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 right now on an eVent bivy is the Borah Gear Snowyside.
References that correspond to the numbers in the text above (some of these may require membership in Backpackinglight.com [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.)