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Author Topic: Boosting my rating Query about the Goretex bivvy in the system  (Read 997 times)

Offline Moondog55

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I am prpearing all the gear I will need for my bucket list trip in a couple of years.
I need to boost the temperature rating of my sleeping system to cope with -40 and lower.
Becuase I will only need this combo for a single trip of about 90 days I see no point in buying a single good bag that I will never use again and what I own is really close to state of the art.
This is a specific query about using my existing system including my Goretex bivvy bag.
System as it stands is my old Everest summit assault bag; expedition cut to fit over an XL sized down suit, rated at -4C on its own under the usual test conditions. Inside that I use a Western Mountaineering Tamarack short sleeping bag rated at -2C and a down parka insulated with a full 600+ grams of 650 FP DryTek down.
When using a snow cave/snow hole/single skin tent I have an XL Full bivvy that goes over everything.
This combination is good down to -30C easily, it is a sauna at -18C/0F.
What I cannot experiment with over here is using the Goretex inside the very big synthetic overbag. it simply does not get cold enough ad OH&S and insurance will not allow anybody with a big blast freeze to allow me to test.
Using 80GSM synthetic insulation in the overbag I am trying to determine at what temperture the Goretex stops allowing vapour to leave the inner bags
This is second generation highly breathable Goretex specific to bivvy sacks and was never used for jackets as it is not waterproof to 10,000mm, even though it does keep off a decent rainfall.

If 80GSM won't keep me warm enough I can add more insulation when I convert the overbag form its current style to a Gerry Cunningham Mountain Sleeper with an integrted sleeve to take the full mattress system of XL CCF pad / XL Ridgerest solar plus the S2S Comfort plus insulated air mat Total R-value 9+

Offline Undersky

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #1 on: March 26, 2017, 09:50:55 pm »
" Using 80GSM synthetic insulation in the overbag I am trying to determine at what temperture the Gore-tex stops allowing vapour to leave the inner bags "

Hey Moondog, it is fun to be planning this far ahead of your big trip, eh?!

Experience with Gore-tex layers in sleeping systems has shown me that so long as there is significant heat gradient between inside of the Gore-tex and outside, and so long as the temperature and relative humidity immediately outside the Gore-tex layer is above the dew point (the temperature where gaseous water condenses into liquid water) then the Gore-tex will allow gaseous water to pass through.

That said, your description raises a few questions: Many campers use Gore-tex as an outside layer, as you indicate that you also do when using snow shelters, but it is much less common for people to use it roughly midway within the layers of insulation. I wonder what the benefit would be. You are not needing it for waterproofing mid-way through your system. Yes, it does provide a small insulating factor, but wouldn't other purpose-built insulation layers of the same gm/cm2 give much more insulation for the same weight?

Also, how many of your 90 days (sounds like a good trip, by the way!!) will you be sleeping in cold conditions? How many of those nights will you be accumulating moisture in your sleep system? How often will you have opportunity to dry everything out using external heat?

If external heat is not to be an option, you might consider one of these two options:

Yukky in many people's minds, and often difficult to live with, but an internal vapour barrier system can keep your insulation in dry condition over a long time period.

Another option could be to think of the inner 3/4 of your insulation closest to you as your primary layer, made of the excellent fully breathable insulation layers you mention, and with none of the less-breathable Goretex. The outer 1/4 might be thought of as your "secondary" layer, and might best be of very breathable synthetic insulation and shell (no Gore-tex) . With this system the primary layer will likely always be inside the dew-point, and will not accumulate moisture. The outer secondary layer will definitely accumulate frost, but this layer can be made dried more easily that trying to dry your whole system, and on warmer nights the secondary layer can be reversed to position the frost closest to the outside of your system. This way your body heat will, on warmer nights, encourage sublimation of the frost, and effectively dry out your secondary outer layer.

Just some thoughts.

Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #2 on: March 26, 2017, 10:45:04 pm »
This bivvy works well down to -25C used it in the Adirondcks there and lower with no condensation.
But there is a huge differential between -25C and -55C.
My thinking is that if I have enough of a push to keep the inner bags dry I won't need a vapour barrier. If I use a VB I need to source a different liner bag and I can't use my down parka to sleep in unless I also make a VB half bag and a new VB shirt.
That -30C is comfort rating so long as I am in a tent and the usual rules are applied. Here in OZ I use a LW Uniqlo down parka and an UL custom sewn though half bag with only 150 grams of fill when it's about -18C [ in adition to my sleeping layer this is] and the main reason for needing extra is simply because the outer bag is so big it has lots of air space inside it
The bivvy weighs about 800 grams but serves a secondary purpose because the shell of my sleeping bag is very [ very ] breathable and not really windproof [ the shell is half ounce ripstop, ethereal stuff 30 years ago when I bought the bag] and the bag itself weighs 1100 grams
I should mention that this bivvy and the sleeping bag have no zip, both are wriggle-in bags to keep the weight as low as possible but the outer shell will have a zipper.
I'm hoping to spend at least  thirty of those 90 days in the outback more if possible and some of them need to be spent in the iconic parks, Yellowstone and Yosemite
I realise that 7 kilos is a very heavy sleeping system these days but I want it to also be as flexible as possible.
the shell will use 80GSM because it is Wiggys stuff, laminated to a heavy tough nylon already and I already have it, a single layer is good to about 14C so on its own it is a summer bag
So this is why the question mainly
if a shell using 80GSM insulation is only good to 14C it will only add 10 degrees to the lower limit of the system [ because in the absence of wind we should sleep comforatbly at 24C] will I need to think about more insulation in the outer bag or simply accept that I will need to use other methods of boosting the warmth if I happen to encounter a real chill

Offline Dave Hadfield

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #3 on: March 27, 2017, 03:12:47 pm »
Sounds like lots of bedding, you should be quite comfortable. I hope you have enough room inside. Me, I like to move my feet around a bit.

Goretex does not work when the fabric is below freezing, 32F. At 31F, water vapour freezes on any surface. Never mind the "pores" -- I've seen frost condense on a spider web, when the gaps are 1/4 x 1/4".

For Goretex to work, and pass vapour to the outside, your body heat has to keep it above freezing.

A simple light wool or felt blanket, as an overbag, is a very useful way to get the vapour outside the down.

Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #4 on: March 27, 2017, 04:59:13 pm »
Why is it tht our respective experiences of Goretex are so different?
I've experienced the phenomenon when bivvying and a I get a frost layer on the ouside of the bivvybag  but never when using it as a sleeping bag protector in a snow trench, tent or hut.

Offline Undersky

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #5 on: March 27, 2017, 11:37:58 pm »
We may be confused by what each other has experienced, but the water vapour is just doing its own thing, irrespective of our understanding. :)

My understanding (and flawed it may be !!) is that the insensible relative humidity on the surface of our skin in the still air within our sleeping systems maxes out at 76% RH. If we get a little over-heated as we get settled in our sleeping bags, then we our body may perspire in a natural effort to cool down, and that RH of 76% can quickly turn into an RH of 100%, but let's say we don't get over-heated for now.

That RH of 76% is much higher than the RH of the air in the insulation a little further away from our skin. Humidity will naturally move from areas of high RH to areas of low RH, so that vapor close to our skin moves out through the insulation. As the RH next to our skin drops, we perspire insensibly a little more to raise it back to 76%. So the cycle continues all night long as we inadvertently "push" moisture out into our insulating layers.

As the moisture vapour moves out through the insulation, further from the body (heat source), it runs into cooler and cooler air. Increasing the amount of humidity and decreasing the temp means that at some point the dew point is reached where that layer of air near the outer surface of our sleeping systems can not hold any more vapour, and some of the vapour condenses into liquid.

Gore-Tex only transmits vapour, so if the dew point is inside the Gore-Tex, the trapped liquid will not escape out through the Gore-Tex layer. Later that night, as your metabolism drops and your heat output drops, and the night-time air temp. drops, the air inside your insulation around that liquid water may well drop below 0C, and that liquid will become frost.

This problem gets worse as more moisture is pumped into the bag on successive nights, if it is not dried during the days. (On one 13 night January trip in a cold tent with no outer shell on the sleep system my -40C synthetic bag accumulated almost 4.5kg of crunchy water :)

If I may, DH, I think that Gore-Tex will still pass gaseous moisture even well below freezing temps so long as that moisture is still in a gaseous state (above the dew point), and so long as the pores in the Gore membrane are not plugged up with frost. I agree completely with your statement that a fairly thin insulating layer, as in a light wool blanket, will serve to keep the air temp immediately outside your main sleeping bag's shell slightly above 0C, allowing humidity to be pushed out of your main insulation, Some of that moisture will condense and freeze in the light wool blanket, but that can be largely shaken out next morning.

M55, could it be that the temperature of the outer layer bivy bag of your sleep system in the "trench, tent or hut" did not dip below the dew point, or that you did not push much moisture out through your sleep system between opportunities for your system to dry out?


Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #6 on: March 28, 2017, 02:38:40 am »
All good points.
I wish I could experiment at -40 before I get there. Perhaps my boundary layer on the bivvy isn't being disturbed or I am simply putting out just enough heat while sleeping to keep the air and the water vapour above the dew point.
I've used VBs and also tyvek partial VBs all to good effect when using bags mrginal for the conditions but as a way of extending a bags lower limit and not to stop the bags insulation accumulating water.
Hyperdry /Nikwax treated down absorbs very little water tho .


Offline southcove

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #7 on: March 28, 2017, 10:32:50 am »
The almost primal wonder and thrill of a hot tent making many of those issues less serious.  And fairly controllable typically.

Everest climber and polar explorers still seem to deal with many of the same frost accumulation issues; very quickly making the temp ratings and lofting (plus weight and packability) of the bags and clothing* much less effective and protective.   G-T is useful in many circumstances but I have come to the belief and understanding over the years, both with personal experiences and written accounts that the 'miracle' of waterproof // breathable is best used in not as many situations nearly, as the manufacturer would have you believe.  I think that probably goes for most laminated, coated fabrics to claim the same properties. 

Life is a series of constant tradeoffs - the usefulness of G-T and similar is certainly one of those situations. 

Not just G-T of course; furs, treated cottons, etc must all be dried out constantly or frost just continues to accumulate.  Enough Polar explorers have found that out too late...

Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #8 on: April 11, 2017, 10:51:53 pm »
Well it's the Northern Spring so I guess I'll just have to wait for next northern winter so one of you brave souls can try out my idea and tell me how effective it is or isn't

Offline chimpac

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #9 on: April 13, 2017, 09:36:13 am »
It is not much fun to use only a bivy in bad weather. I can see where it is needed by a mountain climber if he has to spend the night hanging on a mountain side.
If there is room to pitch a tarp, you and your gear can be kept dry while you cook, eat and sleep. So if you can have the luxury of a tarp then you do not need a waterproof bivy. You do need a bivy bag with a full net top that is good for the hot summer, keeping flying and walking animals out of you bed. The same bag works fine for winter. Anther plus for using the bag with a tarp is that you do not need any pole structure to keep the netting up off your face or in hot summer your naked body.
The net top does not make you warmer (boost rating) but it may let out moisture that may boost your comfort level.
« Last Edit: April 13, 2017, 09:51:00 am by chimpac »

Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #10 on: April 13, 2017, 06:12:41 pm »
Using my existing goretex bivvy means not having to outlay money for a sleeping bag protector tho and if using a tarp I really do need a bivvy as well to avoid spindrift etc.
It's equivalent to using a sleeping bag with a Goretex windstopper shell inside a synthetic sleeping bag and so far I've not found anybody who has done it to comment.
I have to use a sleeping bag cover of some sort as the shell is so lightweight and relatively fragile so why not use the Goretex bivvy bag?
Mind you I have no intention of sleeping under a tarp in -40C, when I come over I will be sleeping in a tent, it remains to be seen if I am cold tenting or hot tenting tho, but most probably hot tenting for at least part of the trip

Offline chimpac

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #11 on: April 13, 2017, 06:33:25 pm »
When you say sleeping under a tarp what kind of a pitch are you talking about.
I use only a tarp in all four seasons pegged usually only on the four corners. The sides are raised if needed in summer or tight to the ground in cold wind. There is no spindrift inside when it is -30C. the edges have fire wood pieces tied to them and are covered in snow.
.

Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #12 on: April 13, 2017, 09:09:44 pm »
No spindrift at all?
I usually use the military pitch, dig a shallow trench, one end to the wind flat to the ground and the other supported on my skis, usually practised once a season, for emergency use only as I normally use a strong tent

Offline chimpac

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #13 on: April 14, 2017, 12:39:49 am »
you mentioned spindrift
quote] Using my existing goretex bivvy means not having to outlay money for a sleeping bag protector tho and if using a tarp I really do need a bivvy as well to avoid spindrift etc.

It stands to reason if any side of a tarp pitch is open there is going to be drafts.

Offline Moondog55

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Re: Boosting my rating Query about the Goretex bivvy in the system
« Reply #14 on: April 22, 2017, 05:54:58 pm »
OK Chimapc Different countries,  different use of the language.
When we Aussies talk about tarps we simply assume they are open, your usage is more like my now defunct Chouinard Megamid.

I am considering getting something similar when I get there as a back-up or I may just get a couple of US arrmy shelter halves to use as a tank in the pulk as back-up just in case the hot tent burns down.