soggy bivy nights?

Two questions: Did you have the bivy completely zipped up (enclosing your head)? and Was there a lot of dew on the outside of the bivy (camping next to the coast, it seems like there could potentially be lots and lots of dew)?

Hi Dave,

I'm assuming your soggy bivy experience occured in Point Reyes based on a previous thread. I had similar problems with a bivy there. Specifically, I was stealth camping near the top of a peak just south of Alamere Falls with a Montbell Breeze Dry-Tec bivy. In the morning the top (outside) of the bivy was bone dry but there was significant condensation on top of my quilt inside, so much in fact that it was affecting the loft. After that experience I returned the bivy and haven't used one since.

I think that area, especially if you are near the coast (not in the woods), is bad news for any bivy. Perhaps Mark Verber, Richard Nisley, or other Bay Area followers of this site can comment.

Dave,
The ocean temp in that area averages around 50F year round. When you sleep close to it, you will experience a larger night time temp drop than just one block inland. The more the temperature drops the higher the probability that the night time temps will be lower than the dew point temperature. Also, there are generally not trees along the beach areas. Infrared radiation losses to the ~ -470K night time sky will also lower the temperature of the bivy material below the ambient air temperature which increases the probability of condensation. The simplest solution is similar to what you did on the PCT in 2004; other forms of “separation” appropriate to bivy use.

Wear a lighter synthetic jacket and vest combination, with mating zippers, appropriate to keep you warm rather than an equivalent heavier jacket. Before retiring for the evening, zip the jacket and vest together to form a synthetic torso quilt of sorts. Place the pseudo torso quilt between your down bag and the bivy. The condensed moisture will now typically occur in the convection “separation” synthetic garment and your down bag will stay dry.

Clear night sky radiation exacerbates the ocean temp cooling by also cooling the top of your bivy below the ambient air temperature. Night sky radiation typically takes about five thin layers of absorption and reradiating to be effectively nullified. Sleeping under a tree is the best way to reduce night sky radiation, a double wall tent is next, and a single wall tent or tarp is marginally effective. The pseudo synthetic quilt will also provide infrared radiation “separation” to mitigate this phenomenon.

It was either dew on your bag due to sleeping out or your were breathing inside the bivy. You can't stop dew if conditions are right unless you use an overhead tarp or tent so the dew collects on it instead of your bivy. Even that may not stop all condensation from the outside air.

I don't think it was body moisture condensing inside the bivy.

I use a hoodless bag and bivy. The only condensation I get is at the foot of the bag due to (I think) silnylon coming too high up the foot end, or when I slept out (no tarp, but bivy only) in Arkansas near a river on a humid night with temps hitting the dew point overnight. Sleeping out in dry areas (Chihuahuan desert) with no tarp does not cause condensation for me. It could if I closed myself inside a hooded bivy.

> I assume due to the warmth I was putting out meeting the quite cold air outside the bivy, and quickly condensing. How can you avoid this?
Technically, not 'the warmth' but 'the humidity'.
However, to answer your general question, your assumption is correct, and you can't avoid it with a bivy by itself under those conditions.
That's why we use a tarp or a tent.

> Night sky radiation typically takes about five thin layers of absorption and reradiating to be effectively nullified. Sleeping under a tree is the best way to reduce night sky radiation, a double wall tent is next, and a single wall tent or tarp is marginally effective.

interesting. This is news to me. I always thought a single barrier would do it. So then I guess thickness and material also play a role?

Dave, do you have a homemade bivy? It'd be interesting to know what the top fabric is.

Inaki,

Yes, thickness and material also play a role. One layer of shiny aluminum coated mylar will block the loss. Polyester and nylon fibers absorb and then re-radiate ~ 1/2 up and 1/2 down.

I think I can see how a tarp might help if dew was forming on the outside of the bivy due to environmental moisture from grass, drizzle or fog)… But how does the tarp prevent condensation from forming inside the bivy ? Surely the tarp doesn't trap enough warm air above the bivy to raise the dewpoint unless it's pitched as a fully enclosed single wall shelter – does it ?

