About the author

Tim Moss

Tim Moss has supported over 100 expeditions across all seven continents. He has climbed new mountains, crossed a desert on foot and recently cycled 13,000 miles around the world. He is a Fellow of the Royal Geographical Society London and a Guinness World Record Holder. He aims to encourage more people to live adventurously. Read more...

90 Comments

  1. 1

    Steve Fuller

    Three others that I’d mention (not that there aren’t a million of them now) – Nemo’s GoGo Bivvy (LE and Elite), Titanium Goat Biivvies (Ptarmigan and Kestrel) and the Miles Gear Pico Bivvy.

    Reply
    1. 1.1

      Tim Moss

      Thanks for those Steve. I think the range of bivis differs quite a lot between the US and UK. If the article’s popular over the Atlantic then I’ll add some more details for the American range.

      Reply
      1. 1.1.1

        nazzarena

        How about the Pieps Alien Bivy Bag and the current ones from Vaude (BIWAK I and II). Thanks

      2. Tim Moss

        Hi Nazzarena,

        The Pieps bivi bags look like non-waterproof emergency covers so good as a back up, in dry weather or underneath a tarp.

        I’ve added the Vaude Biwak I and II to my table. Their hydrostatic head is only 3,000mm which means they are not properly waterproof, just OK for a light shower. They’re light though and very cheap.

        Hope that helps.

        Tim.

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  6. 2

    Karl Ek

    First of all, thank you for a very good comparison and review.
    There is one Bivvy Bag that you didn’t mention. A Bivvy Bag that I believe are in the same category as Terra Nova or maybe even better.
    The name is “Bivy Bag T” and are made by TAIGA of Sweden. http://www.coldskills.com/bivy-bag-t-iir-p-847-c-363.aspx

    Reply
    1. 2.1

      Tim Moss

      Hi Karl, thanks for the link. That does look like a good bivy sack and, as you say, very similar to the Terra Nova.

      It’s heavier though (1.1kg compared to 840g for the Jupiter/Ridge Raider) and more expensive (£300 + £20 delivery to the UK). Another one to bear in mind.

      Reply
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  8. 3

    Ian Lewis

    Nice review Tim -thanks for sharing your experience.

    Can you comment on the US army sleep system or the Dutch army hooped bivi? Both look useful and are readily available through ebay.

    Thanks.

    Reply
    1. 3.1

      Tim Moss

      Thanks Ian. The Dutch Army Hooped Bivis look like they’ll have the same proprerties as a British Army Hoopless Bivi (i.e. tough, spacious, bulky, heavy) but with the obvious advantages of a hoop. For the prices on Ebay (~£90), they’re probably your best bet for a lower cost hooped bivy.

      The US Army Modular Sleep System looks neat and is no doubt very durable. If you’re planning on using the sleeping bags as well and like the idea of an integrated system then I’m sure they’ll be good. Otherwise, my hunch is that a simpler British army bivy bag will be better (i.e. possibly smaller/lighter due to less attachments, easier to use with other sleeping bags and without a opening on the side which could leak).

      I hope that helps. Let me know what you decide.

      Reply
  9. 4

    Jacqui McIntosh

    Trying to choose a bivvy for my husband for Christmas. He’s a ranger with the National Trust for Scotland and spends a lot of nights out on the hill. My question is do all the recommended bags have a mesh window for insect protection? Midges are hell up here!

    Reply
    1. 4.1

      Tim Moss

      Hi Jacqui, most bivvies do not have a mosquito net so it’s a good thing to check. Off the top of my head, I don’t know of any hoopless bivvies which have mozzie nets but the Terra Nova Jupiter and Rab Ridge Raider have them built in. They’re both excellent but expensive. You can, of course, just sleep in a cheap mosquito head net… but that might not be quite such a nice Christmas present!

      Just shout if I can help any further.

      Reply
  10. 5

    Steve francis

    Tim/jacqui….how about the ‘kelty trail’ for a hoopless bivy bag with a mozzie mesh?

    Reply
    1. 5.1

      Tim Moss

      Thanks for heads up Steve. It looks good but I think it’s a little harder to that track down over here in the UK. For whatever reason, the range of bivys in the US seems completely different from the range available here.

      Reply
  11. 6

    Jacqui McIntosh

    Thanks for the help, Tim and Steve. I’m quite handy with a needle, maybe I can modify a bivvy bag to include a mesh window. I’ll let you know how it turns out!

