Bivvy Bag increasing warmth of a sleeping bag?

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Bivvy Bag increasing warmth of a sleeping bag?

Postby tigercat » Sat 09 Jun, 2012 7:17 am

I think a goretex bivvy bag can increase the warmth of a down sleeping bag by about 2 degrees. What is your experience?
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby Moondog55 » Sat 09 Jun, 2012 9:44 am

Depends on circumstances and the construction fabric in the sleeping bag. My bag uses a very light ripstop and I would say the difference was closer to +5 / +8 degrees if a breeze is blowing
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby nq111 » Sat 09 Jun, 2012 10:20 am

Goretex is a partial vapour barrier and good wind shell so the theory fits.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby Franco » Sat 09 Jun, 2012 10:42 am

I would expect a bit more than that but you can also end up with a wet bag...
(in the open it will almost eliminate the wind chill factor and that can be 10 or more degrees)
It really depends how and why you would want to use one.
Originally a bivvy was meant as an emergency stand alone shelter for climbers.
So the focus was on survival not comfort.
If you are thinking of getting one to boost your bag temp,inside a tent , (in my opinion...) you are much better off wearing some puffy clothing or if you don't like the feel of that or your bag is too small for it (keep in mind that down still works pretty much the same compressed to half of its loft...) get a light quilt .
A good solution could be a synthetic one so that it can deal with the moisture coming from you and keep the down in your SB dry.
Light synthetic quilts are available from less than 600g. (you can drape that over you at camp)
On the other hand, you could also look at a light eVent bivvy. Many think that it works better than GoreTex (I do, my walking mate has one...) and you can get them from around 300g.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby wayno » Sat 09 Jun, 2012 11:58 am

an article here reckons bivy bags can create a lot of extra warmth because the air you inhale tends to be prewarmed...

http://www.mountainz.co.nz/content/article/article.php?article=120405_bivouac.php&direct=nz
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby Franco » Sat 09 Jun, 2012 12:51 pm

I haven't read the article (I will...) but yes that makes a lot of sense.
That is the idea behind the face masks of the Psolar type
http://www.exmask.com/index.php
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby creeping_moses » Sat 09 Jun, 2012 1:16 pm

Bivy bags are mad good. I've got a 800 fill-down sleeping bag (mountain hardware wraith SL) that I have used on its lonesome outside at about 0 degree (wearing thermals and beanie) and have been perfectly happy in it, including a windy night on the Snowy Range just below the summit of Snowy South. When I use my bivy (Outdoor Research Alpine) with the bag, I'm normally a sweaty dog. The Bivy bag is only needed in my case when there are medium rainfalls, or anything heavier obviously, and its useful on windy nights to keep your face warm. I think what I am trying to say is that from my experience with bivies, I only use them when I have to, and if you dont want to buy multiple sleeping bags, just get one that is rated to very low temps, and use it to its full potential. So it follows from that, that I walk with my wraith SL bag, which is fairly bulky but I squeeze it into a sea2summit medium event stuffsack which gets it down to a very reasonable size and then carry my bivy which is light and small at all times incase its raining/windy/amazingly cold enough to warrant it. Its even possible to use the bivy without a mat in the case of my sleeping bag if I want to be lighter, but with a neoair, its hardly a burden.

I really want to try the wraith SL on a neoair mat in the snow. People have done it before and written/posted videos of it online and have said its fine, but would be interesting.

Also, I'd stay away from bivy bags that dont have a small loop pole to keep the shell off your face. My mate has one (a macpac one, which is very lightweight and nicely built from eVent) but it literally rests directly on your face, which he has assured me is most unpleasant when its pouring rain. That being said, if you want to use it in a tent, or hut (which seems a little weird to me) the fabric resting on your face might not be so bad.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby Strider » Sat 09 Jun, 2012 1:58 pm

creeping_moses wrote:I really want to try the wraith SL on a neoair mat in the snow. People have done it before and written/posted videos of it online and have said its fine, but would be interesting.

