Bivies and insulation ability

[David Ure]

I was wondering as to what might be the insulating effects of a bivy / sleeping bag combination insofar as the bivy adding a number of warmth degrees to the sleeping bag. For example adding 5 or 10 degrees ‘effective loft’ to a 30 degree sleeping bag (thus now a 25 or 20 degree experience). Hopefully I made myself relatively clear….

[cushing hamlen]

I believe that most manufacturers typically quote about a ten degree boost due to bivy usage around a sleeping bag.

[Paul Ettestad]

I’ve been thinking about this issue also. I have a WM Caribou, a great summer bag that’s rated down to 35 F. I’m going to be up in the Colorado mountains in the middle of September for a week camping around 12,000 ft for a few nights where the temperature may get down to around 25 F or lower. Instead of buying another bag such as the WM Alpinlite I was thinking of combining my Caribou with a bivy and also a silk liner. Does anyone have any comments on whether that will work?

[David Ure]

Paul,

exactly my predicament. I know that Mountain Hardwear sells a down or synthetic ‘upgrade’ that adds a couple of pounds and about 25 degrees. However, I am buying a bivy in two weeks and if it adds sufficient warmth I might be okay (considering I am a relatively hot sleeper already). I would like to hear from some bivy owners out there….i.e. Paul J?

[paul johnson]

i use bivies as, what i would i would term a primary shelter, meaning that i normally don’t pitch a tarp over one.

ID eVENT Unishelter+WM HighLite+baseLayer+midLayer+OuterInsulatingLayer+a ClosedCellFoamPad=25deg for me (i consider myself a cold sleeper). oh…also heavy weight balaclava & 2pr of heavy socks.

i would agree with what i’ve read elsewhere that the WM Highlite is conservatively rated by WM as a +40deg bag, but is really a +35 deg.

so then, the bivy adds (subtracts???) 5deg & my insulating clothing another 5deg enabling a +25deg system. if you’re a warm sleeper, perhaps you can go a tad lower??? your “mileage” may vary.

[Ryan Jordan]

Paul, love your shelter system. “But it could be lighter!”

Yeah, yeah, I hear (say) that all the time.

But the fact remains you have a pretty killer setup for foul conditions at a very good weight.

I have an eVENT Unishelter and I shall now come out of the closet. “I use this 2 pound bivy sack”. Whoo. Feeling better already.

In the winter, all cozied up in that sucker, with Armageddon swirling around you in a vicious storm, something just “feels good” about being inside it, all toasted up in about 8 inches of down, reading a nice book with a Photon light.

The performance, simplicity and weather protection of this system, especially above treeline, is well worth the weight, much of the time.

OK. I’ve shared too many of my feelings. I need to recover this post now.

You claim 5 deg increase in temp rating of your system using the eVENT Uni. I hear those numbers thrown around a lot, and have a hard time substantiating them. Passively, zero wind, you have two issues fighting each other: the accumulation of moisture in the system, which robs body heat to maintain the temperature, or change the phase state of that moisture, and the added insulation provided by the fabric and the air pocket between the fabric and your sleeping bag.

Where a bivy shines: wind.

No question, a bivy can have a huge impact on how warm you sleep in the wind. And I’m not talking about 30 mph gales – even a 3 mph wind in cold temperatures can cripple a light sleeping bag. The bivy has a big impact on sleeping warm in windy conditions because that insulating air layer that it traps – remains relatively still – and thus, warm.

Whew. That was close. Gotta watch those warm and fuzzies. Back to cutting the tags off my Uni…

[Michael Martin]

>> Passively, zero wind, you have two issues fighting each other: the accumulation of moisture in the system, which robs body heat to maintain the temperature, or change the phase state of that moisture, and the added insulation provided by the fabric and the air pocket between the fabric and your sleeping bag.
<< One other factor to add to Ryan’s excellent post…. What about the loft compression of the bag due to the bivy resting on it? A mere 3-4mm of compression would reduce the overall warmth by 5 degrees (F) or so.
[Comments?]