    Reply
  12. 7

    Jacqui McIntosh

    Further to my last post, I have been looking at the Highlander Hawk Bivvy. Do you have any information on it? Many thanks for your help.

    Reply
    1. 7.1

      Tim Moss

      Jacqui, so sorry for not replying to you. I’ve only just noticed your comment. You’ve probably long since made your decision but from what I’ve read, the Highlander Hawk Bivvy is a solid choice. Proper waterproof and breathable material with a mesh face cover and good reviews. In the UK, it’s on Amazon for £50: http://www.amazon.co.uk/dp/B003BBTX8U/?tag=thenexcha0b-21

      Reply
  13. 8

    Jake

    Hi,
    I was considering buying a Rab storm bivi, but could find no information about the Hyperlite storm fabric used in its construction, so I phoned Rab and the helpful chap sent me an email with the details of the fabric.. I was shocked to find that it only has a HH of 3000 and to top this it is a PU coated nylon that is not I repeat NOT breathable!!! Even though the advertising says it’s breathable!

    Reply
  14. 9

    Jake

    Hi ya again,
    Heres the actual e-mail i got from rab concerning their Hyperlite Storm fabric as used in the Storm bivi:-

    Hyperlite Storm is a proprietary fabric of Equip Outdoor Technologies LTD. It is a fully waterproof nylon thanks to the clear polyutherane coating. Hyperlite Storm is not meant to be breathable; it is meant to provide worst-case scenario weather and moisture protection.

    Main Properties
    Composition 100% Teffeta nylon, Coating: clear PU, non-breathable, Yarn Size: Warp 70d / Weft 70d, Waterproofness: 3000mm HH.

    Explains why lots of people are getting wet and having condensation problems in these bags!
    Have fun

    Reply
    1. 9.1

      Tim Moss

      Jake, thanks for taking the time to share that information. I was waiting for a reply from Rab. They said:

      “There are two different fabrics used on the bivi. The base is made from a non-breathable fabric, which is used because of its high hydrostatic head of 10,000mmm. This is important as it is under direct pressure from your body on the ground. The upper fabric is under no pressure and so uses a lighter fabric which is breathable and has a hydrostatic head of 3000mm. This is still higher than many tent outer fabrics and more than sufficient to keep rain out.”

      This obviously contradicts the email that you received but my hunch is that whoever emailed you made a mistake. All the literature suggests that the Storm Bivi is breathable and it would be a pretty poor design for it not to be. A non-breathable waterproof base, on the other hand, is common and good practice.

      That said, several reviews do suggest that the breathability – regardless of the technicalities of the fabric – is poor.

      Reply
  15. 10

    Lisa Mac

    Hi, the links you gave for the Terra Nova Jupiter no longer seem to sell that item? I can’t find it anywhere on the website that you link to. Mind you I’m tired right now, and it’s late, so maybe I’m missing it. Or do they no longer sell it? Regards

    Reply
    1. 10.1

      Tim Moss

      Hi Lisa, that’s interesting. I’ve just done a search and it seems to have almost disappeared overnight. There aren’t even any sellers on Amazon or eBay and it’s been taken off the Terra Nova website. I’ve emailed them to ask about it and will let you know when I hear back. In the meantime, Rab’s Ridgeraider is very similar.

      Reply
    2. 10.2

      Tim Moss

      Got a swift response from Terra Nova: they’ve run out of stock for the Jupiter’s poles so they won’t be in stock again until some time after June.

      Reply
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  17. 11

    Alistair

    Great article thanks Tim… how about the Terra Nova Discovery Lite as a light weight high performance bag please?

    Reply
    1. 11.1

      Tim Moss

      Hi Alistair, thanks for drawing my attention to that one. It’s not listed on the Terra Nova site so I’m not sure if it’s still being made but they’re still available online.

      It’s PacLite so it will be both waterproof and have good breathability so, as you suggest, performance should be good.

      Apart from it’s price (£170 for a hoopless bivy!), the most likely issues I suspect will be it’s size and toughness.

      It’s only 200cm which is shorter than all of the other bivvies here and I’ve seen some reviewers who’ve sent it back because they couldn’t fit inside.

