Having now been lightweight bikepacking for two years I have come to the conclusion that I will always sleep under a tarp so I don't need a heavy duty bivi bag. The tyvec one I made was light and waterproof enough but not idiot proof and I split a seam on it getting out of it. I have now made this one which has a 4oz waterproof nylon base mated to a 2oz waterproof and breathable nylon top, good enough for use under a tarp I reckon. It's boxed to give me plenty of room so I can turn over inside it. I just need to seam seal it and find a way to add mosquito netting. I think I may put a zip all the way around the edge of the netting so that it's totally removable. It weighs in at 265g and packs down small enough.



With Exped Lite down mat and Vango Venom 400 down bag inside.

That look cracking Ray, does it really need sealing given that it'll live under a tarp?

Thanks Stu, thought I'd seal it just to be on the safe side.

Impressive stuff Ray. Liking the colour choice.

pushbikemike wrote:Liking the colour choice.

Thanks Mike. Colour choice was dictated by nothing more than it being on offer, I am after all a tight git . Actually, I too rather like it, it might also come in handy if I ever need to attract the attention of fell rescue but hopefully not.

I'm starting to think that i'm destined to not be able to make a useable lightweight bivi bag. I used this one last night and the "breathable waterproof nylon" turned out to be not so "breathable" after all and I woke to find quite a lot of moisture inside. Oh well, might just have to buy one .

Seeing as you'll be under a tarp have you thought about adding some netting panels? Maybe one across the foot end would be enough to allow a venting 'chimney effect'?

Actually something just occured to me, does breathable fabric only work one way, maybe I sewed it the wrong way round, I think i'll try turning it inside out first. If that doesn't work then i'll try Stu's idea and add some vents.

Imagine your breathable material as a fabric with lots of holes in it. Holes too small for a big water drop to go through but small enough to let small steam droplets out. If your bag contains a lot of cold air the steam will condense and the big drops can't escape.

In theory it shouldn't matter too much which way you did it, I think.

Stu's vents sound like a good idea.

The difference in barometric pressure is what forces the moist air through the pores. So as your body heat warms the air inside the bag(or jacket etc) the pressure inside increases relative to that outside, and so "leaks" air through the pores taking moisture with it. If you introduce vents the pressure equalises and the moisture simply condenses on the inside surface. Ensuring the face fabric is repellent also helps as "wetted out"" fabric will also not breath once absolutely soaked, one of the reasons for WR coatings.
It is a bi-directional process by the way.
This of course assumes it is a coated breathable rather than something like the original Pertex, which would move moisture by Hydrophilic pressure for example.

Ray, I'm no expert on all this waterproof breathable stuff. I tend to lean towards breathability rather than waterproof. Pertex works for me. Tarp keeps most of rain off me and usually have a polycro groundsheet. If you want you can try out my myog bivi bag. The blue one I had on WRT last year. It's all pertex. With a mesh bit for bug protection.
I'm making a new one with a mixture of materials and a bigger zip.

Thanks Mike, if adding venting doesn't work I'll look into other materials.