Breathable Bivvy – Alpine Conditions and Down Bag

Some further detail that may help with advice:

Planned Environments:
Tent camping – Temperate winter (generally wet and windy) and Alpine in Australia and NZ.
Tent and snow cave camping – Alaska (primarily coastal) and the Rocky Mountains in Canada (BC, AB etc) and North America (CO, UT, MT, WY etc).

Me:
I'm about 6'3" and about 200 pounds.

Other gear:
Sleeping mat is a Neo Air X-Therm (largest size).
Sleeping bag is a Macpac Sancturary 1000 XP.

Condesnation from a bivy will almost always outwet condensation from a tent. It's the nature of it. Even the most breathable fabrics will wet up inside.

Some bivys get a slick inside and don't soak the bag. Some do. Everybody's conditions and experience will vary. In alpine conditions, expect a lot of condensation, similar to the amount you already see on the shelter walls.

For your purposes, I would hazard a guess and say a bivy bag will NOT fulfill the needs you outlined. It will get your bag wet, most likely. I would instead suggest a double-wall tent, with a screen separating the rain fly from the interior, as most of these ventilate well.

Whilst waiting for others to chime in, here is some good info I have stumbled across, including a very similar but much older thread on BPL:

Best Lightweight Bivy to Protect a Down Bag: (See the post by Monty Montana in particular)
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=12382

Vapor Barrier Liners [VBL's]: Theory & Application
http://andrewskurka.com/2011/vapor-barrier-liners-theory-application/

I would suggest trying to test a bivy by borrowing a friend's, etc. I would see what other alpinists use as a shelter. Those two excursions should gather enough knowledge to see if a Bivy is the way to go. If you get a bivy, it's hard to do better than the Mountain Laurel Designs Soul e-Vent bivy.

Vapor Barriers are a whole 'nother animal. Firstly, the conditions have to be really good. Secondly, wearing a full VBL suit is… awkward at best, miserable at worst, but if it works then it works.

Definitely try and test your way into VBL's, don't plan a trip and pack a Tyvek suit thinking it'll just work perfectly.

Righto, thanks Max. I realised I totally forgot to mention VBL's and how they might work in the mix.

I was hoping the system might work whilst using a combination. A sleeping bag vapor barrier liner (to stop my body vapour making the sleeping bag or inside of the bivy wet) and then the bivy stopping the envrionment (tent walls, snow cave etc) wet.

In theory that probably works, what might not be so appealing is sleeping in your own sweat all night…Might make me (even less) popular with others on the trip as well, specially if they are down wind…. :-)

More good advice, thanks Max. Will definitely try and borrow a bivvy to see how it works out in Aus. Might just be better off with a synthetic filled sleeping bag that insulates even when damp.

This has, and always will be, my most sage advice; If you're not sure about getting wet, Go Synthetic.

Put it this way. The way modern materials are, you're looking at 6 oz of weight VS hypothermic death (and I'm being very general). Of course, down bags can be fine and will continue to be a standard, but if you're not sure about keeping it dry, err on the side of caution.

i've been pretty satisfied with mine in similar situations to which you describe. i haven't had a situation where my sleeping bag has had any indication of wetting out from internal condensation between the two bags.

I have been very happy with my MLD eVent bivy. It has all of the functionality you require and is of excellent quality. Maybe a bit overkill as its completely waterproof – top and bottom. I've never used it inside of a tent either.

"Ideally I'm looking for something that can be completely closed if required (so that the sleeping bag hood stays dry as well)."

Are you thinking you'll close it up in such a way that you'll be breathing inside the bivy? If so, that'll be problematic. A highly breathable bivy (for what you're trying to do, I might be tempted to MYOG something out of a Driducks poncho) *might* be able to avoid getting condensation inside without a VBL, but in my experience, you'll be swimming in condensation if there's no way to vent the moisture from your breath.

Cheers,

Bill S.

Ah yeah, when I say closed off I was talking more about the style of the bag, rather than how I intend to use it. I would aim to employ it in a similar manner to a sleeping bag (i.e. cinched up over ones head, but with an opening around the face) since I wouldn't be relying on the bivvy for primary weather protection.

