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HitTheTrail
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PostTue Dec 22, 2020 1:09 pm 
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Have you ever had any of the following issues?:
• Tired of rolling off your 20” wide Xtherm inflatable pad at night.
• Or, tired of buying heavy, overrated Big Agnes sleeping bags just for the pad sleeve.
• You want the flexibility of adding more bag insulation without adding a lot of weight.
• Tired of your bag getting wet at night from rubbing against a condensing tent wall.
• Tired of your bag getting wet from the inside from body heat.

Well, the multi VBL option tends to resolve all of them as a single 6.6 oz solution.

What you do is get two Western Mountaineering HotSac VBLs. Put one inside your sleeping bag as recommended to add warmth and keep the bag dry. Then put the other VBL over the outside of the bag and put your inflatable pad under you inside that VBL. This will hold your pad in place and keep the bag dry when it touches the wet tent wall. This works especially well with single wall tents. But even with a double wall tent it will add more warmth and keep the pad in place.
A modified and cheaper version of this concept is to get just one VBL for the inside and put your sleeping bag and pad inside your waterproof dyneema backpack. But, always have a VBL on the inside if you use one on the outside or your bag will get wet from trapped perspiration.

Western Mountaineering’s website says the HotSac is 4.5 oz but my scale says it is 3.3 oz. Probably because they went to a slightly different stuff sack and never updated the website.

I have used this configuration a couple of times and it is not perfect but tends to solve most of the above issues. And, you could probably use the set-up as an emergency bivy without any tent at all if you had to. Any thoughts?

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texasbb
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PostTue Dec 22, 2020 1:21 pm 
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I think I'd be wary of putting a vapor barrier on both sides of anything.  Maybe at the temperatures where an inside VBL is warranted it won't matter, but moisture traps worry me.
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HitTheTrail
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PostTue Dec 22, 2020 1:33 pm 
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texasbb wrote:
I think I'd be wary of putting a vapor barrier on both sides of anything.

The first time I tried I had the outside VBL pulled up high and there was a little moisture at the bottom of my bag. But after that I just had the outside VBL pulled about a quarter way up and it held the pad in place and kept moisture off the end of my bag that was touching the tent wall. And I did say it is not a prefect solution, just partial. winksmile.gif
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Randito
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PostTue Dec 22, 2020 2:32 pm 
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I used a VBL inside my sleeping bag during a multiday winter trip where the lows were around 0F overnight.   Made a significant difference in warmth, especially since my sleeping bag was only rated to 20F -- I wasn't toasty, but I wasn't shivering.  I wore some of my clothes inside the VBL,  including my ski boots.  I felt a little damp emerging from the bag in the morning,  but this disapated quickly in the dry Wyoming air.  I brought two complete sets of socks and swapped them each day,  drying socks on the outside of the VBL, but inside the sleeping bag during the night.  My companions that were putting on frozen leather ski boots in the morning were grumbling more.   During the course of the trip, my down bag stayed dry and fluffy,  while my companion's bags grew damper day by day,  even those with synthetic bags.

If the inner VBL is 100% vapor proof then an outer VBL won't have any vapor to catch and condense on the bag.

If you are going out for a single night in the moderate temps in the Cascades, the advanatage of a VBL I think is much less.
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Eric Hansen
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PostSun Dec 27, 2020 8:47 pm 
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If you are asking about using the vbl alone (without sleeping bag) as an emergency bevy I have this somewhat parallel experience. I tested one of those aluminum emergency sleeping bags. In temperatures in the mid 40's I got 5 hours sleep before waking up a tad chilled and damp. I had on a pile sweater, heavyweight bottoms, pile balaclava. And a foam pad underneath.
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Randito
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PostSun Dec 27, 2020 9:02 pm 
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Eric Hansen wrote:
If you are asking about using the vbl alone (without sleeping bag) as an emergency bevy I have this somewhat parallel experience. I tested one of those aluminum emergency sleeping bags. In temperatures in the mid 40's I got 5 hours sleep before waking up a tad chilled and damp. I had on a pile sweater, heavyweight bottoms, pile balaclava. And a foam pad underneath.

VBLs work best when the inner VBL is as close to the skin as possible.  any insulating layers between the VBL and the skin will get damp.   If there are two VBLs -- the insulating layers between are protected from moisture from within and without.

Good info on practical usage of VBLs here

https://andrewskurka.com/vapor-barrier-liners-theory-application/
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HitTheTrail
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PostMon Dec 28, 2020 7:18 am 
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I skied with my daughter and son-in-law over the holidays and gave them each a WM HotSac VBL as Christmas presents. Then my son-in-law showed me his SOL Escape LIte Bivy that he always carries on his extreme trail runs and has actually had to deploy a few times.

SOL advertises that as being useful both as a VBL inside your bag and as a protective cover outside the bag. I read a lot of reviews and most said it is reasonably breathable and is water resistant so it can be used in both situations. The main complaint was that they are hard to struggle in and out of and that it is not quite wide enough to put both your bag and pad inside at the same time. Since your bag usually rubs the tent wall at the end and the SOL is much wider at the top, you could just put it over the bottom half of your bag. That would add foot warmth as well as hold the pad in place and keep the end of your bag dry. And, you could use the HotSac inside as a VBL if it is really cold out. I may try that.

The only question I have is why a survival equipment company would call themselves SOL? Didn’t they know that acronym has always meant all hope is lost and that you are screwed?
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texasbb
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PostMon Dec 28, 2020 9:17 am 
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HitTheTrail wrote:
...SOL Escape LIte Bivy that he always carries on his extreme trail runs and has actually had to deploy a few times.

SOL advertises that as being useful both as a VBL inside your bag and as a protective cover outside the bag.

SOL advertises this bivy as "breathable," which is not consistent with "vapor barrier."  Of course, there are no standard metrics for breathability in outdoor gear, so who knows what it really is.  The reality is there are some fabrics that would best be described as vapor retarders, but I'm not aware of any study/analysis of the efficacy of merely limiting vapor diffusion rather than stopping it.
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moonspots
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PostMon Dec 28, 2020 8:42 pm 
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HitTheTrail wrote:
The only question I have is why a survival equipment company would call themselves SOL? Didn’t they know that acronym has always meant all hope is lost and that you are screwed?

lol.gif

I hope I'm never that far outta luck!

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"Out, OUT you demons of Stupidity"! - St Dogbert, patron Saint of Technology
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