Make Your Own Bivy Bag (250g)
During last year I was looking at options of how to reduce my pack weight and how sensibly to use a sleeping bag under a tarp in the UK. I was also interested in a light bag for summer use when no rain was expected.
This bag is a lightweight bivy bag with a pertex top and a waterproof base. It is designed for use under a tarp as a replacement for a groundsheet. Pertex is not 100% waterproof and is particularly unsuited to prolonged rain.
This bag is a lot heavier than it could be because I have chosen to include a hood under which a 'pillow' and assorted pack items can be placed and because I have not used the lightest available fabrics.
It is going to be well worth calculating the weight of your bivy bag BEFORE you construct it. This can be done using the calculations here.
The lightest bags on the planet are probably produced by Bozeman Mountain Works with the lightest one being 150g/$300 and the heaviest one being 200g (incl. mesh and zip) / $230
This pattern weighs in at 250g (or less if you trim it) and costs about $50. The only question therefore is how much is 100g of weight saving worth to you?
Mainstream bivy bags weigh in at anywhere between 400g and 1500g (for a hoop etc) and cost typically £50 upwards. A lightweight bivy bag will not always be as durable as a heavier one but the weight savings are huge.
Before you become too carried away with the bivy bag idea it is worth considering that I made a very nice bathtub floor for a tarp and that weighed in at 218g including stiffeners (30g) and guy lines.
You can also see some images of my first prototype.
Fabric Choice - Base
The base fabric needs to be something that is 100% waterproof. 2oz PU-coated nylon is adequate for the task most of the time but it not going to cope with puddles. 4oz PU-coated nylon is much better but it is also much heavier. SilNylon (Silicon-coated Nylon) is fantastic as a waterproof material but is amazingly slippery. This makes it a poor choice in some respects for a base material. I used 2oz PU-coated nylon that weighs in at 80g/m2 and costs around £4 per metre
It is pointless to use a very light fabric on the base because it will very quickly develop holes.
Fabric Choice - Top
For my prototype I used a Pertex ripstop fabric with a weight of 55g/m2. It is the type of fabric used to make kites and parachutes and costs around £6 per linear metre.
For the hood you could use pertex or midge/noseeum mesh. My original design used pertex but I think mesh would have been a better choice for a bag that was being used under a tarp. [You could also fit mesh as part of the hood design].
Costs and Materials - approx. £27
- 2.5m of 2oz PU-coated nylon (or your own choice). Approx. £10
- 2.5m of light Pertex. Approx. £16
- [optional] 30cm of 15mm Velcro
- 1m of 2-4mm cord + a cord lock
- Size 12 ball-point sewing machine needles
- A sewing machine
Pattern - design and notes
The first thing to mention is that this pattern was made for me at 6ft large build with a bulky sleeping bag. The pattern is much longer than it needs to be and the hood area is much longer than it needs to be. The bag is also designed to take a standard 50cm width mat.
The second thing to note is that it is much easier to reduce the size of an item than it is to increase it. My advice would be to use the pattern as provided and then reduce the seams and the bag length if necessary for you. If you have any doubts about anything then the simplest thing to do is to buy some scrap fabric from the cheapest shop that you can find and use it to create a prototype/toile/muslin.
The pattern uses simple geometric shapes so that the material is easy to cut and mark. If you are in any doubt about how to mark or machine fabrics then please take time to read all of Sewing 101. It is always worthwhile to practice o the cheapest fabric that you can buy and not the most expensive...
Most fabric is bought in widths of 1.4-1.6m. I have chosen to keep the design simple by not maximising the use of fabric. This may seem a little wasteful but it does prevent the need to place a seam half-way down the bag on the top and the bottom. The 'waste' fabric can be used to make things like stuff-sacks.
Before you start make sure you are familiar with Sewing 101 and the terms that I am using. Also make sure that you are aware of the right/wrong sides of fabrics
1. Pattern Cutting the Base and Top fabric
Mark the base fabric on the wrong side and cut using a sharp pair of scissors (preferably draper's scissors). For marking up these long lines I tend to use a 2.4m length of 9mmx30mm wood which is straight and will do most lengths that I need to mark. (For details on how to mark fabric see Sewing 101 )
To produce a symmetric mark-up the easiest way is to mark out a rectangle of the required size and then mark the centre-point of the rectangle at the narrowest end. Use this centre-point to calculate and then mark the edges of the foot end.
Cut the base from the waterproof material and the top and hood sections from the pertex material.