Thanks.

John,

You are right, the tarp doesn't enough warm air to raise the dewpoint. What a tarp does is provide a partial IR block which reduces the probability that the bivy temp will be lower than the ambient air temp.

Once a friend and I were caught unprepared by a very late spring snowfall and had to share a too-small tarp that didn't quite cover us both. I stuck the lower half of my down bag in the only "bivi" I had, a large plastic trash sack…talk about "waterproof/NONbreathable"! I slept warm enough and of course the next morning the shell of my bag was sopping wet wherever the plastic had lain across it, but a half-hour in the morning sun dried that out and everything was fine.

After that experience, the slight-to-moderate damping of the shell that occurs even in a "true" bivi sack no longer seems to bother me. I take it as a trade off for reward of carrying such a light and easy to setup shelter.

I have experienced similar conditions on the coast in Denmark. I was surprised by the amount of condensation I experienced on my tarp, but I had little to no condensation on my ID eVent bivy.

I think that Richard has explained why this occurred.

Does it mean that an eVent bivy is the solution, I am not sure, maybe the combo of the tarp and bivy was my saviour. I was using a down bag which was dry.

Based on the string above, is it fair to say that if one wants to sleep exposed (no tarp) near the ocean in Point Reyes that it is better to NOT use a bivy?

> so, if it were warmer out, i wouldn't have had those problems? or not to the same extent?
Right.

> does a bivy, say under a tarp in the rain, work best (i.e. least condensation) in warmer conditions? or is it hard to generalize,
It is easy to generalise, but that may not help you when you are out there! It depends on the humidity and the the temperature and how far the temperature drops at night. But you can't control those things, and don't know them in advance either.
It also depends on how hot you are and how much sweat you emit inside your sleeping bag. I don't suppose it helps much if I say that the cooler you sleep, the less sweat you emit.

What is the perfect solution? There isn't one.

There is one other condensation mitigation factor that I originally thought was too obvious to mention. On second thought, it bears mentioning. An area sheltered from the wind or your tarp pitched to block the wind will reduce bivy condensation. This occurs because, without wind, there is about an .8 clo of additional insulation on the outside of the bivy (boundary layer). This boundary layer will facilitate the moisture passing through the bivy as a vapor. The wind can strip this boundary layer, which in turn will make the bivy surface colder, which in turn will result in a higher probability of condensation on the top inside of the bivy.

Casey,

No bivy at Pt Reyes could be even worse. The dew point could then occur just occur lower in the down insulation.

Richard,
would suspending a sheet of aluminum coated mylar from attachment loops on the inside of one's tarp do the trick?

Tom,

Yes, that is a very good solution if you can't set up under foilage for additional IR interceptions.

Richard writes:

>> …without wind, there is about an .8 clo of additional insulation on the outside of the bivy (boundary layer)…the wind can strip this boundary layer, which in turn will make the bivy surface colder, which in turn will result in a higher probability of condensation on the top inside of the bivy.

That's a very insightful observation, Richard! And I don't think it's an obvious one at all. If I may be so bold as to paraphrase, a breeze can make the outside bivy surface colder and thus more prone to condensation of outside moisture.

Beware the flip side, though. A still air boundary layer outside the bivy will cause a lower vapor pressure gradient through the bivy fabric. Plus, still air conditions will reduce air exchange between the inside and outside of the bivy. Both of these effects may result in increased condensation inside the bivy from internally generated moisture (perspiration, respiration, damp clothing, etc.)

Like Roger said, moisture management in a bivy is a bit of a black art. For (most) still air conditions, I'd recommend sleeping on top of the bivy, using it as a groundsheet, and using an overhead tarp to take advantage of the IR effects mentioned.

