Making a bivy sack

The old conventional wisdom is to make the top layer a waterproof/breathable fabric like Momentum, and to make the bottom layer out of a straight waterproof fabric like coated nylon (for durability). You could make the whole thing out of Momentum, and you would probably lighten it up a bit.

I was going through the same decision myself, so I purchased 5 yards of Momentum, in case I go that way. I also have several yards of coated nylon, Supplex, Spinnaker, and some others, in case I change direction. It's easy to add a yard of mosquito net around the head opening, depending on the design. My current plan is to target for 6 ounces, finished.
–B.G.–

"The big question is what WP/B fabrics can be obtained and from where, and that is what I would be grateful to get advice on."

Lots of threads referring to suppliers if you search this site. If you are set on so called WP/B fabrics, seattle fabrics or outdoor wilderness fabrics are among the suppliers often mentioned here.

Jeremy raises a good question. How is this to be used?

Some folks use a bivy sack perhaps underneath a tarp and surrounding a sleeping bag, so it offers a little splash protection.

Some folks use a bivy sack without any tarp or tent over it, so the bivy sack surrounding the sleeping bag is the only weather protection.

Some folks use a bivy sack without any tarp and without any sleeping bag, and it is mostly as an emergency storm shelter.

Your intended use might dictate the best materials work with. Mine is the last category.
–B.G.–

Generally, we in the MYOG world are not privy to any truly UL WP/B fabrics. I've seen some people using driducks ponchos for fabric which is the lightest and most breathable fabric we can get. Lacks durability you may be looking for.
Most people prefer to go with a 1.1 DWR ripstop or Momentum, a 20D taffeta top. Not waterproof, but far more breathable than any WP/B fabric. You can find a less expensive momentum clone at OWFinc.com called '20D taffeta'.
I see no reason at all to forgo a waterproof floor as mentioned by Bob. Maybe save an ounce, but really you could just get a lighter WP material for the floor.
Good luck with your project!

I made a bivy/bag cover using Momentum on top (www.thru-hiker.com) and 1.1 sil on the bottom (www.rayjardine.com, although thru-hiker has it too). Finished product is 6.5 oz, and includes a 24" zipper and hood/drawstrings at the top.

My usage category is splash/convection protection. The cover breathes much better than my heavier and more waterproof REI Minimalist bivy, but probably not as well as uncoated, calendered nylon would. It also adds a bit of warmth to my bag, although I've not objectively measured that. Using sil spinnaker cloth would probably shave .5 to .7 oz for an additional $15-$20.

I got a bit more involved with the foot box, which ended up a bit smaller than ideal. Next time I'll go with a longer zipper, a simpler footbox, a bit more volume overall to better accommodate heavier insulation and a BA insulated aircore pad, a better contoured hood area and possibly some bug mesh.

Thomas, for your first project it might make sense to use more basic fabrics that don't cost as much when you make mistakes. Plus, you'll find what size and any other tweaks you like. The kit that Quest sells for a bivy is pretty painless. I sewed it as my first project along w/ their cat cut tarp. The bivy kit runs about $50 if you use 1.1 sil floor and .9 ripstop for the top. For a long, spacious bivy with velcro fastened hood, mine weighs in at just under 9oz. I've cowboy camped and tarp camped w/ mine and found it to be adequate for both situations and as others have mentioned, I get a bump of warmth from it too. It's especially nice on windy nights when the breezes coming in under the tarp are robbing precious heat. It really is a simple sewing project. Good luck.

Hi Thomas, if you are getting into MYOG projects here is an invaluable resource:

http://www.specialtyoutdoors.com/tips/sources.asp

Lots of links to materials and also good tips and info like the thru-hiker forums and articles. Everyone has a little different selection in fabric; OWFINC has EPIC and the light 20D nylon; Rockywoods has Paclite and EPIC; Thru-Hiker has Momentum, etc. Like others have mentioned, end use will dictate the best fabric. I find WPB bivies oppressive, and I only carry one on really light trips when I probably won't need shelter and probably won't have an appropriate site for a tarp pitch. My choice would either be Momentum or the 20D from OWFINC. You could also choose some of the seconds in 1.1 from OWFINC and treat it with a good DWR like Revivex.

As Lucas mentioned, use some cheap fabric so you can prototype. If you have some old bedsheets, you could use these to get an idea of the size you need. Don't skimp on the size on your final version; IMO it's better to have too much girth than too little with bivies. This is where most commercial bivies fail for me; they roll with me if my pad is inside, and my bag accumulates moisture with the floor up in the air.

Check out the recent article on laminating lightweight fabrics; I think glued-in patches would be ideal for adding pad straps to the floor of a bivy. Think about the footbox; make sure it's long and wide enough. This is a place some WPB might be really functional, kind of like the MLD bivies with an eVent foot. If you make the floor a bathtub, make sure it doesn't wrap too high or it will impede breathability. Check Thru-Hiker for zippers; AYCE has a good selection of #3, including Uretek #3 by the foot.

What a wealth of information, thanks very much to everyone. If I can do what Jeff did and put something together that weighs 6 ounces, I will be more than delighted. I will look at material costs, but I am thinking of something with a sil-nylon floor and a momemtum top. I have enough sewing experience that I might just dive in without doing a prototype out of disposable material, but that is good advice nonetheless and may make a lot of sense – to use old sheets to put together a pattern or mock-up.

