Bivy Sack

[David Staszak]

How do you use a bivy sack and stay dry? I live in the Adirondack’s and have tried a bivy sack a few times (summer and fall). The condensation has been terrible. Is that the price of going light? Or is it a regional thing: “bivy sacks work best in a dry western climate”?

[Ryan Faulkner]

David,
what kind of bivy sack did you use, was it half breathable fabric like the Vapr or equinox ultralight, or fully waterproof?

I have used the equinox on the east coast in about 95% humidity

[Bob Gabbart]

I use a BMW vapor bivy with a small tarp here in the mid Atlantic region where the humidity is often in the high 90s, and I usually have very little or no condensation. They key to the vapor bivy is that it uses very breathable fabric. Note that you need to tie up the hood on the bivy or you get horrible condensation.

[Richard Nelridge]

David,

Though I do not have experience using Bivy’s, I do know that a breathable Bivy is probably is needed where you live and very possibly used with a light Tarp. The Adirondack’s can be quite warm at times and certainly very humid at times. As a friend used tell me, they don’t call them the Adiraindacks for nothing.

Rich

[Glenn Roberts]

I use a bivy successfully in Ohio, Indiana, and Kentucky in the summer. There are two issues to address: bugs and rain.

Let’s start with bugs. If you button up the sack to keep the critters out, you create, in effect, a roasting pan. So, you need ventilation with mesh. I’ve found a sack that opens to the waist, backed up with mesh (either as part of the sack, or some kind of add-on net – your choice) is a must. This lets in plenty of air, and lets out plenty of moisture.

It even does so in the rain, which is not exactly the desired result. Obviously, if you button up in the rain, you’ll soak the inside of the bivy with condensation from sweat. So I can continue to ventilate in the rain, I combine the bivy with a small tarp (5×8 siltarp.) This means I can leave the sack open as much as I need to, but still have protection from the rain that splatters in around the tarp. I find the tarp a necessity anyhow: you need a place to stand up, or to cook under, too.

My favorite, so far, is the Integral Designs Salathe (my son’s choice is its framed cousing, the Unishelter.) Its WB panel unzips to the waist, and the panel is fully backed by waist-length mesh. This allows me to adjust (and change during the night, if necessary) the amount of ventilation versus protection I want.

So why bother with the WB feature, if I’m sleeping beneath a tarp? Well, because if it’s not raining, I don’t pitch the tarp, preferring to sleep under the stars. The problem is condensation – from outside, not inside – in the form of dew. The WB bivy turns dew very effectively.

At 2.25 pounds or so, it’s not the lightest system. But it’s still lighter than all but the single wall tents (which I’ve had NO luck with in eastern humidity.) It’s also more flexible than a tent, which is mostly either all up or all down. I can spread out a bivy sack in lots of places where there isn’t enough footprint and/or headroom for a tent. It’s also simple: I store my sleeping bag inside it (no stuff sack needed), and making camp consists of digging it out of your pack and spreading it out: “Hi, honey, I’m home.” After which, if you’re the kindly sort, you go help your buddy figure out which of the three sets of differing-length poles thread into which set of sleeves on his tent. (Do it enough times, and he’ll ask, “About this ultralight stuff…”)

[paul johnson]

Glenn’s comments are spot on. I would only add that:

0) sometimes there’s very little that you can do about it. in New England, and you aren’t that far away, you will nearly always have some cond.

1) make sure that nearly all (or all, if possible) of your exhaled breath exits your bivy through some type of opening (mesh = warmer bug weather; partially unzipped openning in cold weather). for me, since i really don’t sweat very much unless HR (heart rate) is elevated, i find this is a key. for me, seems like most of the water lost at night is exhaled, not sweated.

2) VB (vapor barrier) clothing or bag liner – other than VB socks, i haven’t used these yet. they’re primarily intended for below freezing weather i gather from Posts in other Threads. This won’t prevent condensation from the atmosphere, but will keep any sweated water vapor from condensing in and on your gear.

3) sleep under something – e.g., a tarp, a tree, etc. why does this work? i don’t know. what i’ve read elsewhere on this website from very knowledgeable and experienced people is that a “microclimate” is established under the tarp or tree and the majority of the condensation occurs on the “cover” the bivy is situation under rather than on/in the bivy itself. sounds plausible. hope i understood those explanations correctly???

4) weather/atmospheric conditions permitting, adequate air exchange reduces condensation inside of the bivy. obviously, under some weather conditions, this doesn’t help

[paul johnson]

to be honest, i really don’t have a handle on this one yet. sometimes, i’m amazed at how little condensation there is in the bivy – i expected more based upon the conditions. other times, i think that there won’t be much, and in the morning there is. sometimes i get just what i expected – either some or very little.

here’s a thought that i have that i haven’t been able to “prove” to myself yet. hope the real experts out there might respond to this. is this just coincidence, or is there something to it? let me just throw this out there.

often i’m up and breaking camp while it’s still dark, and will be a for a couple or a few hours, so anywhere from 0230 to 0430 generally – if i’m alone, which i usually am AND got to sleep early and wasn’t too exhausted from the day’s trek – i usually sleep, on the trail, <7hrs (at home much less) and sometimes i may take an afternoon nap.

this means that i go to sleep earlier, of course. i do like to hike at night, and am naturally an early riser so this is not an issue for me. often i have very little condensation when i break camp this early. why? not sure…

… some factors could be that often the “classic” early “morning”/post-midnight rain doesn’t start until somewhere around this time, or has only started an hour or two earlier so less time spent in the bivy while raining, and…

… also, on nights without rain, temps are still getting colder at this time and often haven’t reached their overnight lows. so dew point is perhaps a bit more favorable for me earlier in the night? hence, less condensation inside of the bivy.

like i said, i haven’t experienced this enough yet to draw any real confident statistical conclusions. it’s just something i’ve recognized for some time as occurring. haven’t kept track of all of the data (temp, humidity, rising time, rain, hrs in the sack, etc).

frankly, the whole “experiment” is rather uncontrolled and i’m not sure that i’m smart enough to figure it out. but, i’d have to say that, generally speaking, spending the same number of hours in the bag and bivy, the earlier to bed and the earlier to rise, the less condensation i have.

however, if you’re NOT a morning person, then this “scheme” won’t dew…er…i mean do.

what do you real pros out there think? am i all wet here, and it’s just coincidence? or, is there something to this? does this early-to-bed, early-to-rise scheme sometimes help to avoid the most unfavorable dew point hours, thus reducing condensation inside the bivy?

p.s.
also, and i’m just thinking out loud here, i haven’t done lately much winter camping – only a couple of times last winter (overnight in the mid-20’s), getting up earlier may avoid trying to sleep through the coldest hours with minimal UL sleep gear. you’re on the move and producing some extra body heat. anything here??? of course, you don’t have the warmth of the bag, but perhaps the insulating layer while on the move is sufficient for warmth in these, the coldest hours? (yeah…i know. i’ve read that sometimes the hour, or so, after sunrise can be just a tad colder than just b/f sun up).

this early to bed, early to rise scheme has an advantage unrelated to condensation. it has to do with the ‘wee’ hours nature call, viz. i generally don’t have one as i’m getting up anyways and don’t have to worry ’bout it disturbing my nightly repose.

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