2. Hemming the Top section
This bivi bag design features a cord or elastic closure around the shoulder area to help keep the draughts out. It's optional but very useful.
Using a 0.5-1cm seam allowance hem the 90cm edge of the top fabric.
(Orange = Face side of fabric, White=wrong)
Hem the longest edge for a length of about 2cm perpendicular to the hem that you have just created. This will protect the exit-points of the tube that will be holding your cord or elastic.
Fold the hemmed section over again to form a 2cm tube on the wrong side of the fabric. Stitch this using a single line of stitching. Do not stitch right to the edge.
3. Constructing the Main Body
Place the top section onto the bottom section so that the wrong sides of the fabric are touching. (With coated fabrics you will be able to see both coatings).
If you really must use pins then only place pins in the seam allowance. It would be better to use plastic-coated paperclips and even better still if you can just guide the fabric by hand (but this is an 'expert' technique).
Stitch a single straight seam along the foot section and the parallel sections. Do not stitch to the topmost edge but leave a gap near the tube that you have stitched.
Now fold the edges inwards about 1cm so that the seam you have just stitched is covered and stitch again. This will help to reinforce the seam for the lightweight fabric. It will not improve the waterproofing much.
4. The Hood-Free bag (saving 35g)
Turn your bag inside out so that the right sides of the fabric are now visible.
Hem the topmost area of the bag - ideally using a rolled-hem. If you wish you could also sew a loop onto each corner using 10cm of 10-15mm nylon webbing for each loop and using some parallel stitching or flag stitches.
Skip steps 5-7
5. The Hood - preparation
The hood has one edge which is hemmed twice.
To do this hem one edge of the fabric, fold over and hem again.
The hood is designed to have a significant overlap with the main body of the bag. This allows you to have a nice rain resistant gap (if your fabric is waterproof).
6. Attaching the Hood
Place the hood onto the bag so that you can see the wrong side of the hood fabric and the wrong side of the top fabric. Hem the hood as shown using a 1cm seam allowance and a single line of stitching. Fold over and stitch again (just as for body).
Note: The hood is stitched so that there is roughly a 15cm overlap with the main body and this IS NOT stitched. The hood stitching should stop where the hood meets the top fabric - you need to be able to get in and out of the thing...
Reinforce the stitching at the edge of the hood.
7. Hood Velcro Closure
Make sure that the bag is the right side out and that it is possible to enter and exit the bag.
The Velcro is fitted in three strips as shown. The edge Velcro will help to stop side draughts and forms a natural tunnel. It also allows for space to be made in the hood.
The Velcro in the middle is used to close up the bag whilst still allowing fee airflow.
[If you are worried about bugs the easiest thing to do is to add a 30-50cm strip of mesh to the end of the hood bag and then use Velcro further down the bag to secure it. ]
The edge Velcro strips are placed about 2cm in from the edge of the seam and the central Velcro is also about 2cm in from the edge.
Locate the hook half of the Velcro on the top of the bag where the hood overlaps. Use 1 or 2 pins in the centre of the Velcro to secure it. Make sure that the pins do not go through the base fabric.
Stitch the Velcro to the top fabric and make sure that you do not accidentally machine the base fabric or the hood.
Now pin the other half of the Velcro to the underside of the hood and ensure that all the Velcro match up.
Stitch the Velcro onto the hood.
8. Elastic / Cord Closure
You should have left about 2cm of the top unstitched to the base and this should be where you have created a fabric tube.
Make sure that the bag is inside out with all the stitching visible.
Feed the elastic/cord through the tube so that about 3-5cm sticks out of each end. [If you are using elastic it should be 10-20% shorter than the width of the bag].
With one side of the bag (left side?) create a small loop in the cord and pin this so that it overlaps with the seam line between the top and the base of the bag.
Machine stitch over this loop several times so that it is securely attached to both the top and bottom of the bag. Reinforce with scrap fabric if you feel it necessary.
On the other side of the bag feed a cord-lock onto the cord so that it is between the end of the tube and the edge of the stitching.
Again stitch the end of the cord as a loop onto the edge of the bag.
THIS IS A HIGH-STRESS AREA. My bag (without reinforcement) is already showing signs of pain.
You may if you wish seal all the seams internally with seam-sealant.
You should now have a bivy bag suitable for use under a tarp or on a dry night. It probably took you about a day to make.
If you find the bag is far too large for your body size then the easiest thing to do is to create a new seam at the top and bottom of the bag and cut off the surplus fabric. DO NOT GET TOO CARRIED AWAY WITH THIS.