I haven't seen any bivvy bag designs that work in a similar manner to, or are cut like, a sleeping bag though (the bivvy designs I have seen are either open sacks or totally overlap around the head or chest area for complete closure). I figure however that you could fold the closure flap back in such a way that you are essentially left with a hood over your head, but and an opening across the your face to breath out of.

I can see that breathing inside the thing would be a disaster in a cold environment. I'd certainly be trying to avoid that.

I suspect many who mention probems w/ bivies would have been just as bad off as without them. I have awoke to a damp quilt/sleeping bag without a bivy plenty of times – typically when camping in a single wall shelter – even without rainy or overly humid conditions.

Once it is cold enough your sweat just won't pass thru the insulation shell – no matter how breathable. Adding a bivy (of any material) can help to force condensation to occur outside of the insulation (vs inside) where much off it can be wiped off in the morning.

I have also slept outside to test a quilt's warmth – after weighing it the next morning it had gained a fair amount of weight (condensation) even though it appear dry so I think it may be tough to gauge in the woods whether you'd be better off with/without a breathable bivy (w.r.t. condensation anyways)

I believe the value of breathability in a bivy diminishes with decreasing temp (maybe negligible by 20F). Once the cold sets in a wicking type material likely would perform much better at protecting you insulation than a hikely breathable one. See here for an excellent write up with fabric info and other links:

http://40yearsofwalking.wordpress.com/2011/07/04/the-bivy-condensation-conundrum/

Also, I think you are on the right track with vbls. I find them no more fussy than typical rainware.

The way modern materials are, you're looking at 6 oz of weight VS hypothermic death (and I'm being very general).

I think you grossly overestimate the warmth of damp/wet sythetic bag.

Any recommendations on the materials front?

Sounds like I'm after a water-resistant highly breathable fabric, rather than waterproff/breathable…Still not sure about the floor in a snow cave though, I can see that being an issue.

Couple things from my experience. I don't want to seem like I'm refuting other's advice; everything is HIGHLY RELATIVE and at the end of the day, if a Bivy didn't work it wouldn't exist.

#1: When I sleep in a tent with a single wall, I tend to have condensation issues in midwinter. We'll wake up to icicles on our tent's inner wall. However, my other shelter is a Henessey Hammock, which has a ludicrous amount of ventilation. It's obviously colder than the tent, but dry as a bone save for a spot of ice on the netting directly above my face. What this tells me is that the only mitigating factors to condensation, given the nature of materials as they are now, are Temperature and Ventilation.

If you increase temperature, similarly, you must increase ventilation or condensation will form.

#2: I have soaked synthetics completely through and then slept in them. One occasion, a rainfly ripped during a storm, leaving me with a flood of water in my hammock. I shivered, but I was already pushing the bag's limits and was grateful for the synthetic insulation. The same goes for clothing; any experienced backpacker will tell you just how important a fleece is in wet weather.

I have soaked a 20º synthetic bag from ocean spray on the coast of Maine in early april, and I've soaked the same bag in snowmelt in New Hampshire's Mount Moosilauke in February, so I've got a pretty good idea of the expected warmth retention.

I'm a little surprised the idea is up for discussion; I thought the warmth when wet concept was not only ubiquitous knowledge, but pretty undisputed. I can be enlightened, though.

"any experienced backpacker will tell you just how important a fleece is in wet weather."
I will second this. Fleece is the only reliable warmth when it is 3 degree Celsius, raining and 100% humidity. That pretty much describes my coastal winter hiking.
I also agree with Max's other statements and comments made about bivy condensation and synthetic bags. If you have concerns about staying dry go synthetic and ditch the bivy. It will will weigh the same as a down bag with a bivy bag.

I won't debate that sythetic loft will tend maintain its warmth better than down in wet cond'ts. I think the relative amount tends to be blown out of proportion. And I think wet sythetic is definately not warm.

I wet bag sucks.

