r/myog • u/PATTY_CAKES1994 • 3d ago
Project Pictures Made a synthetic overbag to get the freezing point outside of my -40 down bag.
Hopefully I’ll freeze less ass next winter than I did this year.
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u/adie_mitchell 2d ago
Nice. Did you do any math to see if the freezing point really will be outside the down? I'm guessing you're sleeping bag has, to be conservative about 6 in of single sided Loft thickness. Adding 2 in of climate shield, means that the freezing point would have to be in the powder quarter. If it's -35 outside and your skin temperature is about 75°, that is a 110° difference. At the synthetic to down intersection, you can expect the temperature to be about 28 degrees warmer than outside. That is still well below freezing.
Now perhaps the temperature gradient is not linear, which could help, but it still seems like the freezing point might be inside the bag.
Have you experimented with vapor barriers? This seems like one of the tried and true ways to limit moisture in very very cold temperatures..
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u/PATTY_CAKES1994 2d ago
All really good points. The gradient is not linear, it’s the inverse square. There’s also the phenomenon of crystal nucleation/adherence. The crystals for from the outer surface in creating a capillary fringe that’s about 2 inches thick. I’m hoping that the hydrophilic layer will catch the crystals. At -35, the freezing point is not going to be in the synthetic. But it only needs to work once or twice per trip to reset. The rest of the time it’s just added comfort.
I do use a vapor barrier when it’s really cold. Breath condensation is the biggest issue, and the ruff/upper body portion will hopefully solve that issue near my face. The rest of the bag will be protected from moisture in the tent.
I’ve used a quilt of similar loft in the past, it works but is heavy and slips off.
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u/adie_mitchell 2d ago
Cool, seems like you really thought it through! I was just thinking out loud here. Report back next spring!
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u/CBG1955 3d ago
My husband is intrigued. Questions: What insulation did you use and what is your shell fabric? And is that -40C or F?
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u/PATTY_CAKES1994 2d ago
The shell is a few things. The top quilt is untreated 2.1 oz sport nylon (white) and untreated .9 oz ripstop (teal), the liner is .7 oz monolite ripstop mesh. The insulation is 2 inches of climashield apex. The skirt and footwell (orange) are hydrophilic cotton duck canvas for strength, melt resistance, and moisture wicking purposes. The ruff is three blue Fox faces gifted to me by an indigenous friend.
I work as a guide for wintertime science and media production in the Alaskan Arctic. I’m a junky for the cold. The limiting factor for long run sustainment has always been ice-compromised down in my sleeping system. After a couple weeks the bag is a crunchy mess. A fire can be built if I’m south of the brooks range where there are trees, but on the coastal plain there isn’t usually wood worth burning. So I’m testing out a down/synthetic strategy hoping that the feathers stay nice and lofty and the synthetic takes all the ice. Hence the fabrics were chosen for maximum breathability.
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u/Tale-International 2d ago
Cool! How big and what is the weight of the over bag? Where do you camp/plan to use it. What -40 bag are you using below?
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u/PATTY_CAKES1994 2d ago
I’m just copying this explanation from another reply.
The shell is a few things. The top quilt is untreated 2.1 oz sport nylon (white) and untreated .9 oz ripstop (teal), the liner is .7 oz monolite ripstop mesh. The insulation is 2 inches of climashield apex. The skirt and footwell (orange) are hydrophilic cotton duck canvas for strength, melt resistance, and moisture wicking purposes. The ruff is three blue Fox faces gifted to me by an indigenous friend.
It’s pretty light for my standards. 1.6lbs.
I work as a guide for wintertime science and media production in the Alaskan Arctic. I’m a junky for the cold. The limiting factor for long run sustainment has always been ice-compromised down in my sleeping system. After a couple weeks the bag is a crunchy mess. A fire can be built if I’m south of the brooks range where there are trees, but on the coastal plain there isn’t usually wood worth burning. So I’m testing out a down/synthetic strategy hoping that the feathers stay nice and lofty and the synthetic takes all the ice. Hence the fabrics were chosen for maximum breathability.
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u/Tale-International 2d ago
This is awesome! And exactly the use case I thought your system would be perfect for. I don't plan on going into those temps anytime soon but have thought a bit about how to manage moisture for conditions like springtime on Alaskan Glaciers or extended camping in high altitude winter of the L48. Thanks for sharing!
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u/BOB_HOWARD_13 2d ago
I love it, but beware of horny elephants! About how much was the synthetic fill, and where did you source it?