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How much does a bivy augment a sleeping bag?
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Max Dilthey
(mdilthey) - M

How much does a bivy augment a sleeping bag? on 09/02/2013 21:39:14 MDT Print View

In response to the tarp thread I've got going, but a very different question:

Looks like the average low in Washington is 41 in November. Knowing myself, I would sleep 6 hours in that using a wool baselayer and my 45º Mountain Hardwear sleeping bag.

Now, how much does a bivy sack increase my temperature rating? I found a great deal with a free return on The North Face Simple Bivy, and it's fine at 1lb to work with my bike and an MLD hammock tarp to keep me dry. I love cottage manufacturers, but I need something FAST and I would like to return it if it's not working right.

Can I safely bring a 45º summer bag to Oregon and Washington and maybe Colorado and Northern California, in weird places like coasts and cliffs and mountains, if I've got the bivy sack, my MLD Hammock tarp and a good pad and I'm an easy sleeper?


P.S. I know this bivy leaks like a sieve in rain. I just need something for spray and mist and dew.

Dave -
(FamilyGuy) - F

Locale: Up there
Re: How much does a bivy augment a sleeping bag? on 09/02/2013 22:04:53 MDT Print View

Generally speaking, a bivy sack will increase the warmth of your sleep system by about 7C max., which is consistent with my own findings. However, the benefit to warmth comes from its ability to stave off convective heat loss from wind so in very windy and cool conditions, the 7C is more plausible than say, when it is windless and just simply cold.

Michael Driscoll
(Hillhikerz) - F

Locale: Monterey Bay
bivy augment a sleeping bag on 09/02/2013 22:37:41 MDT Print View

I would agree with the above... I have a 45 deg. down bag... plus a Nemo GoGo... and when it gets cold a sea to summit Reactor Extreme Thermolite Sleeping Bag Liner; it says 25 deg. and I feel it is more like 15 to 18 deg... so lets do the math 45 - 7 - 15 = 23 deg... Well I have not tested it to that point but have to 30 deg. and was good to go; but my feet were cold so I opened a chemical toe warmer and put it in a sock down by my feet and slept like a baby the rest of the night... YMMV

F. R.
(fugitiveride) - F

Locale: Syldavia
bivy on 09/02/2013 22:53:41 MDT Print View

I know this is not a scientific answer, but you are just going to have to try it out and see for yourself. One person may feel warm when another person does not despite having the same gear.

eric chan
(bearbreeder) - F
bivy on 09/02/2013 22:59:48 MDT Print View

for ~2 oz more and around the same price you can get an ID microbivy made from event at MEC .. and if it doesnt work bring it back

a bivy will add ~10F or so max ... as long as you dont have condensation issues, then all bets are off


Stephen Komae
(skomae) - MLife

Locale: northeastern US
for the masochists on 09/03/2013 14:42:27 MDT Print View

In my experience, bivvying in any temperature above 50º is for masochists and others who thrive on suffering. Even bivvying sub-freezing is not a thrilling experience, but at least doable. I have an OR Alpine Bivy and an REI Minimalist Bivy.

If you think a rain jacket doesn't "breathe" well enough when you are walking, just imagine how well the same material will "breathe" when you zip your head in there and exhale a liter of water overnight? When you sleep your body loses about a liter of water, which will go... right into your sleeping bag and bivy.

Once the weather goes to freezing temps, expect to wake up with icicles growing above your head. Not the worst, but an experience nonetheless. Sometimes it is too cold to stick your nose out of your bivy, and you are gonna zip yourself in. Icicles result.

That said, I think you can expect about 5º of warmth from a bivy. It certainly blocks the wind and certainly keeps in heat. However, keep in mind that you will get some amount of moisture build-up inside your sleeping bag, which will compromise your bag's loft or ability to retain heat. I try not to recommend a bivy to most people, but it is certainly a workable shelter and something I usually consider taking when the weather gets cooler.

Edited by skomae on 09/03/2013 14:42:58 MDT.

Max Dilthey
(mdilthey) - M

Sweet! on 09/03/2013 15:02:07 MDT Print View

Stephen, I know that to be the case and so I tried to get the most water-permeable bivy I could find. It looks like it's gonna breathe pretty well, and still work to keep the wind spray off. I don't think I'll ever fully zip it unless I was in survival mode.

Yeah, I thought there was some increase around 5-10ºF so it's good to hear this is the case. My 45º bag will be fine.

I accidentally had a mental shortcut and posted the same question twice in 12 hours... Ignore the other post. Sorry!

just Justin Whitson
Re: for the masochists on 09/03/2013 21:56:36 MDT Print View

Stephen, what you wrote is part of the reason of why i don't understand the use of full fabric bivies for most conditions people use them in (not the true mountaineering that they were originally designed for). I think the best bivy is one that is waterproof on the bottom and only has water resistant fabric to about the knees or so, and the rest should be bug netting/mesh and with a cord loop so you can hang it up some. This reduces weight some (unless you're using the newest, lightest nylon fabrics) and breathability is excellent and bug netting/mesh actually does cut down wind some, somewhat surprisingly.

Even under a smaller tarp, chances are, you're only going to really need (on average) just one end of yourself well protected, if you do it right.

Max Dilthey
(mdilthey) - M

Sounds Perfect on 09/03/2013 22:59:05 MDT Print View

Justin, exactly my plan.

Scott Musack

Locale: Oregon
Re: How much does a bivy augment a sleeping bag? on 09/04/2013 17:44:32 MDT Print View

Washington in November might average 41, but the 'felt' temp I'd bet is colder. There is alot of moisture in the air which will make you feel colder.

The bivy will add around 5 degrees. Personally, I would need something much warmer than a 45 degree bag.

Max Dilthey
(mdilthey) - M

Other Factors on 09/04/2013 17:47:09 MDT Print View

I also have a synthetic puffy with a hood and the neat little 5oz down blanket from Montbell for my legs, and a merino wool baselayer. My pad is a Thermarest X-Lite. Knowing myself, I'd take this kit to 20º in an emergency. So, it'll be enough.

The knowledge that I'm saving 2.5lbs of weight over my 20º bag will keep me feeling toasty.