      PacLite is notorious for tearing easily so it won’t be the toughest bag in the world but then the groundsheet is a different material and lightweight will always mean a compromise on durability. (Look out for new Goretex Active bivvies though – that’s a much tougher material that’s lighter than PacLite).

      In summary, if you can afford it and aren’t too tall, then this should be a good quality lightweight bivy. If not, get a Rab Survival Zone or even an Alpkit Hunka.

      Reply
  18. 12

    dave woodhouse

    Hi Tim, nice review. Tell me please do you have any experience of Outdoor Research bivis??

    Reply
    1. 12.1

      Tim Moss

      Hi Dave, I don’t have any personal experience actually (I should probably write to them and see if they’ll give me one to test). The range in the US is very different from the range here in the UK and I’ve not yet tried to cover it.

      However, they look decent to me. A good range using 3-layer Gore-Tex and Pertex Shield+ on the lighter models. Their weights are good, particularly the 500g hooped Helium Bivy. The Advanced Bivy looks very similar to the Terra Nova Jupiter which I’ve been using for over 10 years – both Goretex on top, toughened floor and single hoop (although the newer Jupiters are lighter than OR’s Advanced Bivy). It’s also interesting to see hoopless bivis with zips and mesh hoods which aren’t very common over here.

      I hope that helps a little but just fire away if I can help with anything specific.

      Reply
  19. 13

    Paul Myerscough

    I just returned from a wet trip with my Rab Storm bivi which I used in a MLD trailstar tarp. My biggest problem was getting in and out of the thing. I use a thermorest mat inside which means the bag becomes flat and stiff. Getting in and out at the top end invites clashes with the low roof of the tarp and is almost impossible without dragging myself and the sleeping bag onto the wet ground. Not to mentioned calf cramps from the contortions to get my legs out.

    Surely there is and easier way?

    Reply
    1. 13.1

      Tim Moss

      Hi Paul, getting into and out of bivi bags is certainly part of the fun!

      I suppose options for improving your lot would be:

        Put your mat outside the bivi rather than inside (obviously works better with solid foam roll mats).
        Get a bigger bivi. Army bivvies are the biggest whilst the lightest ones tend to be the smallest.
        Get a bivi with a zipped side entrance
        Get a hooped bivi. They present their own challenges but at least they stay open.

      Apart from that, perhaps you could do some practice drills in the safety of your living room?

      Good luck!

      Tim.

      Reply
  20. 14

    Iain

    Any thoughts on the Terra Nova Survival Bivi Bag?

    Reply
    1. 14.1

      Tim Moss

      Well, it looks very small, very light and very cheap. I’ve not tested it myself (although I’ve just emailed Terra Nova to ask for one) but my hunch would be that it’s either not that waterproof or, more likely, not that breathable. But if you just want something small and light to keep the dew off and protect your sleeping bag then it’ll probably be pretty good.

      The lightweight alternatives are the Rab Survival Zones which are more expensive and a budget alternative would be the Alpkit Hunka.

      Hope that helps for now.

      Tim.

      Reply
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  22. 15

    Palle Nielsen

    Hi, Tim. Really nice blog you got here. I´m a totally beginner in this “bivi-world”. Do you have any experience with the Karabatic Gear Bristelcine Bivi or the Mountain Range gore-tex ?

    Reply
    1. 15.1

      Tim Moss

      Thanks Palle. The Bristelcine Bivi is very light but it’s only water resistant so if it rains you’ll get wet. Do you have a link for the Mountain Range one? If it’s Goretex then you can’t go too far wrong.

      Reply
  23. 16

    Brian Coupland

    Interesting article. Just a heads up to say that the Rab Ridge Raider is now on offer at Go Outdoors

    Reply
    1. 16.1

      Tim Moss

      Brilliant, thanks for sharing that Brian. I’ve added links above or click here.

      Reply
  24. 17

    Tony

    Mr. Moss,
    Your knowledge is worth more than gold! Your gift of sharing it is truly beyond thanks. Would you please do a review of the top 10 bivouacs for USA market? Cheers!

    Reply
    1. 17.1

      Tim Moss

      Thanks Tony. I’m hoping to add a comparison for US bivi bags in the near future but it’s several days’ work to put these articles together so I have to fit it around normal life and work. I’ll send you an email when it’s done though and feel free to ask any questions in the meantime.

