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  1. #1

    Default Preventing Condensation in DIY Bivy

    For those of you that have made your own bivies, how do you prevent condensation? I'm thinking of using silnylon material, but I'm wary of its lack of breathability. My current blueprint has poles that will raise bivy material above the user so that you and your sleeping bag won't be contacting the bivy walls. I've also added some side vents, and I have a bug net that can be used by unzipping part of the top bivy layer on dry nights. Any recommendations would be appreciated, as this will be my first major MYOG project. Thanks!

  2. #2
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    I haven't (yet) made my own bivy, but I do have one, and basically I don't think you can prevent condensation in them under most circumstances. If you are enclosed, breathing behind netting (especially noseeum) then I think your only chance of avoiding condensation is if you are in an arid environment (like the desert southwest) and/or with a good breeze.

    The first is important for a humidity differential which may help draw the moisture through the "breathable" part of the bivy (and noseeum). That's pretty rare in the eastern USA. A breeze is helpful, but is no cure by itself.

    If you leave the bivy open through those side vents (not behind netting), that will be very helpful, but possibly no cure either. You need something actively driving the moist air from your breath out of the bivy, and passive venting is probably not enough IMO. It's especially not enough behind noseeum, which really traps air pretty well. I'd be interested to see tests of oxygen levels behind noseeum on a windless night versus that of say simple mosquito netting.

    My DIY bivy plan will be to mate an unneeded tent footprint with a "box" style mosquito net (Mombasa Outback Travel Net from REI). It too will be suspended above the sleeping bag, not only not touching, but giving room to sit up. But the bigger thing will be to allow more airflow in summer. Where I live, windspeeds usually decrease at night, barring a front moving through, so being in a noseeum tent in summer can be sweltering. I do have a little handheld fan to help (about the size of a deck of cards) but not trapping so much air could help even more. I have used ordinary mosquito headnets around noseeums and they have worked adequately. Whether this will or not, who knows, but I'm game to try.

  3. #3

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    When you used the mosquito headnet around noseeums, was it in an area where noseeums are prevalent? I've heard stories of people getting eaten alive by noseeums through mosquito nets, so I guess I'll have to weigh the cost of ventilation vs. potential small critters inside.

    What are your thoughts of adding a vent near the foot as well? I know that tends to be a big pooling site, but that's often also due to feet hitting the bivy material, which (hopefully) won't be the case if I build this properly.

  4. #4
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    breath outside the bivy as much as possible

  5. #5

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    Ideally, you'd use Gortex or similar for the top.

    I have an early version of the OR Advanced bivy which I often use in the fall. Not sure what the top is made of, but I believe it's breathable. Never had much of a condenstation issue even when it had to be buttoned up for light rain. It's actually a little heavier then my tent, but I can set it up in more places and I usually need the little extra warm and (hopefully) not rain protection. Timing is everything though.
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  6. #6

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    I was hoping to avoid using Goretex or eVent material due to the cost, but I agree that it would be the ideal choice.

    I've been looking at OR's bivies and waiting to hear back from them on their waterproof/breathability ratings. Hoping I can find a somewhat comparable material on Ripstop by the Roll or another raw material website.

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    Quote Originally Posted by glenrunner View Post
    When you used the mosquito headnet around noseeums, was it in an area where noseeums are prevalent? I've heard stories of people getting eaten alive by noseeums through mosquito nets, so I guess I'll have to weigh the cost of ventilation vs. potential small critters inside.

    What are your thoughts of adding a vent near the foot as well? I know that tends to be a big pooling site, but that's often also due to feet hitting the bivy material, which (hopefully) won't be the case if I build this properly.
    I was setting up my tent in the late afternoon for some backyard testing that evening, and I expected mosquitoes to be the problem, so I had the net handy. Turns out, it was noseeums that were getting after me. I didn't feel like going to find the DEET since I didn't want that on my hands while I handled the tent materials. Plus, it would just take me a few minutes to finish setting up. Sample size of 1, so take with a grain of salt, but it worked for me that one time.

