Jan 25, 2014 at 6:59 pm #1312450
I have been thinking over a design for a winter quilt or more precisely a top bag. I am a huge believe in VBL even with temperatures around freezing. I am contemplating a full cuben design but I hit upon a question that I can't really figure out.
Why doesn't quilt manufacturers use a truly waterproof layer on the inside of the sleeping bag especially for winter use? With a breathable layer moisture goes in and cools until it hits the dew point. For cold weather that will be in the insulation. It would seem that it would make more sense to not allow the moisture to go in at all, essentially creating a VBL bag for an extended range. With a quilt it is easier to have temperature control in the event of overheating. So why not eliminate condensation instead of trying to mange it? What say you?Jan 25, 2014 at 9:47 pm #2066283
Stephenson Warmlite sleeping bags are like that
One problem is you really can't wear anything inside the sleeping bag or it will get wet. Like if you wear a down jacket inside bag.Jan 26, 2014 at 7:27 am #2066326
Many years ago I tried and rejected the idea. I was inspired to try it after reading Stephenson's info . Keep in mind that I sweat more than most so that could be a factor.
Outside temps were in the 20s F. I was in a double wall tent. I got so wet and cold that about 1 AM I couldn't take it any longer. Even the slightest movement of air on my wet body gave me a chill.
Removing my wet cold body from the sleeping bag, drying off, putting on dry clothes and getting back in the bag was very very unpleasant.
There were puddles of water inside the vapor barrier bag. If it had sprung a leak it would have saturated that portion of my down sleeping bag. Never repeated the experiment.
On a related note I have weighed my sleeping bag before and after sleeping in it on cold nights (in a double wall tent). The weight is about the same or slightly increased so it doesn't appear that much moisture is staying in the bag.Jan 26, 2014 at 7:47 am #2066329
"On a related note I have weighed my sleeping bag before and after sleeping in it on cold nights (in a double wall tent). The weight is about the same or slightly increased so it doesn't appear that much moisture is staying in the bag."
After how many nights ?Jan 26, 2014 at 8:13 am #2066336
It's been awhile since I did it so I'm going from memory but I think 3 nights was the max with at-home weighings before and after the trips.
I'd have to assume the possibility of the bag drying somewhat during the daytime during the trip so that contaminates the data a bit.
The bag was promptly placed into its stuff sack on the morning following night #3, taken home and weighed. So let's say one night for sure and, to some unknown degree, parts of 2 other nights.
Weather was foggy/freezing with a lot of moisture.
Outside temps were in the 20sF but inside temps would have been near or above freezing. This may be the critical factor.Jan 26, 2014 at 8:25 am #2066340
deletedJan 26, 2014 at 8:27 am #2066341
Interesting. Because that would mean that on shorter winter trips or so weightgain because of moisture would be neglible and thus that there would be almost no effect on the insulating power of that bag. Very interesting.Jan 26, 2014 at 9:13 am #2066354
I made a shirt and pants out of Stephenson's "fuzzy stuff" and tried it on nights when it was about freezing. They're kind of heavy – about 12 ounces each. They didn't seem to add that much warmth. Synthetic or down insulation provides much more warmth for the weight.
I've tried wearing garbage bag "vest" against skin. Since it weighs so little, this maybe makes more sense. I didn't mind the clammy feeling.
On Stephenson's website, he talks about how on arctic expeditions, the sleeping bags gradually doubled in weight from condensation freezing inside bag, and then losing warmth. I think this is the case where VBL makes sense – many nights, temperatures much below freezing. There's a boundary layer of air around your sleeping bag that drops about 10 degrees F. So in temperatures less than 22 F, the freezing point will start being inside your sleeping bag so condensation will freeze. So, the temperature has to be somewhat below 22 F?Jan 26, 2014 at 9:27 am #2066357
Like Daryl, I sometimes weigh my sleeping bag before/after.
Unlike Daryl, I can recall it : )
I have a hybrid quilt/bivy. Down. M50 shell. Weighs 23 ounces. 8 ounces of down. Several trips:
6 nights. One night it rained. Under tarp. Down to 32 F. Gained 1.1 ounces.
