Bivy Bags 2 - Design

Over the years I've now tested various styles and fabrics for bivy bags and have made a few myself. I thought it would be worth comparing some of these more specialist bags to look at how weight is saved and how operating conditions have an impact on the design. Over the years I've found that for much UK walking I just prefer to take some sort of Bivi bag and forget all about tents and tarps. I find them to be a bit more practical to use because I don't have to worry about finding perfect places for pitching, instead I can just find a patch of rocky or boggy ground and set down there and not worry about fragile fabrics being damaged.

This article looks at a mixture of home-made and commercial tarps and their design features. All the home-made ones were made using fabrics you can purchase from these UK suppliers. The summary then discusses what this all means!

100% Gore-Tex Army Bivi Bag (600g!!!!)

A normal Army Surplus bag weighs a lot more than 600g but I took one and put it on an army diet saving 1/3 of its weight. It's now my favourite bag. In 2008 I did Offa's Dyke carrying an ultralight winter tent that I'd made. In 2009 I did Offa's Dyke carrying this Gore-Tex bag and tarp (shown)instead and saved myself over 200g in weight.

This Gore-Tex fabric is nice and tough and so you don't have to worry too much about what it is sitting on. Of course I place it on top of a mat but beyond that I don't sweat it. Gore-Tex is also windproof and waterproof for any conditions you will meet so it's a bomber-shelter, it will keep you dry.

The Army Bags have a draw-string hood design. This is good for comfort and good for when it's not that wet but it does mean that in heavy weather you may need some protection against water ingress, you cannot just zip yourself in and forget about it. If you are not using a tarp then you can just turn the bag over and sleep with the blow-hole facing downwards but this is not a fun thing to do.

Instead I use my favourite setup shown above of a MYOG/DIY Small V-Tarp and a Bivi Bag. This allows me to stay dry, stop the wind and still watch the stars.

100% Gore-Tex means that the bag has maximum breathability. It may not be double that of a bag that has a waterproof base but it's definitely a lot more breathability. The cost of course is that Gore-Tex weighs more than waterproof nylon, a lot more.

MYOG/DIY Hooded 100% Gore-Tex Bivi Bag (810g)

This is a bag that I've made from "3-layer waterproof-breathable fabric" that is not sold as Gore-Tex but looks just like it. It was made using this bivi bag pattern as a starting point.

With an enclosed hood I no longer need to use a tarp and so it's better for extreme use. However it is also quite a bit heavier than the army bag above. Hoods always seem to work out as quite expensive in fabric terms and this adds to the weight. I think this one weighs around 100g more than it should and I suspect the army are using a slightly lighter Gore-Tex fabric since this bag is a little smaller than the army one.

I think this bag needs a little more length and maybe even a zip closure. Both of these things will add weight and I fear that doing so will take it south of 900g and that seems a little OTT to me. I think I see some scissors and redesign work happening for this bag - IF I have some fabric scrap left that I can use.

The fully enclosed hood means that this bag is OK for extreme weather even if it wouldn't be fun. In contrast the tarp/bivi combo shown also works for extreme weather but weighs a lot less.

MYOG/DIY Pertex Sleeping Bag Cover (170g)

Pertex is a very light and breathable windproof/water-resistant fabric. It is at best splash-proof. And amazingly light.

This 170g bag obviously is less than 1/3 of the weight of the Gore-Tex bags above. Of course they are doing different jobs. This Pertex bag cannot be used on its own. It has to be used with a tarp AND a ground-sheet. (This photo is not realistic use of a pertex bag). It would just about be possible to use this as an outdoor sleeping bag cover IF you KNEW that no rain was expected. Basically after a few minutes of rain pertex will wet through.

So why bother with pertex at all then?

Pertex is extremely breathable and windproof - more breathable than Gore-Tex but it feels less wind-proof because the fabric is less insulating. It will keep dew off your sleeping bag and help your sleeping bag to stay dry. That's just what you want when using a tarp. Even so the numbers soon start to add up - 170g cover, 100g groundsheet, 300g tarp (140g with Cuben and other exotics) - and soon you are up to 570g.

It doesn't sound so bad but if you have the money then Terra Nova do the Laser Ultra 1 Tent which is a true double-walled tent with a Cuben Fibre outer and weighs in at 560g.

