More bivy mod brainstorming: cotton or nylon?

[Cesar Valdez]

I started another thread about a planned bivy mod I am about to do, which you can read here:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=73614

The short of it is that I want to put a vapor/condensation vent on the foot box. I have decided on how I want to do this (a strip of breathable fabric running across the foot end), but now I can't decide on what fabric to use, cotton or nylon.

After cutting a strip off of the bivy, sewing the vent on, and then sewing the bivy fabric partially back on as a flap to protect against rain and wind, this should (or I hope, at least) fix or at least very much help with condensation. Condensation is really the only drawback for me when it comes to using a bivy, so once I solve this, I will be good to go. If this whole experiment does not work, might have to switch to a bug bivy, or come up with a better mod.

But for now, cotton or nylon? Weight is not really an issue, either choice should only add about 10g I think (just a guess). Cotton is more breathable, and can absorb 25 times its weight in water, I just learned while doing some cursory research on the fabric: http://tlc.howstuffworks.com/home/question547.htm

So I am leaning more toward cotton. But maybe one of you fabric gurus can offer some perspective and/or feedback?

[James Klein]

I have a feeling cotton or nylon will make no difference in this application vs the stock M50. If the bivy was exposed to very windy conditions, maybe you'd see less condensation (but you'd also be colder than you'd be with the M50).

I think workout clothing type fabric would work much better than cotton – the idea being to pull moisture out and spread it around for evaporation.

If I were going to try something similar I would use:
http://www.rockywoods.com/Sale-Specials/Pertex-Equilibrium-Bicomp-Wickaway-Fabrics
For its weather resistance and wicking properties but I noticed in the other thread you weren't wanting to buy a whole yard of fabric just for this project.

[Diplomatic Mike]

I think you are fighting a losing battle. All fabrics that are breathable enough to totally eliminate condensation, will also not be effective in stopping rain getting in.
FWIW, my best solution has been an all eVent ID Bivvy, with the sleeping mat on the outside.

[Cesar Valdez]

James – Thanks for the input. You make a good point about the whole wicking aspect of some nylon types. Makes me wish I had a synthetic base layer around that I didn't need that I could cut up. And no, I don't want to buy a whole yard of fabric. I was thinking about getting some noseem netting… but I really only need are two strips about 3cm wide and 20cm wide.

Mike – My bivy will be a part of a tarp/ground cover/bivy shelter system, along with the bivy being used in trail shelters and occasional cowboy camping under the stars. I am not looking to have full rain protection with my bivy. I intend on using this bivy during my 2 season (late May to early September) set up. I am looking to benefit from bug/critter, wind, rain spray, and dew/mist protection. And sure, the slight warmth bump is an extra bonus. But thanks for your suggestion about eVent, though I already have two bivies for my wife and I that I am set on modifying. I really do think my idea has a lot of potential to improve bivies. It's also not like we are talking about a ton of condensation either. The most I have gotten on my Ti Goat bivy has been the foot box being damp. I have never had the entire inside of my bivy soaked, and there have also been times that there was hardly any wetness–just a handful of drops at the very end of the foot box that dried out by the time I packed up my tarp and sleeping bag.

[Diplomatic Mike]

In that case, a bug bivvy would probably pick up most condensation drips from the tarp, or rain splatter. If the tarp was big enough.

[James Klein]

Maybe tyvek would be a good option (i don't know much about tyvek.

I don't know about Sweeden but here you can get tyvek envelopes from the post office.

I have a feeling if you add significantly more breathability (just at the foot end) the condensation might get worse. Reasoning is: This will make the inside colder and thus closer to the dew point. I think this is why you notice more condesation at the footbox (this isthe coldest location in the bivy).

[Cesar Valdez]

Yeah, you have a point there. Though I get very little, if any, condensation on my tarp (Zpacks Hex solo). But from what I have gathered, my one bivy (Borah Cuben 135g w/stuff sack, which I will be using in my 2 season system) is lighter than any bug bivy I know of, and then I don't get very solid wind/rain splatter/dew protection, and no bump in warmth (though M50 only provides a small amount of warmth, granted).

[Cesar Valdez]

I am no scientist or anything, but how much will the temperature really be affected? The breathable fabric will still provide some insulation, but allow water vapor to escape the foot box. Plus it is only going to be a small strip of breathable fabric… not to mention that a lot of bivies (mine included) have a net window or net hood at the top, but I have never had any condensation in the head box before.

