Army Gore-Tex Bivi Bag Diet - now 615g !!!

I was looking at Army Surplus and couldn't help but notice that you can buy a 100% Gore-Tex bag for not much money. Nowhere even near close to the £200+ wanted for some makes of 100% Gore-Tex bags. I also couldn't help but notice that the bags were all rather heavy - 900-1000g was usually the quoted weights. I wondered how much weight I could save by chopping one about since everyone says they are rather big.

I took a chance and ordered one. Currently you can find them unissued for £30-50 incl. delivery. You want unissued but at a push Grade 1 will do. "Grade 1" is mil-speak for "Nearly New" but that can be anything from "slightly dirty" to "rather worn".

In fact these things are large, huge in fact. So large that they make some commercial civil bivi tents look rather small. The basic cut is much closer to a bivi tent than to a normal sleeping bag cover. These things are designed to take a beefy soldier, his bag, his kit and his body armour.

When I measured everything up and did the math I realized that these army bags are weighing in a little lighter than you'd expect based on published fabric weights for Gore-Tex (or equivalent) fabrics. Hmmmm!

The reality is that these things have a lot of fabric in them and that means there is a lot that can be cut if you are just a normal shape and are using a normal winter bag.

These bags have a drawstring hood so although you can use them in the open without any other cover it is better to have some sort of covering over the head in the from of a tiny tarp or something. You can turn the bag around and have the blow-hole on the bottom when it rains but that is not a lot of fun even if it works.

Materials

• 1 Army Surplus Gore-Tex Bivi Bag (e.g. Cover Sleeping Bag 8465-99-769-7040)
• 5m of Gore-Tex seam tape - you need at least 2.5m and want room for mistakes. (See Suppliers).
• 3-4m of 4mm (or less) para cord
• 2x cord-grids
• Some clothes pegs
• Courage!

Customization 1

The way to save the most weight is to fit the bivi bag to you and your equipment. The simplest thing to do is to put yourself and your normal sleepwear and your largest sleeping bag into the bivi bag. Get a feel for just how much spare fabric there is.

This model of bivi bag has a central seam on the top of the bag for about half its length and then that splits into a large triangular section that adds bulk over the torso. This will be your main cutting line. (see line of new tape in image below). The second cutting line will be perpendicular to that at the foot end. I've chopped maybe 25cm off.

The first thing to do is to roll up and peg the foot end of the bag. This will give you an idea of how much fabric can be safely lost from there. It is always best to under-estimate by at least 3" to allow for mistakes. It's easy to cut more off and awkward putting it back. When I did this with me (6ft tall and a 5-season bag) I reduced the overall length by approx. 25cm. You may be able to take more or less.

The second thing to do is to cut the top seam (the long grey line in the picture above). On a new cover this will mean cutting down the centre of the top triangular panel that creates bulk around the torso. On the picture above this panel has been almost totally removed.

Now start to roll up and peg the fabric along your newly created seam to reduce the width along the length of the bag. Do this with the bag inside-out so that it will be easier later. These bags have a proper 3D design and so the width you can take in will be different at each part of the bag with most being take in near you head and least being taken in near your feet. Even so I've lost more than 18cm / 36cm diameter of fabric at the foot of it. There is a lot of fabric to lose.

Get things until the bag feels a little loose and is comfortable to wriggle around in. Leave more wriggle-room than you think you need. It's always easy to remove fabric later after you've sewn it and know how much spare you have.

Construction 2

Look at where you have rolled and pegged the fabric and draw a line along the top of the rolled fabric between the pegs - as far away from the body of the bivi as possible. This will be your cutting line and it will not be a straight line so relax.

Now unpeg everything and lay out the fabric flat so that you can see what fabric you will be cutting from both sides. Make sure you are happy that you are not cutting too much fabric. Feel free to roll it all up and re-peg and re-recheck. You should feel that you are cutting less than you could.

Use a pen to smooth out the cutting line. When you are happy lay out the fabric so that the two sides are next to each other and carefully cut along the line. If you are very nervous you may feel safer just cutting the fabric off in say 2"/5cm strips and rechecking after each cut. There is no need to make one big cut.

Stitch the new centre-line (mine have been taped in grey above) allowing 5-10mm seam allowance. Put a few stitches into the foot box to hold it into place. You will almost certainly need to unstitch the foot box that you don't want to over-do it!

Reassemble yourself and your sleeping bag inside the now smaller bivi bag. It should feel a little too loose. You can now fine-tune the shape. Use pegs and pen to mark out where fabric can be lost from.

Leave the existing seams in place (they can always be cut off) and stitch new seams inside the existing ones. This will be the easiest way to shrink the fit.

Trim the seams to leave 5mm of seam. The seam has to be small enough to fit under the tape and strong enough not to be vulnerable to tearing. 5mm is enough.

Iron the seam so that it rests flat and to one side. Iron a test piece first! A Nylon/Wool setting SHOULD be fine but you are on your own on this! The picture below shows where you are heading...

The image below with the distinctive asymmetric 'slope' and 'cliff' on the seam shows that the seam has been folded over to one side. The other option of pressing the seam flat would produce a symmetric seam.

Use some Gore-Tex tape on a piece of scrap fabric and work out what the iron temperature should be. 'low cotton' might be a start. The temp should be hot enough to melt the glue but not the tape.

Now carefully press the seam tape onto the seam. Press firmly, do not iron. Use a single length of tape for each seam.

Finally remove the very heavy cotton cord from the bag closure and replace it with some nice nylon cord. The cotton cord is heavy and works well but the nylon cord will save more weight.

You should now have a nice 100% Gore-Tex bomber bivi bag that doesn't cost much or weigh much!