      Reply
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  26. 18

    Alfie Adens

    Hi Tim, what about the Snugpak stratosphere bivi, I’ve got a loan of one to try this w/end in the western highlands? Great blog by the way.

    Reply
    1. 18.1

      Tim Moss

      Hi Alfie, the Snugpak Stratosphere looks most similar to the Vaude bivi on my list i.e. a decent, lower budget hooped bivi.

      It’s waterproof and breathable although I think it might be using coated nylon rather than a waterproof membrane which means, like the Vaude, it might be a little sweatier and/or lose its waterproofing faster over time.

      Looks like a good bag though and I’m sure it will be fine. Enjoy the Highlands!

      Reply
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  28. 19

    Mike Foley

    Hello Tim. Great blog and extremely informative, iv read it several times.

    I want to buy a hooped bivy over here in Ireland and pickings are slim.

    Im thinking about the vaude or snugpak andwas wondering would an additional snugpak bivy sack around my sleeping bag be an idea towards keeping it dry from condensation or would it be counter productive?

    Should I just hold off and buy a TN Jupiter or RAB ridge raider?? Thank you and once again, tread article

    Mike.

    Reply
    1. 19.1

      Mike Foley

      “Great article” that should have been

      Reply
    2. 19.2

      Tim Moss

      Hey Mike, thanks for the kind words. Were you suggesting a second bivi inside another one? I’ve not tried it but I am pretty sure that would result in a lot of sweaty condensation. Bear in mind that the condensation inside a sealed bivi bag comes from your body so it’s not a case of keeping it out but letting it out.

      The best way to reduce moisture in a bivi is probably just to pitch it somewhere a bit breezy and leave it open as much as possible.

      Presumably you could just pay for shipping for an online order of the Jupiter / Ridge Raider?

      Reply
      1. 19.2.1

        Mike Foley

        Hello again, thank you for the reply.

        I thought as much as regards the 2 bivys alright. The method to my madness was to put the sleeping bag inside the bivy sack to stop the moisture from your breath getting onto the bag, and to stop the sleeping bag having direct contact with the walls of the tent

        Then to be in the bivy tent (because id rather be sealed up when im out) but have the vents open and a tarp over the top. I realise im adding weight and bulk but I carry a tarp and bivy sac anyway even when bringing a tent so I can leave the main pack behind and not have to worry about shelter if anything goes wrong.

        I can order on line but I wanted the “rab ridge master” because of the extra hoop at the foot end and also the side door entrance simple because with the side door and a tarp over the top I have a nice work and lounge area in bad weather. Unfortunately the rab ridge master has been discontinued.

        Iv read great reviews of the vaude however. If it was made of event or goretex then I wouldn’t hesitate a bit

        Thanks again for getting back to me sir.
        Sincerely Mike.

  29. 20

    Denis Fleming

    OR Advanced Bivi was recommend to me by a Swedish friend so checked it out and it does seem like a good piece of kit – do you have any thoughts on this product?

    Reply
    1. 20.1

      Tim Moss

      Hey Denis, I think the Outdoor Research Advanced Bivy looks good. It’s hooped, made with Goretex, follows a simple design and is similar in both weight and price to the Terra Nova Jupiter and Rab Ridge Raider so I suspect it will be a good piece of kit. Let me know what you decide!

      Reply
      1. 20.1.1

        Taith Ellie

        I’ve also been looking at Outdoor Research, and whether or not to go for the Advanced Bivi with all the features, or the Helium one which looks like a basic one but hooped. Having used both simple and fancier bivis, for more frequent use would you say it’s worth splashing out on a better bag?

        I’ve only ever used the ex-army and it’s done the job perfectly fine – wonderful living outside used with a roof, no complaints. On microadventures it held up fine in snow, but I got very wet when it rained on the beach. My hoodie pillow soaked up most of the water, so it wasn’t much fun in the morning, but I was only walking 30 mins back to the car.

        I much prefer sleeping out in the open that in a tent, but I want to upgrade my bivi so that it’s more comfortable if (when!) it rains, if I then need to hike the next day. I’m currently in Chile and have friends coming from the US and UK soon, so I have the option of both markets. Too much choice if you ask me!

        Also, great article, thanks!!

      2. Tim Moss

        Hey Taith, thanks for the comment. If you’re not bothered about a hoop then I don’t think there’s a big difference between the cheap (e.g. army and Alpkit) and the expensive.