The wettest bag I have ever had was a synthetic one, a 7-8 bpacking trip in rain where we saw maybe 2hrs of sun total :(. Coupled with early in the trip I was sleeping a bit warm. I wouldn't call it soaked but it was noticably damp. One of my last nights I froze my azz of in at least 10F warmer temps than I have comfortably had that bag. The down quilts/bags would likely have faired worse but I don't believe game changingly so (I have used down bags in similar weather just not as long).

On one multiday trip that never got above 20F, I had a platupus bladder failure and soaked ~10-20% of my down quilt…I though it was game over as I was counting on this for a >50% of my nightime warmth and was already pushing my insulations limits – it was another cold night but I ended up only shivering the last hr after getting up to take a leak.

What kinda of hypothermia advantage do you think a syn quilt bag would have over a high fillpower down bag of equal weight (not rating) in cold wet conditions?

Fleece is my go to for warm on the move for the reason you mention (and it dirt cheap).

Again, once it is cold enough, synthetic or down or fleece will all gain weight and lose warmth. IMO vbl and bivy really can help mitigate this. Others promote an overbag to help protect the main insulation layer…I have done this bc I am too cheap to have a 0F bag and I could make a quick and dirty 1+season syn quilt to supplement the 3 season bag/quilt I have.

Event fabric seems to be well recieved but it appears to be tough to find one…

If I could find one with a pertex equilibrium top, I'd recommend it.

To be honest, I can't find one I'd buy right now for cold weather. I'd probably just keep using my MYOG M50 bivy (never had it below 35F though) If using a breathable non-wbp fabric I'd want to be able to suspend it above your bag and be ready to lay a fleece jacket on top of bag – you will likely have condensation between bag and bivy (but again, it is my opinion that that w/out the bivy condensation would just occur inside the bag).

So, would something like this (Pentax Endurance) be an option?
http://www.integraldesigns.com/product_detail.cfm?id=894
They also make the same design out of "Lightweight 50D Pertex® Shield 2.5-layer fabric".

This is the kind of design that I hadn't found commercially up until just now. This is pretty much what I had envisioned making myself, it's not fully weatherproof clearly, but ideal as a sleeping bag cover without trapping exhaled air inside. Does seem fairly pricey, but on the flip side making one would be fairly time consuming.

This is another contender:
http://www.mountainlaureldesigns.com/shop/product_info.php?cPath=22&products_id=30

Comes back to the fabric question, water resistant highly breathable (Pertex Endurance), versus waterproof and breathable (eVent or Pertex Shield).

What I'm particualarly unsure of is how colder conditions impact these materials, are some more prone to losing breathability than others?

Get an eVent bivy. Breathability of eVent is unsurpassed.

Thanks all for the discussion so far. I'll be reading up on the fabrics over the next week or so, if I find anything of great interest I'll add a link here.

"What I'm particualarly unsure of is how colder conditions impact these materials, are some more prone to losing breathability than others?"

Once the bivy fabric is cold enough, water vapor condenses before it can pass through. Some fabrics are better at moving the condensed water thru them (kind of like a paper towel). The lighter nylon fabrics typically don't excel in this fashion.

I wonder how well a tyvek type bivy would perform in the above regard.

the following web address is for an article about bivy condensation. The author experience is that air and vapor permeable fabric (Event or similar)perform the best in regards to condensation. He also suggest that a bivy with WPB top and bottom would be better than a bivy with a waterproof bathtube floor.

http://40yearsofwalking.wordpress.com/2011/07/04/the-bivy-condensation-conundrum/

However if you are only interested in protecting your sleeping bag from spray while sleeping under a tarp or from the wet walls of your tent a homemade Tyvek bivy might be a good choice:

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=46661.

Tyvek doesn't have enough hydrostatic head to qualify a waterproof (about 40 to 50 inches),it's water resistant yet breathable. The type 14 tyvek used in the home made bivy is available on Amazon.com

I have an old Gortex hooped bivy and I don't believe the venting around by head reduces condensation significantly. The venting around my head doesn't seen to affect moisture around my feet. I have a preference for a side zipper because it makes getting in and out of a bivy very easy.