7 nights. Last night under stars. Down to 32 F. Gained 2.2 ounces. I think this was just water on the surface of the M50 from dew, not absorbed by down.
6 nights. Some rain. Under tarp. Down to 40 F. gained 0.5 ounces.
6 nights. No rain. Down to 40 F. Gained no weight.
So, in cool weather there is very little water absorbed by down. I suspect the little weight gain I measured was the nylon more than the down. Especially, the bottom of my quilt/bivy which is against the ground and usually gets some visible wetness when I pack it up in the morning. Water in the down actually gets evaporated by body heat.Jan 26, 2014 at 9:28 am #2066358
OK Jerry, but I assume that also the moisture level has somwthing to say ?Jan 26, 2014 at 9:32 am #2066361
"but I assume that also the moisture level has somwthing to say ?"
I don't think the humidity of the air has any effect. It's all water that is evaporated from your body.
The main effect is air temperature. If it's colder, more water will condense inside the sleeping bag. And number of nights if you accumulate moisture each night.Jan 26, 2014 at 10:11 am #2066373
I guess I wasn't clear. What I meant is that moisture in the air determines how easy bodily moisture will condense and it is the combination of temperature and humidity that will determine when condensation will occur (so the higher the temp., the more moisture air can contain or at a constant the temp. the higher the RH, the easier condensation will occur).Jan 26, 2014 at 12:31 pm #2066414
Relative humidity will change the location of the dewpoint – and at a low enough RH and a warm enough temperature, that can mean the dewpoint is outside the bag rather than in the layer of insulation. That can make a substantial difference in how much moisture the insulation absorbs.
But to get back to the OP's question, the big issue with having the lining of your bag be the VBL is that you can't adjust the insulation without venting the VBL, which is a major flaw. To use a VBL effectively you must maintain the temperature and humidity within a relatively narrow range: too warm, you'll sweat. And if you sweat, and then vent to dry out, you dump a lot of heat and start a cycle of too warm and too cold combined with a lot of heat loss. So you want to be able to adjust the amount of insulation without venting the VBL. This works best with VBL clothing, as you can even add layers of clothing if you want. With a VBL bag liner, you can unzip or otherwise open you bag/quilt to adjust the amount of insulation with only a minor opening of the VBL. But if the lining of the bag is the VBL, you have no way to adjust the amount of insulation without venting, since all you can do is open up the bag and thus vent. So it's not a good way to go.
Now if you are using a VBL liner or clothing, you could also have a sleeping bag made entirely from non-breathable fabric, since it won't accumulate moisture and thus doesn't need to dry out. You do need to have part of the shell breathable so that air can get out when you pack it and back in so it can loft up.Jan 26, 2014 at 12:41 pm #2066417
Maybe RH is unimportant.
What's important is the point in your insulation where the temperature reaches freezing. At that point, the water vapor traveling from your body to the outside will freeze, and then it will just stay there.Jan 26, 2014 at 1:27 pm #2066431
Were you able to recall those sleeping bag weight numbers from memory alone or did you have notes to which you referred.Jan 26, 2014 at 2:27 pm #2066456
@ikeLocale: Central Michigan
The relative humidity is so high on the Lake Superior shoreline that I've been disappointed with the performance of my 900 fill down, even with all reasonable precautions (rainsuit VBL, synthetic overbag). This year, I've been playing around with a homemade polycryo liner bag with a 2' lip to cover the top baffles.Jan 26, 2014 at 3:40 pm #2066493
I record measurements in Excel file. Not super ADD, but about half the time when I get back from trip I'll weigh bag, let it dry out for a week, then measure again.
I recently switched to down so I'm evaluating whether it's getting wet. My conclusion is, so far, with a bit of precaution, there's no reason to use synthetic instead of down. Synthetic weighs twice as much for the same warmth. But the shell and lining weighs about the 50% of total, so a down bag only weighs 75% of synthetic. Except I think synthetic loses half it's warmth after a few years so then a down bag weighs 62.5%Jan 27, 2014 at 6:52 pm #2067010
@ngatelLocale: Southern California
I doubt manufacturers would sell enough of them to make it viable.