        I think there’s a big difference between hooped and non-hooped though. The latter are obviously simpler and easier to pitch/strike, not to mention cheaper and lighter. But a hoop is a vast improvement in the rain. Primarily keeping the rain off your face but also shedding water better by virtue of maintaining their shape and they often tend to have fully waterproof floors.

        Personally, I have a hooped bivvy so almost always use it and find it much nicer. Some would baulk at the cost (including me now!) or just prefer the simplicity of non-hooped.

        Outdoor Research contacted me recently so I’ll hopefully be able to review the Helium Bivi. 500 grams and hooped!

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  31. 21

    Fabio.

    Brilliant article Tim! I am considering a divvy and so far the sort list is the Highlander Hawk Bivvy, an american army bivy or a belgian army divvy (of course I am also thinking of the good old British bivvy . . . But I do want a mesh and I am on a budget. Any advice or notes, especially concerning the Highlander Hawk Bivvy and american body (to be used with my own sleeping bag) will be much appreciated. The hills are calling!

    Reply
    1. 21.1

      Tim Moss

      Hi Fabio. You will never go wrong with an ex-army bivi. They’re often a bit big and heavy but always do a great job.

      But if you’re keen for mesh (and don’t want to sleep in a mosquito head net) then the Highlander Hawk Bivi looks good to me. Proper waterproof/breathable material, built in mosquito mesh and good reviews. It’s heavy for hoopless (over 900g) but then so are army bivvies. In the UK, the Highlander Hawk’s selling for less than £50 too which is good value.

      Reply
      1. 21.1.1

        Fabio.

        Many thanks for the advice Tim! I decided at the end to go for the ex-army bivvy: As you say, they seem to tick lots of boxes and are very affordable. Test it/use its few time to see if the mesh thing was something more in my mind that in reality (besides, slug cannot be that bad . . . can they?) an allnighter and take it from there. There is always a fall back position.

      2. Tim Moss

        Great stuff Fabio. Enjoy!

  32. 22

    Dan P

    Hello! Im looking for a lightweight, small pack size, and breathable bivvy that wont break the bank. I was hoping that the kne im considering was in your table but its not! Any thoughts on the Snugpak special forces bivi?

    Reply
    1. 22.1

      Tim Moss

      Hey Dan, thanks for flagging up the Snugpak Special Forces Bivvi. Looks like I need to add that to the list.

      What stands out from its stats is the hydrostatic head of 5,000mm (see Bivvy Bag Buyer’s Guide above for an explanation). That means it will probably be fine in a short shower but won’t keep you dry in heavy rain or if you lie on wet ground.

      At 440g, it actually looks slightly heavier than the Alpkit Hunka which is cheaper and has a higher hydrostatic head (10,000mm) so that might be a better option for you. Back in stock this month, apparently.

      Hope that helps.

      Tim.

      Reply
  33. 23

    Mary Hinge

    Excellent review. My own bag is a WildCountry bag about 8 years old now. I can’t find any similar on the www, but it’s outstanding features are that as well as being sturdy and very breathable, the head end has a plastic window which can be held off the face either by a monopole and guy cord, or by suspending the head end from an overhead branch, fencepost etc, using a bit of paracord atatched to the purpose made loop on the bag. You get the same effect as a hooped bivvi but obviously you need to have something to tie it to! Other pluses are that is is big enough to put your sleep mat inside and that there are small peg loops at each corner if you wish to pin your bivvy down – it’s a good idea in this one as the floor panel is a nonbreathable material, and the upper is a nice olive green goretex material. Keeps your goretex bit clean and unworn if you roll over inside the bivvy. It’s kept me dry in Cumbrian monsoons – I still proof it and use it. The only downside – weight and pack size… my Karrimor one man tent isn’t much heavier.

    I’ve used a HiGear bag once or twice – it’s just as you describe Tim.

    Finally – just landed an Alpkit Hunka XL in sexy kelp….. it’s coming down in stair rods tonight. Maybe time for a field test!

    Reply
    1. 23.1

      Tim Moss

      Thanks Mary. Sounds like an interesting bivvy bag! I’ve just got a Hunka XL myself and looking forward to testing it too.

      Reply
  34. 24

    Francis

    Dear Tim, many thanks for an excellent article. For a light weight cycle your across the Alps I am planning to bivvy. I am deciding between rab alpine (more expensive but lighter), or the highlander hawk (cheaper, heavier, but sturdy, low profile). Given pros and cons, which would you go for? Thanks again.

