Forum Index » Editor's Roundtable » Staying Dry in a Bivy Sack: Techniques for Managing the Moisture Balance Between External Precipitation and Internal Condensation


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Maia
(maia) - MLife

Locale: Rocky Mountains
Staying Dry in a Bivy Sack: Techniques for Managing the Moisture Balance Between External Precipitation and Internal Condensation on 09/18/2012 18:54:45 MDT Print View

Companion forum thread to:

Staying Dry in a Bivy Sack: Techniques for Managing the Moisture Balance Between External Precipitation and Internal Condensation

Ken Thompson
(kthompson) - MLife
Re: Staying Dry in a Bivy Sack: Techniques for Managing the Moisture Balance Between External Precipitation and Internal Condensation on 09/18/2012 22:08:48 MDT Print View

Still a good article. Lot's of talk about bivys as of late so good timing.

Mary D
(hikinggranny) - MLife

Locale: Gateway to Columbia River Gorge
Staying Dry in a Bivy Sack on 09/18/2012 22:23:22 MDT Print View

I agree--good article, quite timely considering the current forum threads on bivies and condensation issues.

David Olsen
(oware)

Locale: Steptoe Butte
bivy pluses on 09/19/2012 09:39:23 MDT Print View

Here is something I here little discussion about.

"One of the advantages of using a bivy sack is that much of the condensation which would have otherwise occurred inside your sleeping bag or quilt instead will occur on the outer shell of the sleeping bag, or on one or both surfaces of the bivy sack’s upper shell fabric. This allows for the isolation of your sleeping bag from the condensed moisture. Before packing your sleeping bag and bivy sack, shake off any external condensation or wipe it off with an absorbent cloth. To prevent any moisture remaining in the bivy sack from wicking back into bag’s insulation, be sure to pack the bag and bivy sack separately."

Ron Jameson
(tigerpaws) - M

Locale: Upstate SC
Bivy on 09/19/2012 09:58:49 MDT Print View

Used my OWARE XL TALL bivy up at Shining Rock W/GG Sinn Twinn had rain worked great. Such a simple set up. I'm 6'5" 240. Only neg harder to get out of than my hammock. The set up Is a lot lighter. Ron

HK Newman
(hknewman) - MLife

Locale: Western US
Re: Staying Dry in a Bivy Sack: Techniques for Managing the Moisture Balance .. on 09/19/2012 13:00:09 MDT Print View

Timely article. Looking to reduce shelter and sleeping bag weights (have a WM currently); don't know if a bivy will factor into that or merely using a water-resistant shell for a sleeping quilt.

William Ashley Hold
(ahold)

Locale: Cornwall
Bivy sack and vapour barrier on 09/19/2012 14:55:16 MDT Print View

I welcomed the mention of using a vapour barrier in a bivi to reduce the transmission of vapour into the sleep system. This is something I have started to experiment with. However my preliminary experience suggests that vapour barrier can be used in relatively moderate temperatures - close to freezing or a little above - and not just in very cold conditions.

Mike Oxford
(moxford) - MLife

Locale: Silicon Valley, CA
VBL? on 09/19/2012 16:20:34 MDT Print View

Interesting that there was no coverage of VBLs. Or perhaps I'm missing the point of VBLs.

Michael Martin
(MikeMartin) - BPL Staff - MLife

Locale: North Idaho
Re: Bivy sack and vapour barrier on 09/19/2012 20:42:26 MDT Print View

I'm a big fan of VB's and have used them extensively for Winter camping. In my avatar photo, I'm about 4 days into wearing a VB shell made by RBH designs for 11 days straight, 24 hours a day, during a Yellowstone ski trek.

I briefly mentioned VB's in this bivy article, because they do dovetail nicely into moisture management. But, I wanted to keep the article to a manageable size so I didn't go into any detail here. If there is enough reader interest, I'll write something else on VB techniques (though I believe Andy Skurka has already covered the topic for BPL and would be a tough act to follow).

Cheers,

-Mike

Edited by MikeMartin on 09/20/2012 10:21:57 MDT.

Steven Paris
(saparisor) - M

Locale: Pacific Northwest
Re: Bivy sack and vapour barrier on 09/19/2012 22:45:21 MDT Print View

Mike,

Thanks for the nice article.

A general comment: No one here likes to carry redundant gear, but an additional benefit of the 5-7 oz breathable bivy is that it doesn't weigh SO much or take up TOO much pack space that it has to be left at home, even if never used. It can be a tool, used under a tarp one night and left in the pack the next. I carry mine less and less, instead moving towards inner net-tents for the more livable space but it wouldn't ruin a trip if I had it with me.

A question about VBLs: is there a benefit to wearing a VBL on one part of the body only? I'm guessing that the human body doesn't give off condensation equally. Can wearing VBL socks reduce condensation enough to matter, rather than a full-body VBL? Coupled with an opening for the mouth & nose, like in your photo above? I hope that makes sense, and sorry for the slight thread drift but it does seem to go hand-in-hand with bivies.

Michael Martin
(MikeMartin) - BPL Staff - MLife

Locale: North Idaho
Re: Re: Bivy sack and vapour barrier on 09/20/2012 00:22:11 MDT Print View

Steven wrote:

"A question about VBLs: is there a benefit to wearing a VBL on one part of the body only? I'm guessing that the human body doesn't give off condensation equally. Can wearing VBL socks reduce condensation enough to matter, rather than a full-body VBL? Coupled with an opening for the mouth & nose, like in your photo above?"

