condensation in WR bivvy is worse (!!?) than WPB one??

I was going to reply with a pithy one-liner such as "Ditch your bivy & get a DWR quilt/bag; get an oversized tarp for weather, otherwise sleep under the stars sans bivy."

However, one of the commenters at the site you referenced said it best:

***

After that trip I sold the bivy and decided to never go back.

It was not a flaw of the bivy itself. It was a flaw in use. Guys like Ron from MLD and Ron from SMD have been saying it for a long time, bivys where never designed for hikers and should not be used for hikers. The more I used one the more I understood why.

I suppose in ideal conditions a bivy can be an awesome way to approach hiking. I encounter to many different conditions throughout a hiking season than what they are designed to be within.

So for me it will be back to a tarp and a ground cloth. I will be trying the MLD Bug Bivy the first of the 2011 hiking season when I know I will be in an area where there are snakes and scorpions but its a bit to short for me so its use will be limited.

John B. Abela
RedwoodOutdoors.Com

I have an OR advanced bivy that's made of Goretex Respiration Positive fabric with an integrated foot vent. I've used it snow camping, mountaineering, and on summer BP trips. Contrary to what I expected, there has been very little condensation. Perhaps it is the foot vent that makes the difference, but from my experience the Goretex fabric has performed as advertised. I have no experience with bivies using a different material so I can't speak to the differences.

@hobbs

although that might be true….its not what i was looking for.

I own a double wall tent and a few tarps, and groudsheets and a couple of bivis…obviously my expectations are not the same for all of these in terms of weight/comfort etc

I am more looking for experience re the DIFFERENCE in performance btw WR and WPB bivis.

Most of the time this would be under a tarp (since i usually hike in rainy areas eg Scotland)

My initial thoughts (which seemed to echo what BPL wisdom says) were that WR bivis are lighter AND more breathable and thus will have LESS condensation. If that is NOT the case then i want to hear about it:)

Mike

Once the outermost fabric surface is at or below freezing…breathability of the bivy fabric makes no difference. Water vapor will condense before it can pass thru.

Once the outer layer of sleeping bag gets below freezing water vapor will condense within the sleeping bag. Expect this to happen under a tarp near freezing if you forgo a bivy to avoid condensation.

The author may be onto something when he mentions: near freezing a wbp material that relies on some form of capillary action to pull condensed water through the membrane may be the reason he doesn't have condensation btw bivy inner and insulation outer. It may also just be that the condensation is occuring before it can pass through the insulation.

I have, actually.

I never had condensation inside my eVent bivy but had copious amounts inside my momentum bivy. I have no explanation for this and only experienced it just above freezing and then below.

Thanks for the excellent article. Here is a BPL thread with lots of good discussion as well: http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=28956

I think the take home message is that condensation is not only a matter of breathability. I have experienced this myself, where I laid out my Pertex Quantum-shelled quilt on my ground cloth and went and sat around a fire for 2 hours. When I came back, my quilt was drenched with drops of water collecting on the outside. The reason? Moderate ambient humidity combined with clear skies meant that radiative heat transfer dropped the surface of the quilt below the dew point and it sucked up all the surrounding moisture. Fortunately it dried out pretty fast when I got in it because my body heat pushed the dew point back outside the bag.

I think factors such as tree cover, the dew point/humidity of the surrounding air, cloud cover, and whether you are under a tarp or shelter are just as, if not more, important than the breathability of your bivy fabric.

and as I have said earlier it was really a literature review to try and answer questions/problems I had observed in the field. It does appear from reviews all over the internet that the WPB's without a polyurethane membrane (eVent and the like) have the least reports of problems with condensation. I am always interested in others experiences as well.

Alex

Good read…for some reason i couldnt find the your name on the blog….

I have had a lot of my own bad bivy experiences, that's why I built me a better one. I think the only reason there are WR bivy sacks out there, is because of the poor breathability of previous WPB bivy sacks. The material I use is polypropylene and has properties that are very different that nylon. Because of these properties, it is WP and more breathable than any Goretex I've tried. There are some conditions that will always cause some condensation, but it I think it will perform as well or better than a WR fabric.

