Condensation inside bivy bags

[Kenneth Reppart]

I understand that below a certain point condensation inside a bivy bag is pretty much unavoidable. Can anyone help me understand at what point that starts to be a problem? From what I’ve read on these forums I think it’s maybe somewhere around 35 degrees. Does the problem go away if the temp drops low enough, say below 25 degrees?

Fabric breathability seems to be a pretty complex issue. If I’m willing to sacrifice some water resistance for breathability what should I look for?

[Jack H.]

I haven't been able to find a pattern really. Just a quick point though, condensation at low temps (probably below freezing) is ice. It's more manageable. I've slept in a bivy probably 40 nights this winter so far. I can only remember one night where ice buildup wasn't present. It's not a huge issue though because i just brush off the ice, pack up and everything stays frozen until the trip is over if I keep it in my pack, or dries if I lay it out to dry. Temps are usually -10 to 10 at night and 20-40 in the day.

[Al Shaver]

Ken,
This won't apply exactly to your question as it's poached from another thread in this category, but it may provide for you some insight into this issue:

I assume your bivy sack will be protected from the rain by a tarp or tent; otherwise life will suck if it rains.
Either insulation will work in a bivy sack, but my concern is that you're using a WPB material. In my experience, WP fabrics don't breathe sufficiently to put over a sleeping bag (UNLESS, you're wearing a VBL sack or clothing inside the sleeping bag to prevent moisture from escaping from your body into the bag's insulation). Even my highly breathable non-WP Pertex Quantum bivy collects moisture on the inside.

WPBs require heat to drive the moisture through them. In a jacket, your body (just mm away from the fabric) does the job. With a sleeping bag, the great distance between your body and the WPB fabric creates a temperature gradient wherein the fabric temperature on the bag side of the bivy is allowed to drop below the dew point (the temperature at which the relative humidity reaches 100%). When this occurs the water vapor condenses into liquid water (or ice) on the inside of the bivy sack rather than passing through the "WPB" fabric.

In this case a synthetic bag would best retain insulative powers as the moisture continues to build inside the insulation during the night and continue doing this into your second night.

Rather than creating this scenario, how about a nice, light down bag with a light, breathable non-WP bivy sack and a tarp or well vented tent for rain protection?

[Mike Hinsley]

I've almost never used a bag without condensation. In fact Bivys in the UK are famous for it.

I have made a bivy with a Pertex Aquabloc top and for a WPB that seems to be the least bad option.

[Vick Hines]

Mike,
IMHO, Al S. has nailed the issue concisely. If your sleeping bag is working, the bivy will have condensation. It is one of those unavoidable issues. There is one kinda-sorta solution that I have had a little luck with: a venting foot section. I make a long spendrift collar on the foot with a drawstring to close it if necessary. Otherwise, it stays open. This does not completely solve the problem. I suspect that one fix might be some form of hoop to hold the foot section up so air can circulate. Sooner or later the fixes to make a bivy work right make it too complicated and heavy. So….

[David Ure]

nm

[John S.]

I have only rarely had condensation with my equinox bivy in the humid south. Only footbox condensation is what I get.

[Tim W]

I bet if you weigh your sleeping bag before and after a few nights in a bivy sack you'll find the condensate. Probably doesn't matter for a weekend but for longer treks, the condensate may drastically reduces the loft and/or warmth of the bag.

[David Ure]

nm

[Chris Jones]

David,

Which eVent bivy do you use? I am considering purchasing one of Integral Designs' eVent bivy bags, and would like to hear an eVent bivy owner's impression of the bag. Also, what kind of sleeping bag do you pair it with (brand/model)?

I saw your post regarding bivy bagging on Vancouver Island. I live in Japan, which also has a relatively wet environment, especially in the mountain regions. As such, I would be keen to hear your detailed feedback.

Thanks…

[Al Shaver]

Expeditioneers on long, cold trips like Denali's West Buttress have reported ice clumps forming around down clusters in their sleeping bags – clearly a bad thing. When temps get below freezing, Vapor Barrior Liner (VBL) clothing or sacks worn inside the sleeping bag come into their own. Once you stop pumping 500ml to 1 liter of water vapor per night into your insulation you can put a waterproof cover on the outside of your bag, It doesn't even have to breathe. I believe this is how Stephenson sleeping bags are made – WP on inside and outside. Stephenson (who also sell VBL clothing are such fans they even recommend them above freezing.

The other BIG bonus of VBLs is that they shut down evaporative heat loss that is otherwise taking place 24/7 on evey body. This allows the user to carry a thinner (lighter)sleeping bag and less clothing if VBLs are worn while active during the day.

[David Ure]

nm

[Anthony Weston]

I've used several Bivy's but so far winter, summer, spring, fall I have not had condensation using the
Montbell Breeze Dry-Tec Bivy Bag 6.4 oz, this was not the case in the past. I'm camping in the Sierra's and Big Sur mostly. My tent is dripping or covered in frost but not much on the bag. I'm using the montbell bivy and a Marmot helium bag but I purchased Helium with the non waterproof, most breathable nylon shell. The montbell dri tec material is remarkable. However it's not for everyone, no zipper, so getting in and out is an art. So far my bivy is part of my indispensible gear.

[Jamie Shortt]

I too have had great luck with Montbell Dry-Tec Bivy. In conditions where I expected significant condensation I woke up completely dry. I rubbed my hands thru the inside of the bivy and found no moisture. I was using a Marmot Hydrogen is low 30's and low 20's. The bivy really seemed to extend the temperature rating of the bag.

