55 forum posts
Sleeping bag liner
55 forum posts
6,037 forum posts
Neither. This is the situation where you need a vapor barrier liner. With fleece or silk, the vapor absorbed eventually gets transferred (due to your body heat) on outward through the insulation (down in your case). If the outer shell of your bag is microfiber, the vapor can get transferred on outward, though if the outside air is cold enough (in the tent, assuming you are sleeping in a tent), you will see frost and/or condensation on the outer shell. If you have a waterproof/breathable outer shell (Goretex or its more vapor-passing cousin Driloft), a fair amount will condense on the inside surface of the shell and get absorbed into the insulation. This is also one of those cases where the wicking of a good synthetic fill, like Primaloft, helps, since the moisture continues to get wicked outward.
A VBL keeps the moisture from getting into the insulation in the first place. It works for some people, but other people find it clammy. Wearing wicking long johns inside the VBL helps wick the moisture up and out of the face hole. Top quality winter and expedition bags use a completely waterproof inner shell around the face hole to prevent your breath from getting absorbed back into the insulation, but that means you have to make sure you keep your nose and mouth in the opening, not buried down into the interior of the bag (which a lot of people seem to do when cold).
Since you mention your bag absorbing moisture from your breath, try to observe how you are placing your nose and mouth at the face hole - is all your exhaled breath going outside, or are you breathing into the interior of the bag? Roughly speaking, you lose about a half liter of water from your body's perspiration over night, and a half-liter to a liter through your mouth and nose from your breathing overnight. So by making sure you breath out the face hole, you cut the vapor to be absorbed by half to 2/3.
141 forum posts
I stopped experimenting with vapor barrier liners mainly because when it's cold, the idea of stripping down to long underwear to get inside a sleeping bag is unappealing. Also, I almost never camp in winter for more than one consecutive night, which significantly reduces their potential advantage.
Or maybe I quit because of the (rather slight) discomfort from claminess. I never made a real decision about VBLs -- just abandoned them. Thus, I'd be very interested in any actual experience that others have with the things.
As far as my tentative impressions went, they seemed to work okay. One often reads that they must be used only in temperatures well below freezing. But two of the five nights I tried them (both duct-taped garbage bags and sewn jobs) were in lightweight bags with temps around 40-50 degrees, and I noticed little difference in claminess or discomfort, compared with the other nights, around zero. Indeed, it well be that my fears of dampening pile jacket and other layers are unfounded.
Apart from the warmth resulting from loft retention, one hears that they add to a sleeping bag's warmth by halting evaporative cooling. An additional warmth of 10 to 15 degrees from this factor sounds like a credible maximum to me, with most of it kicking in in colder weather.
So if they're so great, how come I don't use them? Dunno, exactly.
Individuals vary a great deal in the amount that they sweat though I would think the range of variablity while sleeping would be (much?) smaller than while exercising. Perhaps this variablity influences the effectiveness of vapor barrier liners. In any case, I sweat less than some people. Also, most likely, atmospheric conditions are a relevant factor.
6,037 forum posts
calamity's mention of atmospheric conditions reminded me of one thing I didn't mention - humidity. Waterproof-breathable outer shells, whether your parka or the shell of your sleeping bag (or the tent, for that matter) depend on a gradient of both temperature or humidity to work properly. Same is true of microfiber shells on your sleeping bag. High humidity, and you get condensation and/or frost in huge quantities. Low humidity, and the vapor just goes on out - faster with microfiber than wp/b. As long as you are in a tent, you are better off with a microfiber shell than with goretex, DriLoft, or even eVent. The good microfiber (like Pertex) will shed spills reasonably well, if you wipe them up fairly quickly, and the tent should keep the rain or snow out.
But high humidity and lack of ventilation will allow the tent to create rain and snowstorms inside (another reason not to cook in the tent, along with the carbon monoxide, oxygen depletion and fire hazard problems). One of the big problems we often have in the Sierra, where the temperature is just below freezing, is wet snow, high humidity during one of the typical maritime storms (Cascades has this, too, and the BC coast ranges), and the resulting lots of condensation in the tent, since all your gear is wet on the outside as you climb into the tent. Dragging all that water into the tent makes it hard to keep anything dry. Sometimes a bivy sack over the sleeping bag helps (except for the condensation inside the bivy). It's so much nicer at altitude in the Alaska Range or Rockies where you have dry air (and light, fluffy powder to ski in - note I said "ski in", not "ski on", none of this hard pack nonsense we usually get in the Sierra and you always see at resorts). It's cold enough that nothing gets wet, and properly ventilated tents don't develop any condensation or frost.
141 forum posts
The various alternatives mentioned in this regard indeed have various results, but according to several empirical studies, the variations are, in practical terms, not significant compared with much more simple measures aimed at protection from the elements.
An exception may be the vapor barrier liner, although the relevant theories from physics, and their implications for practical applications, are extremely complex, and somewhat divergent.
As for "humidity," this is patently a misnomer; the correct atmospheric measure would perhaps be dewpoint, although given my relatively "extreme" lack of experience, I'm not certain what, exactly, is being described.
I also note that with regard to at least one specific scenario mentioned, there is a failure to adequately suggest the simple expedient of a sleeping-bag cover of typical design.
Also, perhaps remarks about Sierra Nevada ski conditions have "little bearing" on the subject at hand, though I'm very pleased that views expressed on this topic don't excite additional comment.
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