    Reply
    1. 24.1

      Tim Moss

      Hi Francis, good question. I think it just comes down to cost vs weight. The Rab Alpine will be the better bag at just over half the weight but it’s almost four times the price. If you’re feeling flush and going really lightweight (e.g. no panniers) then go Rab. If you’d rather save pennies or are using panniers then go Highlander.

      Of course, you could get the Alpkit Hunka which is both cheap and light!

      Reply
  35. 25

    Robin.

    Good article Tim. I often use a rab ascent bivi (similar to the rab storm but made of event). It’s quite a clever design in that it has a zip out bug mesh and a cord attached to the top flap That can be supported by a walking pole to hold the mouth of the bivi open. The bug mesh keeps the mozzies out but you get plenty of fresh air circulation.
    I also have a ridge raider. One thing that I think would be worthy of considering in your article is whether each bivi leaves room for a sleeping bag to loft if an air matress is used.
    I had a very cold night in my raider last winter because I used an exped synmat 9 UL in my bivi. I was sleeping in a rap ascent 900 bag but unfortunately the mat compressed most of the loft out of it. I was so cold my muscles cramped up and painfull. The storm was too fierce to risk moving as visability was only a few feet. (Pen y fan) I have since bought a thermarest Xtherm which is not as thick. Yet to try it in winter.

    Reply
  36. 26

    Rob

    Thanks for that Tim. I live in the Alps and I’ve recently started taking my 7 year old son out hiking with overnight bivis and he absolutely loves it. After spending a very shivery night under the stars in october I thought it was about time we invested in some bivi bags and this page has given me all the info I need!

    Reply
    1. 26.1

      Tim Moss

      Thanks Rob. I hope you find something appropriate. If you do buy one online, please do consider using my affiliate links above. Either way, let me know how you get on!

      Reply
  37. 27

    Tim H.

    Hi Tim,

    Thanks for all the great research and information. I need a bivi for a 6 month motorcycle trip through Central and South America. I plan to stay in cheap hotels each night. The bivi will be for emergencies only when I get stranded or can’t find a hotel. I hope to never use it. But if I must use it, it must be waterproof and hooped. It must protect me from getting soaked by a heavy rainstorm.

    I understand as you stated that waterproof and cheap materials (non-gortex) means lots of interior condensation. So here is my question: In an all night rainstorm, 100% humidity outside the bivi, will it be any less humid inside the best gortex bivi versus the cheap AquaQuest “Hoopla” bivi in those conditions? My understanding of porous membranes says no, it will not be any better. Am I wrong on this? I think the expensive gortex will only help inside dampness if the humidity outside the bivy is lower than the humidity inside the bivy. Trying to figure out if the cheap hooped bivi that I hope to never use is the better buy for my application than the expensive gortex hooped (terra nova jupiter or rab ridge) bivi.

    Thanks in advance for your reply.
    From another,
    Tim

    Reply
    1. 27.1

      Tim Moss

      Thanks for the comment Tim and an interesting question. I’m not sure I paid enough attention in school to be able to answer it with science. However, even if you’re right about those nights that are 100% humidity and raining all night, what about all the nights that aren’t?

      Either way, I love your thinking and would really like to hear how you get on.

      For what it’s worth, if you’re really expecting long, wet nights then a tent is probably the only real way to stay dry. You can get some that way less than a hooped bivvy.

      Good luck!

      Reply
  38. 28

    Tim H.

    I looked into this a little deeper and if anyone is interested, here are my findings in simplified terms. Apologies in advance for the length of this. If you completely under the physics of partial pressures and equilibrium and the structure of gortex and why it works, skip to the last paragraph for the “nugget” in this whole thing.

    Gortex material is a very fine grid or filter or screen. Each fiber in the grid is plastic that does not absorb water. The space between these gortex fibers that are woven together like a screen on your window, are open. The opening is large enough to allow gas to pass through the screen but not solids or liquids. A water molecule in its gaseous state (water vapor) is thousands of times smaller than a water molecule in it’s liquid state. So water vapor can move through Gortex in either direction but liquid water molecules are too big and will not pass through the open areas of the grid or screen in Gortex, so it will stay on whichever side of the Gortex it is on.

