I love sleeping outside under the stars and when I first started backpacking, a lot, it was mostly in the desert and drier West. We would just roll the bags out on a ground sheet and all was good. If we were snow camping or in rainy conditions we would then use a double walled tent. Somewhere in the 70’s I picked up a North Face bivy with a urethane coated nylon floor and a breathable taffeta top. I used it sporadically either to add some wind protection, and in theory additional warmth, and to keep the bag clean in certain conditions. I never remember any condensation in any conditions regardless of temperature.
I ended up wearing some holes in the bottom from using it on sandstone and began to look for a replacement when I made the move towards lightweight and ultralight loads. By then I was mostly camping in the East and in the winter months, Oct.- March. I ended up with an Outdoor Research (OR) Basic Bag cover (no longer available) with the hydroseal floor and a Gore Dryloft top. A little heavy at 18 oz. but it did a great job as a “bag cover”. I don’t want a bivy to be a stand alone shelter, I just want it to protect the bag from dew, rain spray when under a tarp, act as a ground sheet, and give some wind protection as part of my shelter/sleep system.
In over 40 nights of use I only had condensation inside the bivy 3 times and those nights were all near freezing. I noted it and was not too concerned about it. I then began to look for an even lighter alternative. Conventional wisdom at the time was a silnylon floor with a breathable nylon top and a good DWR (Durable Water Repellant) like a Pertex Quantum; just a high tech, newer fabric version of what I used in the 70’s. I wanted a side zipper for easier access and ended up with a Mountain Laurel Designs Superlight at 6 oz. with a Momentum 90 top fabric, supposedly the most breathable fabric available at the time according to Ron Bell at MLD.
- North Face bivy, urethane coated nylon bottom, breathable taffeta top, way too big, 17 oz., ’74-‘01
- Outdoor Research Basic Bag cover, hydroseal bottom, Gore dryloft top, 18 oz., ‘01-‘08
- Mountain Laurel Designs Superlight, silnylon bottom, Pertex Momentum 90 top, 6 oz. with long side zip, ’08-
I thought “now I am set, state of the art”. Over the course of nearly 30 nights I have had condensation/frost nearly 50% of the time! What the hell? And it wasn’t just me, my friend Bob who also had an OR basic bivy was having the same issues with his new Rab bivy with a Quantum Endurance top.
A lot of people just assume that condensation is part of sleeping in a bivy but my experience was different and this just wasn’t acceptable so I started doing lots of research on the internet and other places to try and find answers and/or a better, more breathable material. Think of this as sort of a literature review, here is what I have discovered.
First I know that part of the problem is that we are always sleeping out in some of the most challenging conditions for bivys and condensation with temperatures right in the trouble zone, just above and below freezing when the dew points are the lowest or at least very close to the nights low temperature. The warmer it is, generally the less problems with condensation. The worst are the clear nights, when you most want to be out from under a tarp or trees to watch the sky but you need to fear the clear sky! Doesn’t really matter if it is humid or not, even in the desert I’ve had condensation/frost on clear nights.
The problem here is moisture from the inside, not the outside. Moisture will condense on the inside of the farthest out layer of fabric. This layer of fabric cools faster because of radiation heat loss to the sky on clear nights, it reaches the dew point quicker than the other surfaces around it and below it, water vapor will then condense on that cool surface. In the case of a bivy material laying right on the top of the sleeping bag with a very slick inside, calendered surface, like Momentum, it condenses between the top of the sleeping bag and the bottom of the bivy material. [1, 2, 3, 4]
There are several things one can do to solve this problem besides not breathing into your sleeping bag/bivy (which I never do) and wearing damp clothes to bed.
- sleep in your bivy under a tarp, or trees or a shelter where the coolest/radiating/condensing surface is well above you, but then you defeat your view of the sky.
- lift the top of the bivy material off the sleeping bag which moves the condensation surface away from the top of the sleeping bag (see above) and increases air flow to move the moisture away. This is done with hoops or a bungee cord to a pole or other attachment above your head. Fairly easily done but not as nice as just laying out in the bag(s).
- take advantage of breezes to help move moisture away (see above), this can be a fine line when actually trying to use a bivy for wind protection.