What you're describing is a "partial vapor barrier", and they do work very well at reducing moisture accumulation in a sleep system. Anything that slows the moisture input to your sleep system is worthwhile. E.g., I've had good luck with VB jackets and conventional pants, as my legs generate very little moisture while I'm sleeping.

I've had mixed results with VB socks -- they are Ok for sleeping, but my feet sweat too much to use them while moving unless it is extremely cold and I move very slowly.

I've also had very good luck using my wp/b raingear over my baselayer as a partial VB. Wp/b membranes rely on a vapor pressure gradient to achieve breathability. The vapor pressure gradient across the membrane is small when a wp/b layer is worn close to the skin under other layers that keep the inside and outside surfaces at close to the same temperature. So raingear can effectively serve as a partial VB at no additional weight if you carry it anyway. :-)

Cheers,

-Mike

Edited by MikeMartin on 09/20/2012 10:07:04 MDT.

jeffrey armbruster
(book) - M

Locale: Northern California
"Staying Dry in a Bivy Sack: on 09/20/2012 10:24:56 MDT Print View

Michael wrote: "I've also had good luck using a wpb rain jacket as a partial vapor barrier..." That's excellent info and makes a lot of sense.

Edited by book on 09/21/2012 16:57:29 MDT.

Ron Bell
(mountainlaureldesigns) - F - M

Locale: USA
Re: "Staying Dry in a Bivy Sack on 09/21/2012 17:14:44 MDT Print View

Nice Update Article!

Very useful and not overly technical - an easy quick read to get a LOT of solid info.

Clarifies general use situations and debunks many of the DWR bivy myths that so frequently circulate.

I think there may be many BPL basic how to and why articles from the early part of the UL/SUL movement that could be easily updated and presented in the same way.

Given the finite resource for creating new content for any publication today - I would rather see 25 of these articles vs one long in depth STOM or similar long term academic research project.

I think this type article helps to generate new interest in UL and transition new UL converts to the next step. If we can get them there - then there are lots of archive and other info sources available if they want to take it to the next level.

There is a very good reason you see a mix of the same type of beginner/intermediate articles in any sport magazine updated and published every year - to recruit new converts to the sport, add new subscribers and create future mentors and ambassadors.

Michael Martin
(MikeMartin) - BPL Staff - MLife

Locale: North Idaho
Re: Re: "Staying Dry in a Bivy Sack on 09/21/2012 17:27:45 MDT Print View

Thanks for the kind words, Ron. :-)

-Mike

Doug Johnson
(djohnson) - MLife

Locale: Washington State
Re: Re: Re: "Staying Dry in a Bivy Sack on 09/22/2012 15:03:13 MDT Print View

Great article Mike- I really enjoyed it. The pic of me is from a marvelous trip from Stevens to Snoqualmie Passes on the PCT in Washington where the kit worked perfectly. Thank you!

Steven McAllister
(brooklynkayak) - MLife

Locale: Atlantic North East
Great Article on 09/23/2012 12:52:38 MDT Print View

A great article and one that will come in handy for many people.
How many backpackers do you know that don't have the slightest idea about condensation when it comes to backpacking.

This is the article for them.

Anthony Weston
(anthonyweston) - MLife

Locale: Southern CA
Montbell Dri Tec bivy on 09/26/2012 17:50:22 MDT Print View

The best bivy I have found is the Montbell Dri Tec bivy at 8 oz. It has taped seams so it keeps moisture from the outside out. ( I put it in the bathtub weighed it down in 4 inches of water with dry paper towels inside and 4 hours later the paper towels were still dry.) so I'm good for floods.
I use it with a tarp. The dri Tec material rarely gives me condensation problems but after using it for a few years, I purchased a waterproof zipper and had the local dry cleaners sew it in for me, high up on the side. Now I have no condensation. I sleep 90% of the time with it unzipped but if the wind and rain picks up or a blizzard. I'm ok.
I slept thru this blizzard in 2009 with my GG Spinshelter and Montbell Bivy, woke up with drifts all round me, dry and warm, very windy, snow coming horizontal in the night.xh

Jim Sweeney
(swimjay) - MLife

Locale: Northern California
water vapor source percentages, heat exchanger on 10/03/2012 13:28:00 MDT Print View

Mike, thanks for an excellent article. Though I'm sure there would be enormous variation, it would be interesting to learn what the relative percentages of water vapor emitted by the breath and by the skin are during sleep. Suspect that breathing supplies much of the water vapor, and wonder if it wouldn't be possible to have some kind of heat-exchanger which would allow one to breath in an otherwise closed bivy sack directly to the outer air. I use a MLD eVent bivy, and often have it quite closed up to help with heat. (Just zipping up the mosquito netting, with the main fabric of the bivy unzipped near the breathing area, can add a lot of warmth).

The info, and explanation, about choosing a site with a minimal view of clear sky is invaluable. Have noticed this phenomenon when comparing warmth in a grove vs in an open field, but always thought it had something to do with the stiller air in the grove and the warmth emitted by trees (which it may, somewhat, but I'm sure the sky view is more important).