I was cold on a Neoair on my kitchen tiles. Good luck!
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby creeping_moses » Sat 09 Jun, 2012 2:28 pm

Strider wrote:I was cold on a Neoair on my kitchen tiles. Good luck!


I've never had an issue with mine in terms of warmth. Could be the sleeping bag compensating though. One thing I do have an issue with is the fact I'm always paranoid of piercing the neoair. Seems like it is really easy to get some micropunctures.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby hikingoz » Sat 09 Jun, 2012 2:58 pm

I've got a very lightweight terra nova moonlight sleeping bag cover which I find boosts my temperature when in a tent or hut or under a tarp. It is not weather proof enough for stand alone use. In the wind it can flap about, forcing hot air out of my sleeping bag.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby creeping_moses » Sat 09 Jun, 2012 3:07 pm

That terra nova cover looks perfect for hut/tent usage.
I sort of see using a legit bivy bag the same as setting up a tent in a hut...
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby hikingoz » Mon 11 Jun, 2012 9:28 am

creeping_moses wrote:That terra nova cover looks perfect for hut/tent usage.
I sort of see using a legit bivy bag the same as setting up a tent in a hut...


It has vert thin material but is very light. You are quite right, perfect for tnets and huts.

I've used it under a tarp but it is prone to damage from sticks and debris so must be used with a groundsheet.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby bushwalker zane » Mon 11 Jun, 2012 10:04 am

I have a mountain hardware bivvy. I use my old decrepit sleeping bag with it and I get nice and warm! I'd say an extra 5 degrees at least with that one. I usually find myself getting a bit too warm in it actually, so I usually sleep with everything open. But I am a warm sleeper, so no doubt that contributes to the fact.
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Bivvy Bag increasing warmth of a sleeping bag?

Postby Son of a Beach » Mon 11 Jun, 2012 11:42 am

How much warmth for weight do you get for the bivvy? How does that compare to the extra weight of a warmer sleeping bag? I'm not planning to get a bivvy or a warmer bag any time soon, but I'm just curious.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby bushwalker zane » Mon 11 Jun, 2012 3:41 pm

I use a silk liner as well, and I'd say they add about the same amount (for me that is about 5 degrees feeling warmer with each) So when you combine all 3, sleep under a tarp/poncho and you'll be nice and cosy in any winter expedition! My bivy weighs like, 200gms. It's a light weight one, but it does the trick. And it's waterproof of course.

I only use it sometimes, and only when guiding (so I don't need to bring a tent) but if I was a lightweight freak I'd use one all the time. I do like my tent on those personal trips :wink:
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby hikingoz » Mon 11 Jun, 2012 5:39 pm

Son of a Beach wrote:How much warmth for weight do you get for the bivvy? How does that compare to the extra weight of a warmer sleeping bag? I'm not planning to get a bivvy or a warmer bag any time soon, but I'm just curious.


Not much....it's advantage is in overall functionality, not temperatue alone IMO.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby tigercat » Mon 11 Jun, 2012 8:17 pm

Thanks for all the feedback. i think I'll go for the terra nova light bivvy bag.
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Re: Bivvy Bag increasing warmth of a sleeping bag?

Postby Nuts » Tue 12 Jun, 2012 7:21 am

Sleeping bag covers seem to have become more popular with open shelters, tarp camping, not-quite-waterproof fly fabric.. bit of an odd compromise but then 'make them they will come' :wink:
Ive had a few variations of bivy/U/L bivy/bag covers, s2s thermolite, MLD superlite, epic,eVent,g'tex so on.. At the end of the day, the proper waterproof bivy is still the best choice. Not much heavier and provides piece of mind in any tent (ie no stress in a howling gale). Also, being w'proof they make an acceptable packliner. As for warmth, it adds to cutting wind and slowing airflow. Probably on par with eg s2s thermolite overall (warmthwise) but much better if you have a 'meshy' tent. A tent chosen to compliment a bivy can be lighter, in a pinch you don't Really need the tent (ie if you cant find a tent spot, just lie down..).
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