Pertex is also rather fragile as a fabric, it doesn't take much to pierce it or abrade it. To use Pertex you have to keep it safe. So maybe 100% pertex covers are not the way to go, so why not go for 50% Pertex?

MYOG/DIY Pertex/Nylon Bivi Bag (260g)

This Bivi Bag is the prototype that was built for the make your own bivi bag project. The bivi is designed to be used under a tarp and is no use anywhere else. It includes a hood - which adds weight but the hood also gives me a bit of wind protection and under a tarp and you really want that - at least in UK mountains!

I've used 2oz PU-coated nylon for the base because it's nice and light at 55gsm. Unfortunately it's not sufficiently waterproof and if you put it on a damp patch it will let in water. I'd avoided SilNylon because of how damn slippy the stuff is. However, the de-facto ultra-light standard for bivis used under tarps is in fact Pertex + SilNylon.

Under a tarp you are trying to cut down the wind-chill a little and provide a moisture barrier against splashes and dew/condensation. A pertex top on the bag does this. Obviously half the bag is now not breathable but the breathability of pertex is good enough that this is not so critical. Mostly though of course it's a weight issue...

Oware USA DWR Draw-cord Bivi - (202g XL)

This is what you might call a specialist bivy. If 100% Gore-Tex is one end of the specialist spectrum then this is the other end. Oware USA cater for USA Ultra-lighters and their products reflect these specialist needs.

This Bivi has a 45gsm (1.1oz) SilNylon base which is both waterproof and about as light as commercially available SilNylon goes for our purposes - any lighter and it's not really strong enough or waterproof enough for groundsheet or shelter use.

The top is nylon which has been treated with a DWR (Durable Waterproof Repellent). This bag is not waterproof as such. It's less water-resistant than Pertex. It is however very breathable.

What this bag will do is provide some protection against dew and maybe the odd rain-drop but not much else. It will provide some protection to your sleeping bag but only a little. Obviously this bag always has to be used with a tarp.

I have as an experiment tried to use it as a normal bivi and in foggy conditions it was overwhelmed. Since it was advertised as a bivi for desert camping I'm not too upset by that.

This picture shows some interesting design features for the bag. The first thing to note is that the mesh is at the top of the bag and is a reasonable size - head-size. Finally the drawstring is at the end of the bag. This is a bag that you slip into and out of rather than climb into and out of. That's quite nice. Once you are inside you just close the cord and the mesh wraps around your head.

I think this bag is almost useless for UK conditions - largely because we can almost always guarantee long damp grass touching the DWR. However, it would still work OK underneath a tarp and in the summer it would be a nice bug bag that's not too hot.

DIY/MYOG AquaBloc + 4oz PU Nylon (495g)

This Bivi is once more based on my bivi plans and is my Mk2 Bivi Bag. It has a 4oz PU Nylon base. The base is obviously twice as heavy as SilNylon but a bit more practical for mountain use being more abrasion resistant. It's also a lot less slippery and so tends to stay put more! It has a proper zipped hood and so can be used fully closed.

I've used this one in some rough conditions and it works very nicely. AquaBloc is waterproof but not stormproof. I think in the worst of conditions it is likely to fail unlike Gore-Tex but not many people choose to go bivi'ing when there's a storm coming. Such people use Gore-Tex ;-)

Now there are quite a few commercial bags using Gore-Tex or equivalents that come in at this weight such as the Rab Super Light Bivi at 465g which is eVent and PU Nylon. Many say eVent works better than Gore-Tex for Bivis. There is also the Terra Nova Discovery Lite which at 300g is very light thanks to Gore-Tex Paclite fabric and a very slim design.

Alpkit Hunka (397g)

The Alpkit Hunka is a proper 100% waterproof breathable bivi bag with some very nice design features. The fabric used looks very much like what you will see on an ultralight waterproof-breathable jacket - imagine Gore-Tex without the mesh inner. Alpkit do publish numbers for it and they are quoting 10,000 mm hydrostatic head (more than storm-proof) and MVP of 10,0000g/m2/24hr breathability which compares well with Gore-Tex and eVent (15,000 - 25,000g/m2/24hr depending on fabric). Basically it's good enough.

