Bivy Condensation

I like having a waterproof/breathable bivy in case my Hexamid blows down in a storm. Last summer the wind was blowing so bad I figured it best to just take the Hex down before it blew down… I spent the rest of the night in my bivy on top of my Hexamid.

Food for thought.

And, yes, I think you would get more condensation inside your bag if you just used it under the same conditions but not inside a bivy. The warm, moist air is coming from inside your sleeping bag. The cold air will chill the inside moist air of the sleeping bag just inside the outer skin. Cold air can not hold as much moisture, hence it drops out of the air into water droplets inside your bag. Having your bag inside a bivy offers a transitional space where the air will not be as cold up against your bag so you should get less condensation inside your bag. Correct me if my logic is wrong, but that's what I think happens.

Bill D

You are thinking along the same lines as me Bill. I had a little success with leaving my bivy opened up for the first few hours to allow some ventilation, then zipping it up for the last few hours until waking. This approach seemed to help, and may be my routine from now on.

My first thought was to just roll up in the tent. Next comes the question: just windy or windy and wet? Big difference, of course!

Dale,
In my case the rain had stopped. Of course, when I took the Hexamid down I didn't know for sure it would not start raining again. But even if it had been raining, with wind like that the Hexamid would have come down if I had not taken it down. And being in a waterproof bivy I would have been okay anyway. So, I guess my strategy worked.

Well as I always say, it all depends but I would generally think that in the Sierra and drier west you would be just as well off just on a ground cloth, less condensation, especially if you are under a tarp. Every so often I re-post this article I wrote a few years ago.

http://40yearsofwalking.wordpress.com/2011/07/04/the-bivy-condensation-conundrum/

I cowboy camp 90% of the time in California and I almost never get condensation/dew on my bag except for wet/foggy weather (where I would need a shelter anyways) or when camping right on the coast.

I prefer to not use a tarp at all unless rain is threatening. I really enjoy sleeping under the stars! You are correct though, when I am under the tarp in normal conditions, I don't get the bivy condensation.

Interesting variations by region and climate. Rain doesn't threaten here, it just *is*. In the western forests of the Cascades and Olympics we're always living near the dew point. Being in a bivy and breathing into the interior really adds to the mess.

I've thought about a tiny battery powered blower to move the air through a small shelter. It wouldn't take much a with the low cubic volume of a bivy.

I decided I'm going to cut my 6 oz waterproof bivy in half.
I just bought some kite grade tyvek which weighs less than regular tyvek on amazon.
I'm going to create a reversable bivy with half tyvek and half
montbell dri tec waterproof material.

When it's raining a deluge, I will use the bivy under my 5oz tarp with the
eventmontbell dri tec side on the bottom so if a river changes course under me I'm still dry. The tyvek top can handle spray and condensation just fine and it breath equally well as the eventmontbell dri tec side.

When it's dry out and here it's sounds counter intuitive but when the ground is dry then I will use the tyvek side as the bottom which is 90 % of the time. The event on top breathes well, the tyvek handles rocks and pointy sticks better than any other material.

I'll save weight by never bring a ground cloth again and I'll have great protection if it does rain and the tyvek doesn't slip around and I'll never have to worry because it will be wide enough that my neoair will fit inside the bivy and on most nights I'll just cowboy camp and not set up the tarp at all.

Since I have experienced the great breathability of eVent in my REI Kimtah rain suit I now feel that eVent is the only WPB material I would want in a bivy bag.

OTOH, I would only use a bivy bag for sleeping in snow shelters. I'm a dyed-in-the-wool tenter. But having a bivy bag as a safety shelter if a tent gets shredded is also a good idea – IF you can haul the extra weight.

Anybody know if there's much difference getting condensation inside a bivy when it's just draped over your bag (so there's not much space between bivy and bag) vs. having the area around your face elevated with a bit of cord attached to something above you (so there's now a some space between bivy and bag)?

