Adding foot box vent to a bivy

[Cesar Valdez]

So I was lucky to find a nice bivy on GS here, and I should have it in not too long I hope. I also own a Ti Goat Ptarm bivy that I have been pretty happy with, so now my wife and I can each have a bivy if we go out together and choose to take advantage of trail shelters rather than share a tent–this would also save us a lot of weight, as the weight of both bivies combined is only about 335g.

I have mentioned before (as have many others that live in wet/humid climates) that condensation is an issue. It has mostly been a minor annoyance of having a wet foot box and a damp foot end of my sleeping bag, and they dry out during breakfast once hung up. But then I had an idea to mod a bivy to eliminate or help lessen condensation, which is to sew in a small vent in the foot box.

My idea was to use a piece of scrap nylon (just regular ol' nylon, i.e. not treated and very breathable) that I have leftover from a nylon sleeping bag liner we made last year. I emailed John at Borah gear about this, and he was nice enough to give me feedback even though I didn't buy a new bivy from him but a used one on GS, so big ups to John–thanks again man.

My email:

"I saw a good deal on BPL for one of your Cuben bivies and I could not pass it up. I hope it is in as good condition as described, which the seller claims is "like new". Anyhow, I had some questions about sewing the material because I want to do some mods to the bivy, the foot box vapor vent.

I decided to put a 5cm x 10cm rectangle in the foot box. I am not sure what fabric to use for the vent, but I will probably go with just a normal piece of scrap nylon. Should I do this by hand or use a sewing machine? How well does M50 take to mods/repairs? Is it better or worse to mod/stitch than silnylon? Do you think this is a good idea overall?"

John's reply:

"If you wanted to add a footbox vent, and want to use nylon, then make sure it is a very breathable piece of nylon. Otherwise there wouldn't be much difference over the normal M50.

For sewing it, you can use a sewing machine, but its important to make sure the tension is set for such thin material. If your machine doesn't like the material, it wouldn't take too long to just sew it by hand since its such a small rectangle your sewing on. M50 is a bit harder to sew than silynlon, as it tends to like to bunch up in the machine since its so lightweight. It is very strong stuff for the weight though, so it takes well to sewing."

So my question is… has anyone else every done something like this, or this exact mod? If so, how did it turn out?

Otherwise, just wanted to share what I think is a good idea, and hope we can pull off. I must admit a bit of anxiety cutting either of my bivies, but lucky for me my wife is pretty handy with a needle and thread. The vent will be small, we'll do everything by hand.

I intend on taking lots of pics and writing an article on my blog, and will provide a link here once that happens of course.

[John S.]

Bivy makers seem to run the nonbreathable fabric bottoms too high up the footbox end. I would consider either altering the fabric of the foot box or cutting a slit across the footbox, adding a bit of noseeum netting and having an overhang of fabric over the slit.

Also, make sure feet are dry before bed and you are using a completely dry sock. That may take 5-10 minutes of airing them out.

Another alternative is a vapor barrier over the feet, but that isn't always fun unless it's way cold.

[Cesar Valdez]

"Bivy makers seem to run the nonbreathable fabric bottoms too high up the footbox end. I would consider either altering the fabric of the foot box or cutting a slit across the footbox, adding a bit of noseeum netting and having an overhang of fabric over the slit."

I don't have any noseeum netting, unfortunately, nor do I want to buy a whole yard of it so I can use literally two little patches of it–but that would be ideal.

I like your slit across the footbox idea. I initially thought going across was best, but then I thought a little box in the middle of the footbox might be best, now I can't make up my mind. But yes, I will save the piece of bivy fabric I cut off and sew at least part of it back over the vent to help against any wind/rain spray. This should only add around 5-10g total to each bivy, though this is just a guess at this point.

"Also, make sure feet are dry before bed and you are using a completely dry sock. That may take 5-10 minutes of airing them out."

Yes, a good piece of advice. I always have sleeping socks with me that are in a water resistant stuff sack, and my go-to pack is my hybrid Cuben Zero that I have seam sealed on the inside, making it pretty much water proof–also great for saving weight by getting rid of the need of a pack liner and/or pack cover.

My normal routine before getting into my sleeping bag and bivy is to sit on top of the bivy/bag/sleeping mat and get things ready for the next day, and I do this with my shoes and socks off to let them air out before putting on my sleep socks.

"Another alternative is a vapor barrier over the feet, but that isn't always fun unless it's way cold."

Another good tip, if you can put up with VBL. I am personally not a fan, but have resorted to it in the past, and it is a very warm solution when it is very cold. I only do some winter backpacking anyhow, I am more of a 3 season backpacker (though I do enjoy winter trips too).

Thanks for the input :)

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