Tuesday, 5 July 2011

Elephants Foot Bivi Bag.

I had to bow out of the TGO this year but I was up until a month before going with my friends Seth and Tom. Tom had just purchased a brand spanking new PHD minim 400 with a dryshell outer to keep the moisture out of the down.

They were both sharing a Go-Lite Hex 3, with no inner just a floor that I had made for them. A single skin shelter with two people in can suffer a lot with condensation. Tom was having issues with soaking his new bag at the foot end even with the dryshell outer so he asked me if I could come up with a solution. And here it is...

The green fabric is a two layer waterproof breathable fabric.  I have not a clue what it is but it is alright on the breathable stakes.  I got it to make overmitts with, and the prototypes (coming soon) worked well.  The black strip is Pertex with a drawcord around the top to stop the bag coming off.  The pertex strip is there for two reasons.  Firstly if you can put the drawcord above your knees it doesn't have to be on that tight and therefore  compression of the down is minimised.  Using Pertex for this last bit reduced the weight of the overall item.  Secondly the green fabric although breathable is no where near as breathable as the Pertex. Having a semi sealed sack of this fabric around the foot end of your bag could lead to condensation issues.  The Pertex panel gave the bag that extra needed air flow.  Tom reported back that it worked well, no issues with condensation even in the damp cold conditions that they faced.  The only issue was that I made it to fit me not taking into consideration that Tom is a couple of inches taller.  Therefore the draw cord was a bit low for him.

I don't think that something like this would suit everyone, I can think of another blogger who can destroy a footbox in down quilt with moisture without enclosing it in a membrane fabric bag!

At 84g and a small pack size it wasn't a great extra burden to carry and resulted in Tom's bag staying dry for the duration of what was a very wet couple of weeks.

I know a lot of people use their waterproof draped over the foot of the bag to keep the damp at bay.  As I'm as not as tall as Tom I don't tend to have problems hitting the end of the shelter so I can just use a thin Pertex bivi without issue.  I'm not sure how well a wet waterproof would have worked over two weeks so perhaps Tom made a call using this?


Alan R said...

Even using a waterproof that is wet outside and only slightly damp on the inner will transfer that damp through a sleeping bag outer and into the down. Not recommended. Your idea is far superior. Nice one.

Shed Dweller said...

I think over a long trip you're dead right, you can get away with it on an overnighter or weekend. I was dubious regarding the condensation but Tom said it was fine, it was a joint project so I can't take all the credit.