Cheers,

-Mike

I also wonder if we have to consider individual physiology here. I've begun to put together the fact that I am a pretty thirsty fish, and the fact that even my eVent bivvy is regulary soaked in condensation under a variety of conditions.
I don't pee more often than anyone else, so I must be sweating it out. Kind of gross, but may well be relevant…

> I also wonder if we have to consider individual physiology here. I've begun to put together the fact that I am a pretty thirsty fish, and the fact that even my eVent bivvy is regulary soaked in condensation under a variety of conditions.
I don't pee more often than anyone else, so I must be sweating it out.

You got it. As I wrote before:
"It also depends on how hot you are and how much sweat you emit inside your sleeping bag. I don't suppose it helps much if I say that the cooler you sleep, the less sweat you emit.
"

I very much appreciate the technical insights from Richard, Mike, and Roger. That's why I come here. However, rather than trying to keep up with that, I think I will generalize a little for the sake of those that aren't following the science.

A more breathable bivy top, especially one that directly vents water vapor like eVENT, will reduce or eliminate condensation in some fringe situations. Nothing will prevent condensation in a bivy (or tent, tarp, etc.) under the worst conditions. Thus most techniques discussed are focused on minimizing it.

For where and when I usually camp (3-season Rocky Mountains), eVENT eliminates condensation in a lot of situations. It would be far less effective on the coasts.

Dave T's posted a question on 11/28/07 regarding how he could reduce bivy moisture for a specific night time temperature, location, bag, and bivy. There was consensus on overhead IR loss blocking. There were divergent opinions whether 1) sleeping in or on the bivy and 2) blocking or unblocking the wind would theoretically reduce bivy moisture for his specified scenario.

I just created a simple thermal model of this problem and have attached it below. I did it in an attempt to understand where I may have screwed up in my original scenario subjective analysis. This model shows one thing that I hadn't considered in my original analysis. The condensation theoretically occurred in Dave T's bag at the 78F point in the insulation on that 30 degree night. I had intuitively thought that it would be close to the outside cover and the 30F ambient temp. None the less, once the moisture is condensed, the more heat applied to the condensation, the faster the secondary evaporation will occur. An open tarp should not significantly hamper a vapor pressure gradient from moving the evaporated moisture out of the bag. Although temperature and humidity levels both determine the vapor pressure gradient, the relative heat difference is a much larger factor in the equation. Secondarily, natural convection will disperse vapor moved to the outside of the bag.

I understand that there are many other complexities that could be incorporated in a bivy thermal model to make it more accurate. For this forum, I think the simplest possible model that will answer the question is better. This allows a much larger audience to understand and consequently participate in the analysis process.

I tried to find any scientific study that looked at the issue of bivy condensation. My multi-day search didn't turn up anything particularly relevant. I did find a 2002 copyrighted scientific paper called, Moisture Accumulation in Sleeping Bags at Subzero Temperatures—Effect of Semipermeable and Impermeable Covers by George Havenith. The abstract of this study was: "Because subzero temperatures are expected to affect the vapor resistance of microporous membranes, the effect of using semipermeable and impermeable rain covers for sleeping bags on moisture accumulation in the bags during 6 days of use at -7°C is investigated. Moisture accumulation is related to the vapor resistance of the materials. The best semipermeable material gives the same moisture build-up as no cover. Semipermeable cover materials are effective at reducing moisture accumulation in sleeping bags at moderate subzero temperatures." No theory or model was covered, just climate chamber tests. Gore-Tex was the semipermeable cover used in the study. There are now WPB bivies available with much lower moisture vapor resistance than the 2002 Gore-Tex bivy used in the study.

Please point out any errors, or areas requiring clarification, that you might see in this simple model. Also please point me to any relevant papers that you are aware of.

The pad part and the bag part make a sleeping system which has its own resultant clo value. This resultant clo value is calculated knowing the clo values of the constituent parts plus the sleeping position. In this model, I assumed he was sleeping on his back. That is the reason for the label "Pad +Bag (back sleep)".

Hi Richard,

Nice work. There are two parts of the charts that I don’t understand.

1 The phrase “it would close” in the second paragraph.
2 The “Part clo” column and its relationship to Pad + Bag (back sleep)

As always, thanks for your time.