- sleep with a hot water bottle to push even more heat through the sleeping bag and move the condensation point to the outside of the bivy. Good theory, how practical is harder to say. 
- sleep in a lighter sleeping bag so your body heat will push the moisture out beyond the bivy. A sleeping bag that is too warm (well insulated) keeps the dew point closer to the heat source, you. Same theory as above.
- If it is really cold (below at least 20 degrees) you can sleep with a vapor barrier liner or VBL clothing which keeps the moisture right next to you. [6, 7]
- Change the material in the top of the bivy to one that actively wicks or helps move the moisture through the material.
Changing the material is what I did going from the OR with Dryloft to the MLD with Momentum but it might have been the wrong way! I have scoured the internet and there is no miracle fabric, all bivies have condensation/frost from time to time but the ones with the least mentions of condensation are made of all eVENT like the Integral Designs All eVENT bag cover (now discontinued) and the OR Advanced Bivy, both with vapor and air permeable top fabrics. [8, 9, 10]
The ones with a breathable nylon top with a good DWR and minimal silnylon floors, but without high bathtub floors, are the next best. From reports there seems to be something about the waterproof bathtub floors that leads to more condensation. This is apparently why the ones made of all eVent (top and bottom) have fewer reports of problems with condensation.
The worst offenders are any of the polyurethane membrane/WPB materials that are only vapor permeable like full Gore-Tex, Montbell DryTec, Mountain Hardware Conduit, etc. I have used or been around many of these and they all have had bad condensation problems.
One of the things that seems to happen with the vapor and air permeable fabrics (eVent, Gore-Tex FLO2, Gore-Tex Respiration Positive, Exchangelite and the less waterproof Gore-Tex Dryloft now Windstopper ) is that the membranes are laminated onto fabrics, and many lined with Tricot, that seem to help wick or pass the moisture through the material. I feel that this is why my old OR bivy had so few condensation problems, it has an almost cottony feel to it. The problem with these fabrics is they are heavier and the lightest of these bivies on the market are 13 oz. and up, with most around 18 oz. and they are crazy expensive ($200 plus).
So this is the conundrum: it is a tradeoff between ease of use, just roll the bag out where ever you want, with less condensation but at a higher weight and cost or be more careful in site selection and set up with a lower weight and cost.
Here is a spreadsheet with a selection of the bivy’s most applicable for light and ultralight hiking and most mentioned in internet discussions. It is not exhaustive and models come and go, the important part is which ones are made with which fabrics. Currently Rab/Integral Designs and OR seem to be offering the most options with eVent or eVent like fabrics. It’s not the lightest but the best deal on an eVent bivy used to be the Borah Gear Snowyside. The Snowyside has not been available for awhile and there is some internet talk that Borah is going to discontinue it for something with another fabric.
References that correspond to the numbers in the text above (some of these may require membership in Backpackinglight.com and with the new BPL website some of them no longer exist [BPL articles but not forum threads] to view):
- Night time condensation on tarp and tent fabrics BPL article
- Dew, condensation and frost
- BPL Article on Condensation in Single Wall Shelters
- MSR video on Condensation in shelters
- Cold Weather Clothing, good information on dew points in insulation
- BPL article by Andrew Skurka on Vapor Barrier Liners
- A good discussion on using vapor barriers BPL thread
- BPL State of the Market Report on Bivy Sacks from 2006
- Waterproof Breathable Fabric Technologies BPL article
- A thread on the different fabrics as it can be impossible to find details BPL
- New fabrics on the horizon BPL article
- A BPL thread on winter bivy’s and using Tyvek or other materials.
- Lots of threads on BPL on bivy condensation
- The Book of the Bivvy by Ronald Turnbull (This is not a technical book about bivy’s but more about the technique and joy of sleeping out in the open in a bivy. There are only a few pages on actual styles and fabrics.)
Very interesting research. Thanks for the good work!
I bought the MLD Superlight last summer and love it. It especially complements my Golite 20 Quilt as it holds the quilt in place when I turn over and cuts down on the wind. I have also noticed condensation on some nights, as you mentioned, when the temps hover around freezing but not enough to wet out the bag.
Nice well written article.
