Layering insulation and using goretex bivvy shells

[Moondog55]

Time for putting on your thinking hats here.
Assuming most here are like me and never had to sleep in -40 temperatures and can't afford to own a -40 rated bag.
If I need to add insulation to my system and will be re-making my synthetic overbag by adding a short side zipper and a sleeve for the mattress system do I still take my Goretex bivvy bag and use it inside the synthetic shell?
I have not found this discussed anywhere and while I've asked the question over an a couple of US sites I've not had an answer there.

[slparker]

can you hire a -40 bag where you are going?

[GPSGuided]

Where are you going to encounter -40C? Alaska? Far north Canada? Surely you are not going to be sleeping out in the open, right? What's the accommodation/shelter?

[Moondog55]

can you hire a -40 bag where you are going?

Not a LW one that I can carry in

[andrewa]

Wouldnt you be better with a vapor barrier than a goretex layer? Whilst I'm sure the goretex will slow down some heat loss, don't you really want to just stop it dead! Anecdotally, I do sleep in my goretex layers when I've been cold, and it does make a significant difference. I usually chuck my goretex on under my down jacket as per your thinking, and then quilt over.

A

[DarrenM]

Lightweight -40 bag?.....I'd consider staying warm an absolute priority. All these lightweight multiple layers add up to very little compared to down. That kind of cold requires $$ and quality down IMO.

[Moondog55]

Where are you going to encounter -40C? Alaska? Far north Canada? Surely you are not going to be sleeping out in the open, right? What's the accommodation/shelter?

Algonquin Park
Average night time temperatures are higher than that but I have been warned it can get much colder that -40 occasionally, down to -56F once I was informed
Also no point in hiring gear if I have it already
People tent, hot tent and use lean-to tarps and hammocks at this gathering apparently.
This is the gathering I missed 3 years ago when I asked all those annoying questions about woollen clothing and Arctic footwear

Bloke I am going up with is a tarp camper but maybe I'll have some funds to buy a new tipi tarp

Clothing etc I am good, and footwear I'll buy over there as surplus.
I only had the thought about using the Goretex bivvy inside the overbag today when I pulled said overbag out to check it out
A vapour barrier would work perhaps better but I cant use the VB and my down parka at the same time unless I also make a new hooded VB pullover shirt and a VB half bag
I wouldn't want to run the risk of the down in my sleeping parka becoming wet by wearing it inside a full VB liner
My thinking was that the insulation on the outside of the bivvy sack would stop frost from forming inside the Gore membrane and allow body moisture to pass through into the synthetic which would be easier to dry out

[Moondog55]

Lightweight -40 bag?.....I'd consider staying warm an absolute priority. All these lightweight multiple layers add up to very little compared to down. That kind of cold requires $$ and quality down IMO.

G'Day Darren
Nothing wrong with my current system down to -30C IMO
It's that extra 10 degrees I'm worried about
My current SB is going off soon the One Planet for a rejuvenation and I'll add 150 grams of down [ a shame they only have 850 fill power down and none of the 900 that my old J&H Ultra was originally filled with] and that Ultra Flouro with the Western Mountaineering Tamarack and my Everest down parka combined give me in excess of 100mm of insulation at a minimum and more over the torso; the parka alone has 75mm of insulation on the chest and 50mm on the arms and the SB is cut to allow full loft
They are $eriou$ money and $eriou$ly good down

[DarrenM]

Lightweight -40 bag?.....I'd consider staying warm an absolute priority. All these lightweight multiple layers add up to very little compared to down. That kind of cold requires $$ and quality down IMO.

G'Day Darren
Nothing wrong with my current system down to -30C IMO
It's that extra 10 degrees I'm worried about
My current SB is going off soon the One Planet for a rejuvenation and I'll add 150 grams of down [ a shame they only have 850 fill power down and none of the 900 that my old J&H Ultra was originally filled with] and that Ultra Flouro with the Western Mountaineering Tamarack and my Everest down parka combined give me in excess of 100mm of insulation at a minimum and more over the torso; the parka alone has 75mm of insulation on the chest and 50mm on the arms and the SB is cut to allow full loft
They are $eriou$ money and $eriou$ly good down

Ahh, bit more perspective.
Good idea with the One Planet bag. I had my OP bag stuffed with 1150gms back in the day. Warm but bulky and probably less than 800 loft at the time.

[Moondog55]

These are the components of the system
Been posted before but worth showing again
It was only the other day that I thought about using the Goretex bivvy over the SB but inside the shell
The total system hasn't been accurately weighed yet but would still be much lighter than the US/Canadian hire bag [ Cecile has one of these I bought from Hallu when he left for home:- it's a monster] and not cheap to hire either

[slparker]

What about a VB (tyvek or something) over your sleeping sheet, then your SB and synthetic overbag, then a tyvek VB over the lot. No possibility of your down getting damp from body vapour or external condensation and tyvek is relatively cheap.

[Moondog55]

I have tried the Tyvek system before but it doesn't work with the parka
I already have the big MD Foxhole bivvy
We are in the middle of a big renovation semi-disaster ATM so I can't get everything together and show photos for a week or so

[nq111]

It is an interesting thought exercise this one. I'm trying to get my head around it!