Thanks for all of the replies! Any recommendations on a good e-vent bivy that doesn't weigh a ton? I REALLY like the fact that my current bivy is just a touch over 6 ozs., and combined with my Hexamid Solo, is a shelter system under a pound. If I too heavy on a different bivy, it defeats my system to a certain extent.

I've started using Dave Mile's new "Pico Bivy". I pushed Dave to consider using Tyvek 1443R (the lightweight Kite making Tyvek), Dave bought a sample and tested it, he refused to use it, its simply not waterproof enough for him. Having used the Pico half a dozen times I noticed his top fabric will get damp inside if it is folded over on top of itself (double layered around the retracted hood), otherwise its very very breathable.

Another minor damp area is around my feet, simply not enough circulating air for very dewy conditions. I've been toying with the idea of a retractable hood/flap over my feet on the Pico so that I get good through ventilation.

Dave's bivies are excellent and the space between the sleeping bag and the top layer of the bivy combined with the huge open end around your face ensures any breeze wafts out humid air whilst leaving your bag to fully loft and insulate you. PLUS becaue the mossie net and hood are held aloft with a hoop, they dont require you to stake the thing in one place and find a way of lifting the netting off your face.

I'm seriously thinking of going for an Uber bivy for really foul weather – its effectively a mini self erecting tent!

I was on a canyon trip west of Delta two weeks ago. The days were warm and the nights were mildly cold. I woke up under my SMD Haven tarp with frost all over my WM bag. The doors were open so there was some exposure to the clear night sky. I slept in my base layer. As I moved around in my bag in the a.m. it seemed that the frost began to melt and my bag got wet. I suspect it was the stratified warm air in the insulation hitting the outter shell of the bag causing the frost to melt. I would guess it was in the 20s that clear night. I was fortunate enough to have a clear day and so I spread my bag out to dry before packing up for the day's walk. My boots were frozen that a.m.

This last week I was up the Cochetope. I slept under the SMD Haven and I also used a Borah bivy. The Borah's top fabric is M90. It was a long clear 9 degree night and I kept the doors closed. I was surrounded by noisey elk. I slept in my down jacket, pants, booties, and base layer. My boots were in a plastic bag in the bottom of my WM bag to keep them from freezing. The top fabric of the long Borah Bivy isn't nearly long with boots being stored and the top edge only came up to my armpits. I woke up multiple times through the night very concerned about the frost build up inside the bivy bag. I was fortunate enough to have a clear day to spread my sleeping and bivy bag and tarp out to dry before packing up for the day's walk.

About 4 weeks ago I was up by the Butte. I slept under the SMD Haven with doors open and just my WM bag. I slept in my base layer, a mid weight layer, and my rain gear. It was a moderate high 20s-low 30s night with a mild wind. I had no condensation issues and packed up and started walking right after breakfast and glassing.

I have no conclusions yet, but I do want to experiment more with vapor barriers to see if that will reduce the condensation issues or if it is strictly a matter of temperature differentials. The answer is probably already here in this forum and I just have to search it out. I don't remember any condensation issues years ago when I was in a unit that issued Feather Friends bags that included vapor barrier bags and bivy, but maybe we didn't sleep much.

Thanks Alex, good read and good referals to pertinent articles.

Ian I also appreciated your recent observations. Lots of factors involved but I do think vapor barriers at least in cold weather do help. In more moderate conditions I am moving towards thinking that the lighter the sleeping bag the better for your body heat to more effectively move the dew point out past the sleeping bag and maybe the top of the bivy.

Alex, great info! Your situation was very similar to what I experienced this summer. I really don't want the hassle of setting up a tarp when I don't need to, so I will have to experiment more with selective sites. I found that leaving my Superlight Bivy opened up for awhile helps to reduce the quantity of internal condensation. I might even experiment with sleeping with my body under the tarp, and my head out for the view and see if that helps.

