thanks for the succinct observations.

Weight is a consideration that is driving me to a light highly breathable top versus a heavier eVent bivy.

I figured in a tarp environment I would avoid wetting out the bivy with good site selection and the ventilation inherent with the tarp, but had not considered that a frost, inside or out, would create the same situation, regardless of the top-sheet material.

I am beginning to see that with a tarp/bivy combination the bounds of comfort and safety are set by dew point and temperature. That a freezing rain/snow environment would best be avoided, or handled with a double-walled tent.

But for the rest, I'll take my chances with a light DWR.

Hi Greg

> I am beginning to see that with a tarp/bivy combination the bounds of comfort and safety are set by dew point and temperature. That a freezing rain/snow environment would best be avoided, or handled with a double-walled tent.

You know, that's not a bad summary.

Cheers