I recently bought a MLD Superlight Bivy with cuben fiber floor and Momentum 50 top and the half-moon netting.
I have had it out a few times and so far no condensation. Both times the lowest temps were in the low 50’s so by no means am I able to say it is not going to suffer any issues. I am very much looking forward to this winter when I can get it down into the sub-freezing range and really see how it will do.
Any thoughts on how you suspect it will handle condensation?
I would think the Momentum 50 should breath better than the 90 and so less problems, it really depends on if you are under another cover or not (trees, tarp or tent). I look forward to hearing how it works. In your initial nights were you out in the open?
A quick update on the above mentioned bivy.
After more than a fair trial I ended up selling it. I used it for around 350 miles over the last part of my 2011 hiking season and used it in a lot of different conditions. The final decision came when I was on a hike in the Trinity Alps of Northern California. The weather dropped unexpectedly (~15f lower than expected) and I woke up around 1am and the inside of the bivy walls were covered in condensation. It was a drastic temperature change over around a five hour period while I was sleeping. I just did not enjoy being a long way from nowhere at 1am and having my sleeping bag soaked and the gear I had inside of the bivy damp. 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
I recently used a Sierra Design bivy with a Drizone top / 70D nylon bottom in Big Bend area for 7 nights and had condensation every time. I used a WM 15* bag, under a tarp, and never zipped up the bivy. The condensation always collected evenly on the bottom/wall of the waterproof material and never the top. Some nights created more than others and the only frost on the bag I noticed was in the foot area. There were too many variables in the weather for me to point to any one cause. Low temps ranged from 17* to the mid 20s. We had some clear nights, high clouds, and even some ice fog and high winds that coated the basin with ice.
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The best bivy I have found for less condensation is the Montbell breeze-dry, taped seams, breathes well. I’ve tried quite a few.
While the Breeze Drytec has many fans, I think mostly in the western US, my experience was really bad condensation here in NC under clear conditions but just below freezing.
Thank you for this article.
í see its from 2011, i was wondering if your advice is the same.
Could you share what you feel is the best bivy these days? (2015)
Thanks for your time, your advise is appreciated !
njoy the best day ever 🙂
Yes, generally my advice is still the same. eVent is still the most breathable of the waterproof options and will have less condensation but will always result in a heavier bivy. The Borah gear is still the least expensive option ($170 with side zipper) but it comes with a weight penalty at well over 20 oz. You could probably get John to make you one with a cuben or silnylon floor and shave off 4-6 oz.(?). The MLD eVent Soul is 13 oz. with a cuben floor but at $355 I find it just too much money for such an item.
As of 11/15, Borah now offers the eVent bivy with a sil-PU nylon floor that comes in at 12.5-14.4 oz. depending on the batch of eVent material. For $159 dollars, for sure a bargain.
In non-waterproof fabrics it appears the most breathable ones are Argon and Nobul but that comes from this thread at BPL
I use a Gore-Tex bivi bag and there is NO condensation whatsoever on the inside of the bag. The bag is a French army surplus bivouac bag, extremely high quality and made out of Gore Tex. This highly breathable material means that when sleeping inside the bivi with my down sleeping bag the inside of the bivi is dry in the morning, even after 8 hours of straight use in temperatures that range from near-freezing to +15 C. They key is to buy a bivy that uses the most breathable fabric available, but it will usually cost a bit more.
Bob, I am glad that you have found a bivy that works for you in your conditions. Gore-Tex is better than some materials but there are still plenty of reports of condensation with Gore-Tex too and it still not as breathable as eVent. Even with eVent there are reports but consistently less. The point of my piece is both material choice but also site selection especially in cool to cold conditions.
I have always had problems with condensation in my superlight mld bivy. On multiple day hikes it’s been so bad that my sleeping bag almost collapsed. And that is really bad and potentially dangerous in near freezing conditions. I now bring an synthetic quilt as an overbag instead. 200 grams more, but keeps the down bag dry and ads warmth. Only concern for me is bugs in warmer conditions.
After having done a fair amount of research in preparation for my first bivy purchase, your article has been, by far the most informative and objective comparison. As with almost all types of camping equipment, gear selection is always going to be a tradeoff between comfort, cost, ease of use, and weight. The information you’ve presented however is helping me take a good long step toward finding the solution that will be most appropriate for the kind of camping I intend to do. Much appreciated and keep up the good work!
