Hi there
I have seen a few posts recently regarding the jerven bag and it has intrigued me
My question is, what actually is it and what does it do? ...
It's a multi-purpose shelter. It can keep you dry and warm, and it can hide you.
It's basically a rectangular sheet of waterproof fabric plus some (possibly optional) insulation. The fully-opened out sheet is about 2.2m by a little over 2m. The fabric is printed with an camouflage pattern on one side and aluminized on the other to reflect heat. Both the camouflage and the heat reflecting coating are excellent. The sheet is fitted with five zips. More on the zips later.
The fabric does not breathe water vapour, water cannot pass through it at all. Nor can air, and the makers claim that it can help to prevent your scent escaping.
There's a liner which is either permanently attached or detachable, depending on which product you buy. There's a choice of liner weights, at least two (80g/sq.m. and 200g/sq.m) but there may be others now. I went for the heaviest, non-detachable liner. More on that later too.
The zips need some explanation. All the edges of the sheet have one half of a zip attached. The clever bit is where the halves are attached. Imagine that you fold a rectangular sheet to make a sort of bag which is open along three sides. Imagine now that each of those three sides has its own zip, and you can zip them all up to make an almost totally closed bag. That's the Jerven bag. You can use it like a big rectangular sleeping bag. It's very roomy like that, in the morning I've often lost my Thermarest!
When you unzip the zips all the way along they split at the 'bottom' so that the zip halves don't pull on the fabric when you open the folded sheet right out. I find them a bit fiddly to get back together but they're good strong zips. All three zips are double-ended, so you can open up one corner of the closed bag to get some air. I usually fold an underneath corner back to lay my head on some sort of a pillow on the ground which means that the water vapour from my breath doesn't get in the bag.
Earlier I said five zips, and that's only three so far. About where your shoulders are when you're lying in the bag there are two short slits about a foot long in the sheet and liner. The slits have zips. You can get into the bag, leave two of the three main zips partly open at one corner for your head, and undo the short zips to put your arms out through the slits. It's great for sitting up toasty warm in the morning and making a brew without getting out of the bag. You can sit in a chair with just your arms and head sticking out. I've done that for many hours in bitterly cold weather and been warm as toast.
There are eight eyelets. These are placed near the four corners of the
folded sheet, so that when it is opened out there is still one at each corner, but also two pairs very close together in the middle of two of the edges. I think the eyelets are probably the weakest point of the design, I always worry that water could get in through them but I've never actually found it a problem in practice. Having the eyelets means that you can open out the bag and use it as a tarp or as a reflector behind you when you sit in front of a fire. I wouldn't want to get it too close to the fire in case sparks made holes in it.
I've used mine a lot in cold weather, and I've never noticed any condensation at all inside it although I often wake up with the outside covered in ice. I usually hang it up to air in the mornings to let any moisture inside it evaporate and keep it fresh.
The model that I have is the 'Thermo Extreme' which has a fixed liner of 200g/sq.m. Sometimes I've been very hot in it. For that reason, if I were going to buy again I'd probably buy one with a detachable liner and two different weights of liner. I find that the Thermo Extreme is just too hot for use in warmer weathers. In February on the north coast of Spain for example, I got much too hot and sweaty in my sleep and had rather an uncomfortable night as a result:
http://www.jubileegroup.co.uk/JOS/misc/dscf3783a.jpg