These questions are posed to anyone who has used a vapor barrier liner in their sleeping bags. 1- do you wake up in a pool of sweat? After hiking anywhere from 22-30 miles a day, I find that my body produces sweat even after stopping and at rest. It seems like it would be uncomfortable sleeping in your own sweat, but maybe I'm overthinking it? 2- Has a VBL helped reduce condensation on your bags and/or your bivy bag? I know there are people that swear by them, but I would like some first hand reviews from some BPL-ers!

Robert:
1) You get pretty moist inside a vapor barrier bag. I'm talking a non-breathable, water proof vapor barrier. The warmer you are, the more moist you will get. The cooler your are the less moist you will be. If you are cold and shivering you will not get moist. The spectrum ranges from being in a pool of sweat to shivering and being dry, directly proportional to the temperature next to your skin.

2) if you breathe to the outside and your body is in a vapor barrier bag from the neck down, you will get little to no condensation inside your sleeping bag or your bivy because there will be little to nothing to condense… at lest nothing added to the ambient humidity.

I used a vapor barrier bag for three weeks on Mt. McKinley many years ago. Each time I rolled over (and I roll a lot), the movement would suck cold air into the VB bag and chill the moisture that had been trapped on my skin. I never had a desire to use a VB liner again.

Bill

Keep in mind it's totally possible to have large amounts of condensation even if you aren't adding your own moisture to the system (like with a VBL.) I laid my bag and bivy out one night with clear skies and temperatures in the low 40's, then enjoyed a campfire for a few hours. When I came back my bag was soaked – totally wet with visible water droplets inside the sleeping bag shell. The radiant cooling from the clear sky dropped the temperature of the outer layer of fabric below the dew point and the bag quickly accumulated water. Fortunately it wasn't that cold out so I stayed comfortable anyway and the bag was mostly dry by morning from my body heat.

So. If you use a VB liner inside your sleeping bag your down will be in better shape due to less condensation, but you will be moist and likely less comfortable… moist, but warm… except for when you turn over and suck cold air in over your moist skin.

Perhaps a compromise solution would be to have a water proof breathable bivy both inside and outside of your sleeping bag :)

At least that way the moisture crossing over into your down would be reduced to maybe the same rate that moisture escapes from the exterior bivy. :)

Bill

Thanks Bill! For the 'normal' hiking I do in the Sierras, I think I will stick to not using a VBL. I can always drape my bag on a rock sometime during the day and let it dry out. I can't stand that clammy feeling I get while even in a rain jacket while hiking, let alone lying in a puddle of my own sweat:( !

I'm also thinking about switching to a quilt, or a bag that can be opened wide like a quilt, to allow for ventilation and keep me from breathing into my bag as much. I try to avoid it, but sometimes when I zip it all the way up and breath out of the small opening created, my head will get turned around facing into the bag, which isn't doing me any good for condensation prevention!

Anyone else have some good info?!

I have often wondered whether a bivi (or tarp or tent) made from a fabric with a metallic reflective (low emissivity) coating would suffer less from condensation on clear nights. In theory the temperature of the coated fabric should be a few degrees warmer than an uncoated fabric. Anyone tested this?

Example fabric: Silver Metallic Ripstop from http://www.seattlefabrics.com/nylons.html

Bill D – unfortunately, using W/b layers inside and out doesn't do much for you, because of the dew point issue. Your body produces moisture; it moves through the inner W/b layer nicely because it's going from higher humidity inside to lower outside the fabric (but still inside the sleeping bag). That moisture travels through the insulation until it reaches the dew point. If you are in warm dry weather the dew point is outside your bag, and all is great. In colder/damper weather the dew point is within the insulation, and the moisture condenses before it leaves the bag. So the RATE of moisture production is not the issue, it's where is the dew point – inside the bag, inside the bivy, or outside it all. A VBL, if used effectively, keeps moisture from getting into the insulation in the first place; but it has no effect on where the dew point is. But VBL's can be tricky to use; that's a whole separate discussion.