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Waterproof inside layer for winter bag
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Hiking Malto
(gg-man) - F
Waterproof inside layer for winter bag on 01/25/2014 19:59:33 MST Print View

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?

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Waterproof inside layer for winter bag on 01/25/2014 22:47:16 MST Print View

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.

Daryl and Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Re: Waterproof inside layer for winter bag on 01/26/2014 08:27:18 MST Print View

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.

Woubeir (from Europe)
(Woubeir) - F - MLife
Re: Re: Waterproof inside layer for winter bag on 01/26/2014 08:47:18 MST Print View

"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 ?

Daryl and Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Re: Re: Re: Waterproof inside layer for winter bag on 01/26/2014 09:13:35 MST Print View

Someone,

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.

rOg w
(rOg_w) - F

Locale: rogwilmers.wordpress
deleted on 01/26/2014 09:25:54 MST Print View

deleted

Edited by rOg_w on 02/05/2014 20:49:44 MST.

Woubeir (from Europe)
(Woubeir) - F - MLife
Re: Re: Re: Re: Waterproof inside layer for winter bag on 01/26/2014 09:27:31 MST Print View

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.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Waterproof inside layer for winter bag on 01/26/2014 10:13:41 MST Print View

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?

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Sleeping bag weight data on 01/26/2014 10:27:40 MST Print View

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.

Woubeir (from Europe)
(Woubeir) - F - MLife
Re: Re: Waterproof inside layer for winter bag on 01/26/2014 10:28:34 MST Print View

OK Jerry, but I assume that also the moisture level has somwthing to say ?

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: Waterproof inside layer for winter bag on 01/26/2014 10:32:03 MST Print View

"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.

Woubeir (from Europe)
(Woubeir) - F - MLife
Re: Re: Re: Re: Waterproof inside layer for winter bag on 01/26/2014 11:11:20 MST Print View

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).

Paul McLaughlin
(paul) - MLife
Re: Re: Re: Re: Re: Waterproof inside layer for winter bag on 01/26/2014 13:31:37 MST Print View

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.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Waterproof inside layer for winter bag on 01/26/2014 13:41:11 MST Print View

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.

Daryl and Daryl
(lyrad1) - MLife

Locale: Pacific Northwest, USA, Earth
Re: Sleeping bag weight data on 01/26/2014 14:27:35 MST Print View

Jerry,

Just curious.

Were you able to recall those sleeping bag weight numbers from memory alone or did you have notes to which you referred.

Ike Jutkowitz
(Ike) - M

Locale: Central Michigan
VBL bag on 01/26/2014 15:27:33 MST Print View

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.

Vbl

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Sleeping bag weight data on 01/26/2014 16:40:08 MST Print View

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%

Nick Gatel
(ngatel) - MLife

Locale: Southern California
Re: Waterproof inside layer for winter bag on 01/27/2014 19:52:08 MST Print View

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.

janos mathiesen
(janosm) - M

Locale: phinney ridge
@jerry on 01/27/2014 22:21:36 MST Print View

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.

just Justin Whitson
(ArcturusBear)
Re: Re: Re: Re: Re: Re: Waterproof inside layer for winter bag on 01/27/2014 22:40:45 MST Print View

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.