The question of what I want out of this is indeed an excellent one. Of course, I want it to do everything! Truth be known, unless it is full on raining, I just sleep under the stars and want a bivy to mostly add warmth and reduce heat loss due to the wind. I would carry a tarp if I thought rain was a possibility (and a really light tarp would be yet another project — I completed a Jardine tarp last summer, but could imagine a lighter and more minimalist one.

I have never tried to put my pad inside my bivy, as I am learning that some people actually do. I am still scratching my head over this, it might make sense if I acquire a Jacks-R-better quilt …..

Thanks again. I need to now take notes, reread all the replys and check the sources all of you have kindly suggested!!

Is there any reason to not make the bottom tyvek? That's what I was planning on doing.

Tyvek is not really waterproof, although silnylon isn't that waterproof when pressure is applied either. I think the silnylon is tougher, and the waterproofness can be enhanced by coating it with a silicone sealant/mineral sprits mix. Mix it much thinner than you would for seams, so it can be lightly brushed on. The mix has a bit of tackiness, so if used on the inside it can reducing the slipping around. Another big advantage for sil is how compactly it rolls up.

Tyvek is turning into another term like refridgerator. There are many types ranging from complete VB to a one way goretex like material. Some times we have to "dry in" a house before winter so that we can stay busy. Our need depends on the durability.

My new toy is a baker style tarp with a bathtub floor made out of titanium UDL30. With a name like titanium how can it be bad?
BJ

I'm talking about hard surface HomeWrap. I haven't heard of anyone doing MYOG with anything else. A couple of the cottage industry guys sell it for ground cloths and it's all over eBay. Do you think that type would be suitable? Packability might be an issue, but I thought Tyvek was pretty bombproof compared to silnylon. Maybe I'm wrong, though. For me, the price savings alone justify using Tyvek, as long as the performance is comparable to 40d PU nylon which I thought it was.

Anyone know how waterproof Tyvek is and how much it weighs? Trail durability?

At least that is my impression.

I bought some off Ebay and cut it up for groundsheets. It does the job of protecting my pad from getting poked and punctured (which is what I mostly ask of it), but it is noisy as all get out, and I have vivid memories of a night when the wind came up and I was up gathering rocks to hold down all the corners and edges so it wouldn't rattle and drive me insane. Not compatible with restful sleep, for me anyway.

So, I am not thrilled with it. I would make the bottom of my bivy with silnylon I think and figure to replace/repair it when it gets trashed.

I have heard that if you run your sheet of Tyvek through a wash/dry cycle it softens up and becomes more like fabric. Is that something you tried? I don't think using tyvek as the bottom of a bivy would result in the same kind of noise as you described, but I could be wrong.

Wash it. It works. You can also just wad it up repeatedly. That works the same way, but the washing machine is faster.

Bad aesthetics, Always looks like trash from a jobsite. Reminds me of work. Not what I want to be thinking about when I don;t have to. I'd go with some silnylon myself. Here is my Giant Meteor bivy. Made this large enough to accomodate a large Neoair and my winter bag. Can scoot down for cold weather and can store my bag at my feet and sleep more under the netting in warmer weather sil bottom and momentum top. 10 ounces. quiet.

Hey Thomas,

I'm actually looking to start a similar project. However, unlike you, I don't have any sewing experience. Are you using plans to make yours or are you designing it yourself? I have a mother in law that would be willing to help me sew it, but I'm thinking it would be good to have plans to show her.

The first bivy sack that I made was a complete disaster about 30 years ago. It was in coated nylon. The second one that I made was coated nylon bottom and Goretex top, but I made it a few inches too short. The third one that I made used the same materials, but I made it wide enough for two sleeping bags and 8 inches extra long. It was decent. Now I'm getting ready to do my next one, and it will be at least 50% Momentum fabric. I just have to decide how roomy to make it.
–B.G.–

Bob,

If you make this next one too short by accident as well, let me know. I'm only 5'5". ;)

No plans, but I like Ken's design (big enough to include the pad inside — in my case an Exped DownMat). I also like his comments about Tyvek looking like trash from a jobsite, ha! I will have to run mine through the washer (they tell me to do this without detergent) and see if that gets me feeling any fonder for the material.

If your mother in law is willing to help, go for it man. I wish I had had somebody with experience to show me the ropes with a sewing machine when I was getting started. Don't just let her do it all, get her to train you!! Sew stiches on some scraps, then do a few stuff sacks, then go for broke man!

Thanks for the comments. That bivy was only the third project I did. Sewing gear is easy. Tailoring clothes is another matter. I used little binder clips instead of pins. Practice a little, and go for it. I use a 50 year old machine. Straight stitch and zig zag. New needles are key to your success. PM me and I will help as much as I can.

very interesting tip, thanks!

I'm also thinking of a tyvek bivy. On some threads I read you can simply bond it with some 3M 77 and it holds up as well as sewing…an interesting alternative for the sewing impaired.

Make sure you wash the tyvek before you start cutting/sewing/bonding.

After washing tyvek it will "shrink" significantly. Actually, Im not sure if the fabric actually shrinks or if it's just all the folds and creases that you get that effectively reduces the size.