    So the gortex is a porous membrane with a grid or screen size large enough to allow water vapor, a gas to pass through but small enough to prevent liquid to pass through. The laws of physics prove that nature wants to equalize the environment on each side of that porous membrane, the gortex. If there are 100 “pieces” of water vapor on one side of the gortex and 80 “pieces” of water vapor on the other side of the gortex, the side with the 20 extra pieces acts like it is of higher pressure and wants to equalize with the other side so both end up with 90 “pieces” of water vapor. The side with 100 pieces and thus more pressure pushes 10 pieces out to the other side and then the pressure on both sides is equal so they no longer change. The pressure difference in this example is small so the rate at which the equalization occurs is slow. If it was 100 pieces of water vapor on one side and 15 pieces of water vapor on the other side, the difference is much larger and thus so is the pressure and then so is the rate or speed of change to the equalization state.

    When you create water vapor in your tent/bivi through perspiration (sweating) and respiration (breathing) and evaporation (your wet jacket and boots you brought in with you) you build up a certain number of “pieces” and “pressure” of water vapor on your side of the gortex porous membrane (inside the tent/bivi). If there are more “pieces and pressure” of water vapor on the other side of that gortex (outside the tent/bivy), it will try to equalize, bringing more water vapor into the tent/bivi until it is equal on each side of the gortex or in other words, the same humidity inside the tent/bivi as outside. And of course the opposite holds true, if it’s more humid inside the tent than outside, nature will cause the conditions to equalize, reducing the humidity inside the tent until it is equal to the humidity outside the tent. The rate or speed of that change occurring is dependent on the magnitude of the difference in the pressure on each side, which equates to the percentage or number of “pieces” of water vapor on one side compared to the other.

    So what does all of this mean. Gortex works well and relatively quickly shedding your water vapor to the outside of your tent or jacket when there is a big difference between the humidity (water vapor) inside your tent/jacket versus outside your tent jacket. As the outside weather gets more and more humid and approaches the humidity inside your gortex tent/jacket, the shedding effect of humidity slows down and eventually stops when humidity is the same inside your jacket as outside your jacket. And, it can even reverse, pushing humidity from the outside air through the gortex to the air inside your jacket/tent if the water vapor (humidity) outside is greater than the humidity inside your tent/jacket.

    So the nugget I found in my research that pertains to my original question is this. The humidity outside is seldom 100% at ground level when you are getting rained on. The humidity is 100% where that water vapor is turning into water (in the cloud) and then falling to the ground where you are but that may be happening 1000’s of feet above you and your tent. If you are in mist or fog, then the outside humidity is at or close to 100% and you will also be at 100% humidity inside your gortex jacket or tent very soon. But during a rainfall, the air outside of your tent could be at 65% humidity, still pretty comfortable compared to 80%-90% humidity inside a non-breathable tent/bivi. The gortex tent/bivi won’t exceed the outside humidity. See below a website address that explains this pretty well if I did not. http://andrewskurka.com/2012/breathability-its-importance-mechanisms-and-limitations/

    Reply
  39. 29

    Freya

    Magnificent article, thanks! Which one do you think is more breathable: the Alpkit Hunka or the Highlander Hawk? Both are made of nylon, but the Alpkit is more water resistant (and cheaper) and thus might be less breathable. I’m thinking of fixing a mesh net in the Alpkit, so I would basically get a cheaper, lighter and more water resistant version of the Hawk – or at least that’s the idea.

    Reply
    1. 29.1

      Tim Moss

      I’ve not used the Hawk yet I’m afraid but the Alpkit one is good and the breathability is fine. Alpkit stuff is always decent – not the best but reliable – so that would be the safer bet. The hydrostatic head of 4,000 on the Hawk won’t be good enough in heavy rain.

      P.S. Sewing on a mesh net sounds great. If it’s too fiddly though, a mosquito headnet would be a crude alternative.

      Reply
  40. 31

    kevin steffey

    The information is useful. It helps me know what I should buy. Thank you for sharing

    Reply
    1. 31.1

      Tim Moss

      Thanks Kevin. Glad it was helpful.

      Reply
  41. 32

    Bobby Peer

    Thanks a lot of your helpful post. That’s exactly what I need. The Rab Ridge Raider is quite suitable for me.