But yes having no zip means you must slide into the bivy and it doesnt leave much extra room.

I am really interested in hearing if anyone else has found this bivy to work this well.

[Anne Flueckiger]

OK this has helped me with the questions I've had about my Western Mountaineering VBL bivy. I've never been able to use it w/o extreme condensation (tried using it inside and outside my 20F down bag). Yet I have lent it two years in a row to participants on a Mt. Kilimanjaro climb whose bags aren't keeping them warm enough…they have then been toasty and not sweaty (they put it over their bag, down or synthetic). So I guess my bag has been warm enough that the bivy was overkill when I tried it out? Anyway I carry it on long ski outings as an emergency blanket. Now I'm looking into an eVent bivy…

[Chris Jones]

David,

Thanks. Just out of curiosity, when you mentioned the Montbell SS#3 were you referring to the Burrow Bag (synthetic) or the Down Hugger (down)?

As you mentioned (and as is detailed on the ID Web site), the bag you have is 100% eVent. How does it fare when water has pooled underneath the bag? I have read that hydrostatic pressure from water pooling can introduce moisture into a bag. To that end I have been eyeing the South Col eVent Bivy…

Anthony/James,

Recommendations noted. I do not think that the Montbell Dry-Tec Bivy is a true Bivy in that one can fully enclose oneself in the bag. In fact, the MB site refers to their bags as "Sleeping Bag Covers".

I am looking for something that offers protection from the mozzies (e.g., a built-in bug net), which I have a very low tolerance for.

It would be nice if Montbell were to offer "true" bivys manufactured with Breeze Dri-Tec. From what I've read so far in the reviews it seems like an outstanding material.

Chris

[Chad Miller]

I think the e-Vent bivys work great for temps above 20 degrees but once it gets colder they frost up on the inside.

From my personal experience I don't have condensation issues with my E-Vent bivy (Integral Designs Micro Bivy) in temperatures down into the 20's. Once the temperatures get much colder (19-0 degrees) then there is a buildup of frost on the inside of the bivy. During temperatures in the 30-50 degree range I have never had any condensation.

Now keep in mind I'm not wearing VB clothing and my bivy dose not have any side zipper to provide more ventilation.

[John Carter]

The Montbell bivy doesn't have a bug net, but think about how much more shelter you could get with the Montbell for the same weight as the ID South Col. For example, a Montbell Breeze + SMD Wild Oasis would weigh 21-22oz, significantly lighter than the South Col. Plus you'd have more versatility, and could stretch out in a bug free zone.

Don't get me wrong; I love the simplicity of just a bivy, and I've purchased an ID Crysalis, ID Micro bivy, and a Nemo GoGo bivy. But I keep coming back o the same conclusion: they would be quite cramped and limiting in prolonged rain or in heavy mosquito areas. So I try to bring a tarp or tarp/net combo, but the total weight becomes heavier than some single wall tents. The Montbell/Wild Oasis combo, while not as simple as the South Col, provides much of the protection and more versatility for less weight. If the Wild Oasis were to blow away, you could always flip the Montbell over so the entrance faces down to keep rain out.

[David Ure]

nm

[John Carter]

"And drown in the condensation. ; )"

Well, since this thread was originally about bivy condensation before I took it off-topic, you've got a point. However, I was referring to a technique Ryan Jordan has successfully used, and spells out in his review of the ID eVent Overbag on this site:

"If you are using one of these bivies as a standalone shelter, the Overbag/Bivy gives you the option to flip the bivy over for additional weather protection in case of hard rain. I found that to be particularly advantageous in a few situations, even in the summer…"

I presume Ryan would sleep on his side or stomach the rest of the night to keep his mouth near the opening.

I would only consider this in an emergency situation, personally. I brought it up simply to point out one has other options if one's tarp were to fly away, or if you got caught in the middle of the night in an unexpected downpour without your tarp setup. You don't have this versatility with a pertex bivy, and since this is a worst case scenario, one would benefit from the tarp the other 99.9% of the time it doesn't blow away =). Also, I'd think the opening in the bottom of an inverted bivy would be as breathable, if not more, than the ID Chrysalis or the Nemo GoGo, both of which force you to breathe directly into the bivy.

I think perhaps the biggest advantage of the ID eVent bivies (besides their more robust design and use of eVent) is the beaked hoods that allow you to sit up in the bivy and still be protected from the rain. You wouldn't be able to do that in a Montbell. But again, I would only use the Montbell as part of a tarp/bivy combo.

[Matthew Robinson]

to the original poster: i dont know how much bivy experience you have or money :) but in temps down to 32F ive used a 45F synthetic bag and the amk 2.0 bivy (slightly modified) alongside midweight baselayer and been just fine (was windy too). I also think just airing out your gear in the mid-day is the best option. aloha!

[Ryan Gardner]

Why is it that the footbox gets more condensation than the rest of the bivy? Do the feet really sweat that much more?

[John S.]

In the equinox bivy I have it seems that the silnylon comes up too high and doesn't allow enough ventilation to prevent footbox condensation.

[Roger Caffin]

> Why is it that the footbox gets more condensation than the rest of the bivy? Do the feet really sweat that much more?
My guess is that the rest of the bag gets more heat from your body. The footbox is the coldest part.

[t.darrah]

The foot box of the bivy is the furthest point from the bivy opening resulting in less air movement. Often times hikers/climbers will store damp footwear at the inside base of the bivy to help prevent freezing, this will also result in a greater possibility of increased condensation.

[Author]

Posts