I have been using the Uber Bivy for a few years now and it has been 95% condensation free on roughly 75-100 uses in all seasons. 26degrees is the lowest temp I have used it in and most nights have been 8-10,000ft. 35-45 low temps. I have been very happy with it!
that is good to know, Mile’s bivies do seem to perform well but they are heavier than most.
This data point may be pertinent: I made a bivouac sans sleeping bag cover on the Kahiltna glacier on my way back down from 16k’ on Denali after coming down with a bad chest infection and didn’t have a tent or bag cover, or even a plastic garbage bag liner (which in retrospect was insane) but regardless, I was trying to catch up with a group of guys from Tennessee who I could rope up with to cross the last part of the lower glacier to the landing strip where ice bridges and crevasse hazards were greatest. The ideal crossing time was the very early morning when things would be most frozen.
This is all to say, I bivouacked for a few hours in the evening until early first light (~3-4am) with neither a VBL/trash bag liner nor a tech-fabric bag cover and I had a large amount of frosted condensation all over my -30F 800+ fill down bag (which had a DWR coating on the outside nylon). My take away is that over a long expedition where you hope to maintain down loft (especially down, even more so then synthetic) VBL’s and or plastic sacks should be used frequently to keep moisture from penetrating insulation _from the inside_. When I slept with two other climbers at camp at 14k’ there were three of us in a 4 season tent and it was reasonably warm, so I don’t think this moisture entrapment was as large of an issue with 3 dudes snuggled tightly in a small double wall winter tent (due to relative warmth, mind you it was still down to -40F some nights outside).
Andrew Skurka has a few articles about this, and clothing/sleep systems he used in northern climes to go reasonably light while maintaining insulation loft (search for ‘Skurka Icebox’); all things being equal I’d much rather have an ultralight fully waterproof bivy/bag cover and use a VBL hot sack (while being much warmer but clammy) then carry a heavier bag cover and know that my insulation will eventually wet out (especially down over time). Only my $0.02; YMMV; if you’re not doing a multi-week expedition at altitude or below freezing hopefully your margins will be better.
Color matters. Black bivies radiate the most, are coolest and, so, condense more than other colors. White radiates the least, is warmest, etc. Superlite is black; the Miles is white
Hi Steve, while you are correct to some degree, from my reading all colors are seen as “black bodies” when it comes to radiation as discussed in the first reference article above. I think the reason Miles bivies have little trouble is mostly the fabric but possibly also a bit the color.
Given that the fabric is being used when it is dark the color will not matter. Without light hitting the material black body radiation is pretty much the only concern. It’s remotely possible different cloth materials have varying conductivity rates. That would create a difference in moving the energy from inside to outside. Would make a good science fair project.
Color will matter greatly the next day when you put the object out in the sun. The heat absorption curve looks like an inverse log curve. Going from white to orange it climbs very steeply. By the time you get to Orange it’s really high, and continues to increase across the rainbow colors as you go toward black. So, any dark color is going to take light and convert it into heat pretty well. But, that is very different from an object emitting heat radiation.
Excellent commentary on bivy’s and bivy fabrics.
I went away from bivy’s in the early 90s…just too many problems. I bet if you measured the moisture coming off of different individuals at night it would be a huge swing. I think my body could form its own cumulous cloud from the moisture at night. My GF at the time didn’t have nearly the same amount of condensation using the exact same equipment.
I went to a UL shelter and never looked back. Of course these have their own issues with condensation.
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I have used a VBL with my sleeping bag in temperatures only a few degrees below freezing (maybe 4-5), largely to avoid condensation forming on the bag, which can happen even in my tent.
Hi – thanks for a great article! I’m curious if you have any updates? I want to experiment with bivys and am worried about the condensation. In particular, I’m keen to know if you have updated bivy and/or sleeping bag recommendations.
have not done any in depth research of late but just was looking for a friend and it appears that many UL folks are buying the Borah Gear bivies or spending the dollars for one from Mountain Laurel Designs who has the only eVent topped ones I know of.