Key to the thought process to me is a) what does the goretex bivvy achieve and b) how does the down jacket end up performing - in particular with reference to the down getting damp and collapsing some.

For a) I would guess the goretex would act as a sort of vapour barrier to increase warm by reducing the heat lost by evaporative cooling. Goretex is not all that breathable (hence why so many walkers put on their goretex jackets for warmth around camp despite it having little insulation value). However the goretex will still let some moisture escape. The outside of the Goretex bivy is going to be within the synthetic bag, and hence have fairly humid, still air around it (once an equilibrium is reached, which may take some time) so probably won't breathe all that well I would have thought. Gore-tex really needs a flow of dry air over the outside of the shell to breath well as I understand it.

For b) the issue is if the goretex doesn't breath very well (which is what I would guess will happen), the down jacket is going to get pretty damp, pretty quickly being worn inside the bivvy. So this system may give a decent boost to warmth for the first few hours, or first night, but than a damp down jacket is not going to be such good value. If the goretex actually does breath really well (I doubt it!) then it isn't going to add much extra warmth to the system (because evaporative cooling is happening).

So assuming the thought exercise is to get the most warmth with a synthetic outer bag, down jacket and goretex bivvy I would try either:

1) Sleeping with just thin baselayers on, then the goretex bivvy as a vapour barrier right up to my neck (and done up fairly tight around the neck to prevent the loss of humid, warm air), then the down jacket draped over the outside of the bivy across the chest/neck and then all encased in the synthetic bag. From a moisture management perspective, the down would be better on the outside of the synthetic, but I would rather have it sandwiched within the synthetic bag so it didn't fall off too easily. Plus the goretex bivvy should slow the amount of moisture impacting the down.
2) Sleeping with just thin baselayers on, then the goretex bivy up to under the armpits (sealed up as well as possible again), then the wear the down jacket, and then the synthetic bag around everything. This would have to be more comfortable then 1) but you lose the benefit of a vapour barrier over the arms and neck. The down jacket should breathe 'ok' over the arms but hard to know given that the vapour produced is going to freeze up somewhere in the insulation at -40c (i.e. won't escape into the air as water vapour). I have been surprised how significantly drier my down bag is after a week in Tassie when using a full vapour barrier to sleep in - and that isn't anywhere near -40c.

I can't see any value in a waterpoof shell to the sleeping bag - at -40c no moisture is going to fall on the bag, only ice crystals and it is going to stay frozen (brush off easily). The battle to focus on is managing the moisture vapour from within.

Then the logical solution from the above (provided I am not totally off track) would be to ditch the heavy 'sort of' goretex vapour barrier and get a proper one, like an SOL emergency bivvy or a Western Mountaineering hotsac. Actually no way I would want to tackle anything like -40c without a full vapour barrier - for the dual benefits of a significant boost in warmth and the preservation of the insulation from the damp.

[Moondog55]

Thanx for the thoughts
My actual experience is that Goretex breathes very well in these dry conditions [ especially this "2nd generation" stuff; before they put that second lamination on the membrane] being a totally different fabric from that used in clothing, but I was worried about the frosting up inside the membrane
The trouble with your scenario is that the down parka simply isn't big enough to drape over the sleeping bag; the Ultra Flouro is a big bag.
Without the half bag and parka inside it there is also a lot of free dead space to warm up
While a VB would be and is a good idea it won't work for me unless I buy another slimline SB to use inside the outer bag and I cannot afford that
I explain that in post #7
I need the parka as sleeping wear to get me to -30C
I am considering the 2 part VB tho but even a garbage bag works for the feet end but a VB shirt for me would need to be hooded to keep the duvet hood clean and dry
Note tho that the owner of the site WinterTrekking advised that he uses a Big WPB bivvy sac and not a full VB
This is the real reason I posed the question
Will the centimeter of insulation in the outer synthetic bag allow the water vapour out though the Goretex membrane?
The other problem with using the down parka over the down SB is the lofting power of the different downs, the ultra uses 900+ down and the parka uses 650 loft power down and thus would tend to compress the sleeping bag.
I am convinced that in the real world there is no point in really high lofting downs in garments that will be worn constantly for long periods of time
I do have a possible partial solution in one of Evans Tyvek SB covers as a partial VB but using it gives the definite possibility of the duvet getting wet

[GPSGuided]

Algonquin Park

Ok. That's not that far north from Toronto but given mid-winter with a nasty front, the whole area can get that low. Ummm... Will you guys tough it out in those temperatures and weather? I know I'd be running for a cabin or home when it gets that nasty.

[Moondog55]

I get the impression from talking to them online that they just tough it out.
I'm assuming tho that if it gets to -55 I'd be able find room in one of the larger hot tents and they would keep the stove going by taking things in shifts.
But better safe than sorry where that's concerned.

[Moondog55]

OK So I just ordered 2 meters of 85GSM Climasheild from Simon.
I'll take the overbag apart and resew with another layer of insulation on top and add a short side zipper and a mat pocket big enough to take a mattress combination about 100mm thick
Even if I don't make it again to the gathering I'm covered for the next Ice-Age I guess
I did find some very lightly insulated quilted fabric at Spotlight which is on sale for $5- a running meter and we bought some for LW vests and hats for the tadpoles and that may have done but it was only an average of 4mm thick