There more to a bivy than fabric. My design also keeps me dry in a downpour while venting. Check the water test video a customer did, I always get a good laugh from it. I guess trying it is the only way, so I started a rental program.

A bivy is not the best choice for all trips (no shelter is). However, too many discount them due to bad past experiences. If the rain blew in under a trap, would we give up on tarps ;)

Condensation happens on any solid object highly breathable or barely breathable. Once the condensation happens especially if it freezes, water vapor can not pass through.
I suspect that because a WR material is thinner it freezes quicker than the thicker WP materials.

A side note. I have used a SMD Meteor bivy (upper half bug mesh) as a bivy under a tarp in winter. I use it as ground cloth, a way to add warmth be reducing breeze and to block spindrift.

On cold mornings, I find the bug mesh gets a lot of ice in it from frozen breath.
Makes me think about how much condensation can happen with a full coverage bivy. I have never experienced much if any condensation on the WR fabric, but I do hang the top of mine to allow some airflow along the top of the bivy.

Okay, here's what I think.
I think that a waterproof bivy is fine, as long as you have a VBL on, so you don't shed your body moisture to be trapped into the bag.
This way, the insulation layer is protected from moisture build-up from both sides.

Ultimately, the waterproof bivy prevents water from going thru it, so if water condenses on the outside, or even if it gets rained on, it is of no consequence.
All we need to do after that, is make sure that no water condenses on the inside of it.

I think the answer is something like a AMK SOL aluminized polythelylene emergency bivy over the sleeping bag or quilt, and some VBL coverage over your body. This will prevent most/all moisture from collecting in between the VBL and the SOL bivy, but there will always be ambient humidity in the air which may be a concern. The radiant reflective barrier of the SOL bivy captures the body-heated air and retains much of it inside the bivy, since it doesn't breathe. It traps the heated air in, and so it keeps the interior of the waterproof bivy warm, and over the dew point.

The result is that your body produces its own "micro climate" inside the VBL layer, the insulation layer of your bag/quilt is kept free of condensation, and anything which condenses outside which is beyond your control is kept out by the waterproof bivy.

This is the same way that a heater rod works in a gun safe or a closet for de-humidifying. It just keeps the temp high enough to stop condensation, by giving out a little heat in the confined area.

A AMK SOL emergency bivy weighs like 3.5 ounces or something like that. It's not heavy, and it stuffs very very small for packing. And for a person under a minimalist tarp coverage, it could be a welcome addition during rain or humid seasons. You just have to accommodate it with the VBL inside the sleeping bag, so that you don't create a problem by using it.
You could even use one SOL emergency bivy outside, and one as the VBL layer inside your sleeping bag.
And you will likely find that you don't have to carry as heavy of a sleeping bag with this arrangement, too.

I know people are resistant to VBL, but if you understand them and use them properly, they can solve some problems.

I have had nights with just the mesh closed and frozen breath on the mesh. The same night I had no condensation on my polypropylene fabric. I think part of this is because the mesh is held away for the head in the cold air. The fabric lays directly on me and can get enough heat to drive the moisture through the fabric. My Goretex bivy always had ice on the inside below freezing. Condensation is worst of very calm nights.

Ice inside the bivy isn't so bad. It means it was warm enough to drive the moisture out of the outer layer of your bag/quilt. All you have to do is shake the bivy out.

Ice is definitely better than water! But no ice is even better :)

If there's no ice or water condensation in the quilt/bag either, then I agree. That's a lot to ask for when a vapor barrier isn't used.

Water vapour will condense on no-see-um mesh if the conditions are right. Just look at the grass in a meadow after a cold, still night.
The Blizzard Bag is a proven life saver. There are a few inferior products that share the same name.
Like Dave earlier, my all eVent ID bag has proven the most condensation proof bivvy i've ever used. The breathable floor is a major factor.