[paul johnson]

David,

There you have it. Dr. J has spoken (for which I thank him for correcting me). My 5deg for my bivy is just something you often hear – i “parroted” it not knowing any better. My apologies for misinforming you. Hadn’t realized (i’m fairly inexperienced in the UL arena) that this is perhaps just “hype” that I’ve read elsewhere. Perhaps, even being a cold sleeper, the reason i’ve been able to go down to +25 is NOT b/c of the bivy, but my other insulation? [what else could it be?!!] The +25 for the system works for me (forgot to explicitly mention the heavy wt. long johns – it’s part of my winter base layer), but, as Dr. J pointed out, apparently NOT for the reason i stated.

(BTW, I cut the tags off within 20min of rcving it. I’d really like to get mine shortened to ~70″, but was afraid to try it myself for fear of ruining it. Anyone have any suggestions as to how to get this done professionally and still maintain all of the integrity/weatherproofness of the bivy?)

[Mark Verber]

As other people have noted… the bivy is a big help when there is any sort of wind or draft. There isn’t a big advantage if you are using a tent which seals well, but a bivy can be a huge help if you are tarping and the wind is blowing.

I have used silk liners. I have not found them to be that helpful in colder weather. In very hot weather they can be nice instead of a sleeping bag. I have found that a super warm hat and a warm part of gloves / socks is a much more efficent way to extend my sleeping comfort. I easily get another 10-15F with wearing a high loft insulating hat and warm gloves.

You should also remember to eat enough. In places that I don’t have to worry about bears I like keeping a snack to eat in the middle of the night.

–Mark

[Richard Nelridge]

David,

To extend the lower temperature performance of your sleeping bag and lessen the build up of moisture from perspiration, consider using vapor barrier clothing in conditions below 32 degrees F. You can use a Vapor Barrier Shirt, Vapor Barrier Pants, Vapor Barrier Socks, and Vapor Barrier Gloves. Particularly consider the Shirt, Pants, and socks. The Shirt and Pants will take the place of much of your thermals (or other clothing when sleeping); you would have to experiment regarding your metabolism to find what works and how much clothing you need at different temperatures. With the Vapor Barrier clothing you keep the moisture next to your skin. Thereby you can can wear other clothing outside the Vapor Barrier Clothing, but inside the sleeping bag without fear of wetting out your clothing or your sleeping bag insulation. In the process you can carry a lighter sleeping bag because the vapor Barrier Clothing will give the equivalent to an additional 10 to 15 degrees F of insulation. My Stephenson’s Warmlite (www.warmlite.com) fuzzy lined Vapor Barrier Top and Pants together weigh about 9.8 oz. The fuzzy lining of the Warmlite products make them more comfortable than many Vapor Barrier materials because it absorbs perspiration. As I recall the Warmlite Vapor Barrier Top, Pants, Gloves, and Socks cost $97 including shipping.

Rich

[David Ure]

I would think that the fact that the ‘space’ between the bag and the bivy would be minimal, as compared to a tent, there would be less space to ‘heat up’ thus helping to create a several degree gain in temperature rating. In combination with the abilty to provide wind protection, as discussed by Ryan, I figure that a 10 degree benefit would not be out of the question.

I appreciate all of your input.

Thanks for the suggestion on the sleepwear Richard.

[David Olsen]

One trip stands out in my mind. We were sleeping under the stars in Eastern Oregon’s Hart Mountain Refuge one winter in strong winds. We had good pads and synthetic bags that were rated to -20f
(they actually worked to about -5). The temp was
above freezing but with the wind we were very cold
and couldn’t sleep even with winter clothes added. Just crawling into a bivy made all the difference, ( a
tent would have worked just as well). Being able to
seal out drafts around the head with a bivy is especially helpful. It also keeps your pad from blowing away if you sleep with it inside your bivy.

[Author]

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