The is a solution that has worked well for me in winter. A Cuben EE quilt inside of a Nunatak Arc Specialist.Jan 27, 2014 at 9:21 pm #2067048
@janosmLocale: phinney ridge
Relative humidity, as I understand it, is totally the point here. Relative humidity is a gradient ending at freezing (I think!). Air loses it's ability to hold moisture the colder it gets with an ultimate result of zero moisture in the air. My ignorance being fairly profound I don't remember if this absolute condition occurs at freezing (32 f) or some other point but I do know there is a trajectory and correlation with lowering air temperature and relative humidity. Anyhoo….a I see it our insulation layers are sensitive to a temperature gradient and dew point occurs as an algorithm of available moisture and temperature.Jan 27, 2014 at 9:40 pm #2067056
just Justin WhitsonMember
I agree with Paul about the efficiency of VBL clothes over bag/quilt liners or similar.
Ike, i wonder if this would be a case wherein lower quality down, especially if over stuffed, might work noticeably better than the super high quality and super downy stuff like you currently have? In the lower quality down, there are more feathers, and i would think that a higher ratio of feathers would help to keep up some loft under those overly moist conditions wherein the down tends to collapse and stick to each other.Jan 27, 2014 at 9:45 pm #2067059
just Justin WhitsonMember
Janos, it would be well under 32 degrees F i think. But i think you're generally correct in that the colder it gets, the less humidity tends to be in air. Ime when its below 20* F or so, tends to start feeling much dryer (but this inland and away from larger bodies of water).
Certain conditions, might strongly change that tendency though, like being near large bodies of water?Jan 28, 2014 at 3:31 am #2067091
AFAI can remember, the colder it gets, the lower the absolute humidity necessary to reach a certain RH (so also lower then freezing), but also less moisture is needed for condensation to occur e.g. at 32 °F air can contain max. 3,77 g. moisture/kg. air but at 0 °F it's only about 0,7 g.max..Jan 28, 2014 at 5:23 am #2067100
@ikeLocale: Central Michigan
"Ike, i wonder if this would be a case wherein lower quality down, especially if over stuffed, might work noticeably better than the super high quality and super downy stuff like you currently have?"
I've been thinking a lot about that lately. Never used to think about condensation at all with my trusty old WM puma, other than to shake the ice crystals off the shell each morning. In comparison, with the high performance 900 fill, I figuratively walk on eggshells trying to keep moisture out. VBL suit, synth overquilt, and every effort to avoid dampness near it. I still notice loss of loft the first night out.
For extended trips in extreme cold, I'll still take the old bag despite a nearly two lb difference. It is worth it just for simplicity's sakeJan 28, 2014 at 7:27 am #2067128
I don't think RH has much to do with it
Picture worth 1000 words:
The "22 F Air Temp" line shows the temperature gradient from skin (92 F) to ambient (22 F). This includes the boundary layer of air surrounding the sleeping bag that drops the temperature about 10 F. The point where the temperature reaches 32 F is right at the outside edge of sleeping bag.
The "0 F Air Temp" line shows it for 0 F outside air temp.
Water vapor is sweated from your body and goes through sleeping bag to the outside.
For the 0 F case, the water vapor reaches 32 F inside the sleeping bag. At that point it will freeze. Water (frozen) will accumulate inside the sleeping bag.
For the 22 F case, the water vapor reaches 32 F just outside sleeping bag, so it will exit sleeping bag, no water accumulation.
So, if the air temperature is above about 22 F, you don't have to worry about VBL.
For temperatures below 22 F, VBL starts being more useful to prevent water accumulation inside bag.
This is more theory based, as I avoid temperatures below 22 F. Starts being too difficult keeping drinking water from freezing. I suspect 99% of backpackers also avoid the really cold temperatures for many days.
On the Stephenson website, he talks about arctic expedition. Temperatures much below 22 F. Many days for water to accumulate. That's when a VBL is really useful.
For you 1% that go on arctic expeditions (or Minnesota right now) VBL is really useful, otherwise it's not.Jan 28, 2014 at 8:53 am #2067155
Your chart helps me understand why I usually get so little condensation inside my very small, all fabric inner tents. The air inside the inner tent stays warm enough to maintain its vapor state until it exits the inner tent and comes into contact with the fly…..at which point it is shocked into wetting itself.
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