    Frankly I don't think touching the tent or bivy walls causes the moisture. I think breathing in the bivy and transpiration of water vapor off the skin, esp. on the feet., causes moist air. This air condenses on the interior single wall of the tent/bivy (or in your bag - depends on where the dew point hits). Your feet touch it and the moisture goes from the inner tent wall to your sleeping bag shell - and maybe further into the insulation, depending. I don't think moisture enters the bag from the outside, generally.

    As for your design, yes, a foot vent would help. From what I've read, the best you can do to avoid bivy condensation is to have a mesh top, and not just from torso on up. At least a stripe of mesh down the middle to the feet. Maybe an inverted T, going laterally at the base of the feet. Cesar of "Cesar and the Woods" did the mesh stripe and said it helped a great deal.

    http://cesarandthewoods.blogspot.com...ification.html

    [Note, of course, that this will then generally imply the necessity of adding a tarp over the bivy for rain protection, since mesh isn't going to keep out rain. You don't want to your bathtub floor to become a real bathtub!]

    Also, check out this blog post:

    https://40yearsofwalking.wordpress.c...ion-conundrum/

    HTH
    TZ

  8. #8

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    Thanks! These are some great resources. I'll play around with the extended mesh top for a bit. I'm slightly against tarps at the moment, as that will just be another thing I have to carry. It'll be fun to look into how to optimize breathability without requiring a tarp.

  9. #9

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    I only use the bivy on short trips when I'm pretty sure it's not going to rain or at least not much. Maybe a little early morning drizzle kind of thing.

    For long trips where there's a greater chance of consistent or more frequent rain, you really need to add a decent sized tarp. With out a trap, getting in or out of a bivy sack while it's raining will get you and most of your gear wet. Plus you need a sheltered area to organize under. So, in this case you might as well just have a bug bivy which is mostly netting. Then the condensation isn't an issue. I find a bug bivy also improves a bag's temp rating by a noticeable amount.

    The SMD Gatewood cape and serenity bivy work along those lines, but I find that combo a bit cramped.
    Follow slogoen on Instagram.

  10. #10

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    Ah good point about getting in/out while it's raining, and sweet recommendation about the serenity bivy. I really like that design. It seems almost large enough to be considered a tent and fit all of your gear inside. I'm a pretty small person, so I'm aiming to make a bivy large enough to fit me and my pack inside. I've found I can usually fit myself and my pack into standard sized bivies.

  11. #11

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    I have four bivies which are all tubes with a hole in the end to slide into. One is long enough gear could go into it too, but it would be all the way at the foot, making it a pain to deal with, so I never do that.

    I have a Gortex bivy I used to use for cowboy camping in the winter. When tents used to weigh 3-4 pounds, it was a 1 pound alternative.
    I have a North Face bivy (that's the long one). Don't really like this one.
    I have an Outdoor Research Advance bivy, this one I use the most in the fall.
    I have an Outdoor Research Bug Bivy, which is great on a nice summer night as it's like cowboy camping but with bug protection.
    The OR bivies come with poles to keep the material off your head, the first two don't have any support.

    I used the Gatewood cape with the OR bug bivy for rain protection. I did eventually buy the Serenity net, but like I commented earlier, I find it a little too cramped, being 5' 11". But at least the Serenity net opens up along the side so it's easier to get into and out of.
    Follow slogoen on Instagram.

  12. #12
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    Quote Originally Posted by glenrunner View Post
    Thanks! These are some great resources. I'll play around with the extended mesh top for a bit. I'm slightly against tarps at the moment, as that will just be another thing I have to carry. It'll be fun to look into how to optimize breathability without requiring a tarp.
    For real life experience with bug bivy and tarp, check out Evan's backpacking video logs of his recent AT hike. Tarp and bug bivy the whole way.
    humor is the gadfly on the corpse of tragedy

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