The drawstring hood design is quite nice. They've provided a proper shaped hood and then added a drawstring. This hood will do a good job of keeping the wind off your head and out of the bag.

So 400g for a Gore-Tex alternative; what's the catch? The catch is that this is a close-fitting cover, very close fitting. So whereas this 400g will hold a 2-3 season and allow it to just about loft it's no good for a 4-season bag and I'd still worry about the loft on a 2-3 season bag. In contrast the 600g Gore-Tex Bivi bag provides lots of room for loft.

Although I love the design and the weight of this bivi bag I find that it's a little too constricting for me and I don't think it gives quite enough room for my down bag to fully loft.

Hunka does come in an XL size and you probably want this one rather than than the standard size. However at 600g it's no longer so svelte.

Wild Country Scout Bivy Bag (647g)

This bag has a 3-layer waterproof breathable laminate upper - aka. unbranded Gore-Tex - and a waterproof nylon base. It's a fully enclosed bag with a front-zip hood. In fact most Gore-Tex Bivi Bags look like this in design and features. It was my first bag and has seen some rough weather so I know it works just fine.

This one has been modified to add a bug net (with proper tape for sealing the stitch-lines) to make it a bit more friendly for British mountains.

Summary?

What are you trying to protect yourself from? That is the key question that drives much of the design.

If you are using a Tarp and the Tarp is keeping the weather off you then you may not actually need anything else. Certainly I've done lots of tarp camping without a bivi bag and it's just fine. UNLESS. Unless there is fog or lots of wind. In both of these cases your sleeping bag will struggle to keep you warm and keep out moisture. Likewise if rain or snow can blow onto your sleeping bag it will fail.

If the ground is wet you need to keep yourself off the ground. Thus it is that most tarp users - especially in the USA will have some sort of Bivi Bag with the tarp and this will generally be SilNylon for the base and Pertex for the top. The SilNylon protects against the damp and the Pertex is very breathable and will protect against splashes and wind and snow flakes. Obviously such a bag is specialist - it always needs a tarp cover.

Any bag designed for use without a tarp then has to fit the same design criteria - it needs to be waterproof on the bottom and breathable at least on the top. It needs to be enclosable to keep out most or all of the weather and it needs to be windproof and waterproof all over. Hoods tend to win out over drawstring closures but drawstring closures are very light and will do most of what people want. Most people who go out and Bivi tend to do so when the weather is expected to be dry. It's only the more hardy that will go out in other weathers and really need the option of fully enclosing the Bivi.

Most commercial all-weather Bivi bags that you buy will have a Waterproof base and Gore-Tex or eVent upper. The waterproof base is not used because it's the best option from design and performance, it's used because of price and weight. Gore-Tex is a heavy fabric because it is a laminate. It is also an expensive fabric - for argument's sake 3-5x the cost of the waterproof nylon base fabric.

So if you are a manufacturer trying to hit a price-point and a market segment you will use a cheap waterproof base and an eVent/Gore-Tex up. This gives you the headline "Gore-Tex Bivi Bag" but without the full cost.

This is why it's so interesting to compare the commercial stuff with the military stuff. The military have stuff designed and made to order. They can have what the hell they like and they can get it at a price that comes with saying "I'd like to buy 20,000 a year". The British Army Bivi Bags are designed for Western European weather and all-year use and they are 100% Gore-Tex. There is a reason for that and sleeping out in a 100% and 50% Gore-Tex bag will tell you the difference. The difference is that you get better breathability and therefore better functionality.

Using a Gore-Tex bag underneath a tarp may seem a bit silly but perhaps not. Under a tarp it means that the fabric outer stays mostly dry and that means it will be easier for water to evaporate off; it creates a greater difference between inside and outside.

So it may well be that for many people the best shelter solution for lightweight camping would be a 600g Army Bivi Bag together with a 140g Cuben Fibre Tarp; but in another way that's also quite silly, especially when 750g gets you comfortably into the territory of tents. The versatility and flexibility would be hard to beat; it's just that the Tarp and the Bivi Bag are designed to do similar jobs and so one kinda makes the other redundant. Still for expedition use I can see that a Bivi is a useful piece of kit to carry and a tarp is also useful; after all camping in a bivi in the rain is not a fun thing to do. A micro-tarp will make a big difference but it's not the same as a warm tent....