    Reply
    1. 32.1

      Tim Moss

      Excellent. It’s a great bag and they’re reduced on Amazon at the moment: http://amzn.to/1MmfGaT

      Reply
  42. 33

    Ozzie

    I like the Army Gortex Bivy Cover; the side zipper is very convenient, and it is nicely breathable. However, I also have the RAB Bivy (the model is yellow on top and black floor. I think the floor is nylon and the yellow top is event. The floor is the problem — I have never had an experience with it that gave me a dry night — the condensation (I believe caused by the nylon floor) spoils the whole experience.

    Reply
    1. 33.1

      Tim Moss

      Thanks Ozzie. I think you might be describing the Rab Storm bivi. Other readers have had problems with it too (see here).

      Army bivvies don’t usually have any zips – normally just a large draw cord at the top – so that’s interesting to hear. I’m glad you’ve found it useful.

      Reply
  43. 34

    Sherpa

    Very interesting list, thanks for the effort! A few additional, lightweight contenders (I hope I didnt overlook anything in your list): Terra Nova Moonlite; Montbell Breeze Dry Tec UL Bivy; SOL Escape Bivi; SOL Escape Lite Bivi. If you have any insights on these, they would be very much appreciated.

    Reply
    1. 34.1

      Tim Moss

      Hey Sherpa, thanks for flagging up those other bivvies.

        Terra Nova Moonlite – I’ve seen this before but it’s currently out of stock so I’ve emailed Terra Nova to see if it’s coming back. It’s really light for a waterproof/breathable bag so I’d be interested to try it. If it’s still being made then I’ll add it to the chart.
        Montbell Breeze Dry Tec UL – seems to be an American bag that’s not stocked in the UK. I’ve not included US bivvies here because their range is, for the most part, completely different. Perhaps I should do a separate article for them.
        SOL Escape Bivis – these look really interesting as cheap, lightweight bags. The reviews on them look alright so I’ll see if I can get my hands on one. They also look like US products but seem to be available here in the UK too so I’ll get them on the table next time I update it.

      Thanks again,
      Tim.

      Reply
  44. 35

    Jas Holden

    Hi Tim,

    I wonder if you have heard of a new waterproof and breathable fabric called Hipora or Hypora that is coming from Korea? Several top brands of gloves for skiing and motorcycling have this. I recently bought a bivi bag with this Hipora from a Korean dealer on Ebay America for $35. I have not had a chance to give the bag a good test since I sweat very little and have never had condensation in the US Army goretex bivi. This is the first use I have heard of for Hipora other than gloves.

    Somewhere on Youtube there is a video of a man wearing the gloves and putting his hands into a bowl of water and staying dry, and then pouring hot water into the gloves and having a cloud of steam form around the fingertips.

    Anyway, here is a link to the bag I bought, it weighs 12 ounces.

    http://campkor.com/product/buck703-hipora-tech-sleeping-bag-cover/

    Reply
    1. 35.1

      Tim Moss

      Hey Jas,

      Thanks for the heads-up. I’d not heard of Hipora/Hypora before but it certainly sounds interesting. I’ll keep my eyes peeled for any Hipora bivvy bags appearing in the UK.

      Let me know how you get on with your bag.

      Tim.

      Reply
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  48. 36

    Zac Caldwell

    Hi Tim,
    I’m going on the Fjallraven Classic hike in Sweden and only 15 years old. I’m looking for a bivy bag that can withstand the conditions of Sweden in August as well as being very small and compact. I have done some research and most of the products out on the market look great but reviews on most say otherwise. Most of the issues are condensation or not waterproof and too stuffy from being too small. The Outdoor Research bivy bags look very good but the Helium had little structure and looks like could rip of and rough objects hit the surface. The Advanced bivi of theirs looks amazing and everything I’m looking for but way out of my budget around £100-180. Do you have any recommendations? Any help would be greatly appreciated. Thanks again,

    Zac

    Reply
    1. 36.1

      Tim Moss

      Hey Zac, thanks for the message. If you want tough and inexpensive, go with an ex-army bivvy. They’re the toughest of the lot, breathe well and are huge.

      Failing that, the Alpkit Hunka XL would be your next best bet.

      Unfortunately, hooped bivvis that are tough don’t come cheap. The Helium is excellent but you’re right, it could rip on rocks.

      Does that help?

      Thanks,
      Tim.

      Reply
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