I think you raise an interesting point, but I am skeptical. I guess we'll have to wait and see what happens after I test it out. If the mod goes poorly, I could always just sew the vent up to close it back up with the M50. Thanks for offering a potential issue, however :)

[James Klein]

My point more against mosquito netting type fabric and only if it was a on the footbox end I have no clue how much colder. Just that it would be colder.

I don't think the will be much of a difference btw M50, uncoated nylon or cotton for heat loss.

If the conditions you camp in usually only result in mild condensation in the foot box area – I bet that sleeping with you head all the way out of the bivy (netting included) would eliminate most of the problem.

But I agree, you idea is worth a try — if it doens't improve things just sew it back to normal — Good luck!

[Nick Larsen]

Just a thought, but why add the ventilation at the foot end that is most exposed to the elements?
Why not just make the whole top(head) half of the bivy out of nano-see-um, and keep the feet as waterproof as possible? Then you don't have to worry about flaps that will double the weight in that section, and probably defeat the purpose of the breathable material when it lays against it.

[Zachary SCOTT]

I have pondered this same problem. I had an idea based on some tents, and their ventilation but haven't yet decided if I want to take on the project. Imagine something along the lines of these images at the foot of your bivy.

No zipper like a tent to adjust but have noseeum or nanoseeum mesh under with a small flap and some type of lighwieght carbon shaft to prop in up. This would provide ventilation and protect from the elements. The problem I have thought of is if the shaft would be effective where a bivy has no tension or taught-ness like a tent.

[Cesar Valdez]

Nick – The foot box is where I get the most condensation, and while the foot box is closer to the end of my tarp, I have not ever had severe spray for one. Next, this is only a 3cm x 20cm (or so, this might change slightly) strip and will have a flap of M50 over it.

The top of the bivy has a 7inch bug window that I think is adequate for ventilation on the top end. I am fine with double weight for a patch that I am guessing is around 10g or so. You bring up a good point about the flap laying against the vent, and this is something I have taken into consideration already. I will attempt to sew it in a way that perhaps resolves this issue, was thinking of maybe shortening the flap so that it could only cover say half of the vent, or maybe a tiny Velcro patch to keep part of the flap open–we'll see. Thanks for your input :)

Zach – That looks like a good idea for a vent for a tent. I don't think it would work for a bivy, as you also point out, but we'll see if I can work something out with the flap, as I just mentioned to Nick some possible solutions.

I just had yet another brainstorm as I sat and wrote. I was at a camping equipment store killing time with a few friends a few weeks ago, and saw that they had some small fabric balls for sale that supposedly soak up moisture. We were all skeptical, and moved on right away. But maybe this could solve bivy condensation? I will have to pay that store a visit again and see how much it costs in terms of both money and weight.

[Cesar Valdez]

This is what I was talking about in my last post, in case anyone is curious or does not know what these are:

http://www.amazon.co.uk/Trekmates-Absorba-Balls-Moisture-Absorber/dp/B001690WHA

It says the boxed product weight is 299g, so that works out to around 50g per ball. But it would be multi-use gear, as it would also absorb moisture in your pack too while you hike. Plus with all the weight I am saving on my shelter system anyhow (355g total for tarp, bivy, stakes, and ground cover), maybe this is worth the weight if I only take one ball.

Not sure yet, but this is and interesting idea, methinks…

[Diplomatic Mike]

Do you always have adequate insulation from the ground under your feet?
Sometimes an empty pack isn't enough, if you are using a torso pad.

[Cesar Valdez]

Good question, I should have included this info in the OP, sorry about that.

I have been using a double ground cover that is just a piece of space blanket duct taped onto a garbage bag. After getting my new bivy, however, I decided that maybe this is overkill, as Cuben is pretty strong against rips (which is what I am really worried about) and any punctures I can fix easy with duct tape. And damage might not even be an issue, as I will at least be using a plastic ground cover, which is just a garbage bag (42g)–and of course I am always careful with camp location selection.

So it will be garbage bag, bivy, and sleeping mat and bag inside the bivy.

EDIT: Forgot to mention pads I use. I will be using either a Neoair Xlite or a full length thin foam pad. I usually don't put my backpack under me, but sometimes will fold it and put it under my head as part of an improvised pillow.

[Diplomatic Mike]

It might be worth trying an extra section of thicker foam under your feet to see if this helps. It might save you from cutting up your bivvy.
I used to get condensation at the feet end if i was using only a torso pad.

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