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Euler
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Euler
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PostSat Sep 16, 2017 2:48 am 
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For years, I've carried an inflatable pad* and a blizzard bag in my pack on day hikes, to better my situation if I have to spend an unplanned night out when it's cold. I take these things for summer hikes and snowshoe trips in winter. I believe that the pad weighs 13-18 ounces (it was the lighest for given R-value as far as I could tell; I don't remember the exact weight) and the blizzard bag weighs 13 ounces.

I wonder what would happen if it rained. Then, water would run over the outside of the bag. It seems like the water would be close enough where I'd lose heat to conduction, which would be very bad. I could lie on my back, with my knees and hands holding up the bag, but even if I could stay in that position while sleeping, I would think that my knees and hands would quickly become very cold, probably frostbit, if conduction is happening. Is conduction in the rain, or even not in rain, something I should be concerned about, with the pad & bag system? Even if it's not raining, it seems bad to have the sack directly next to skin. Do I try to rig a stick or my pack or something to the head of the bag so that none of it touches me from above, like with a bivy sack?

Another idea is to replace the inflatable pad with a closed cell foam sitting pad, with the idea of sitting up, and having some way of holding the bag so it doesn't touch my head in the rain. I'm not sure what that would be. The sitting pad would be 2-3 ounces and I could even bring two of them with acceptable weight and volume. But, trying to sleep sitting up would not be great. Also, I might be too exposed to wind, or, if in a snow cave, not able to sit up. Maybe I could lie partly on the sit pads and partly on my pack.

If I need more weight, then so be it; if I can get good protection with less, that's better.

I'd appreciate some advice.

* I accept the risk that it might puncture; I also carry patches.
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texasbb
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texasbb
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PostSat Sep 16, 2017 8:52 am 
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It's certainly true that you need a gap for the reflective coating to have its desired effect, but good luck getting a blizzard bag to touch you all over.  IOW, I think you're worrying about something that's a small effect overall.  If you can stage some sticks or branches to hold the bag off you, that would certainly make it more comfortable.

FWIW, I'm more inclined to carry plenty of insulation and a way to keep it dry (usually a 5' x 8' sil tarp for summer hikes).

Also FWIW, I'd definitely prefer a CCF pad over an air mattress for emergency use where I know I'd be setting up right on the ground.
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Euler
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PostSat Sep 16, 2017 10:50 am 
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When it's raining, the water will weigh down the bag, and wherever the bag touches you, there'll be a lot of heat loss through conduction, no? This seems to be more about survival than comfort.

The inflatable pad's R-value is 2-3 times that of one CCF pad, and its volume is 1/10 to 1/100 of the CCF pad. For this I pay a penalty in more weight. I don't think one CCF pad in 20 degree weather on snow is going to cut it. Also, I like having the pad inside the pack, instead of having a bulky thing outside, especially for scramble routes. A sit pad is an option, though - even two of them may be small enough to fit inside a pack, although this is still far bigger than an inflatable pad. Two sit pads are much lighter than an inflatable pad, though - two weigh 4-5 oz as compared to 13-18 oz.
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RandyHiker
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RandyHiker
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PostSun Sep 17, 2017 9:46 am 
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Euler wrote:
This seems to be more about survival than comfort.

Gear like a blizzard bag is exactly about surviving an unplanned night out.  You aren't going to be comfortable unless you bring significantly more insulation.

A blizzard bag or space blanket or tarp does some things to reduce heat loss: 1) Protects your clothing from rain and/or ground moisture , which significantly improves it's insulating value.  2) Reduces heat lost from convection , particularly from wind.

Note "Space blanket" type items also have a reflective layer to reduce heat loss from radiating heat to the open sky.  Personally I think this is a minor issue in the PNW , so I favor a SilNylon tarp over "Space Blanket" type items as these tend to somewhat fragile and easily torn.

Minor note: Frostbite isn't a high risk if it is raining -- hypothermia is the prime danger in the Cascades.  Frostbite becomes most of a danger when temps are below 25F.

The factor that is very deadly with hypothermia is WIND seek shelter from wind. This is a far greater heat loss risk than rain drops dribbling down the sides of the bivi bag.
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Euler
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Euler
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PostSun Sep 17, 2017 11:15 am 
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Randy, thanks.

My main question was about heat loss through conduction. 35 and rain is a scenario that got me thinking. Another scenario is being stuck out in the open in high wind, e.g., on the Muir snow field on Mt. Rainier.

The frostbite question, I also wonder about.

The blizzard bag is pretty thin. In 35 and rain, why would the blizzard bag, or a lightweight bivy like the OR Helium, for that matter, not generate substantial heat loss through conduction, enough for hypothermia over the course of a night? There's cold water running almost directly on skin.

It helps me not only to know a fact or opinion, but also "why". If I get clear understanding of even one of the two questions, then I'm ahead of where I was before.

I mentioned survival because someone else mentioned comfort. Being comfortable would be a bonus, but I don't expect it.
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RandyHiker
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PostSun Sep 17, 2017 2:02 pm 
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You'll need to crack open an engineering textbook on thermodynamics to make precise calculations about how much heat each form of heat transport will remove.

The amount of heat lost from rain drops dribbling down the outside of the blizzard bag would depend on how cold the rain was and the total volume of rain.   So typical cascade drizzle would involve a low mass of water and a lesser heat loss than a downpour. 

The amount of heat removed compared to conductive and covective losses to the air follows similar principles the temprerature of the air and the mass of the air that passes over the surface of the bivi bag.     

Liquid water has roughly 800 times the density of air -- so a liter of air flowing over the surface of the bivi bag will remove roughly 1/800th of the amount of heat of a liter of water.  But the volume of air flowing over the surface of the bivi bag is far more than 800 times higher than the volume of water flowing over the surface of the bivi bag -- and that volume gets very large when wind speed is high.
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DIYSteve
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PostSun Sep 17, 2017 2:36 pm 
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The analysis is more complicated than measuring surface convection because a significant function of a Blizzard Bag or equivalent is vapor barrier effect.

I favor CCF for emergency bivi pad because of reliability. You can supplement thermal performance with tree boughs.
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Steve
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PostSun Sep 17, 2017 3:14 pm 
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RandyHiker wrote:
You'll need to crack open an engineering textbook on thermodynamics to make precise calculations about how much heat each form of heat transport will remove.

Liquid water has roughly 800 times the density of air -- so a liter of air flowing over the surface of the bivi bag will remove roughly 1/800th of the amount of heat of a liter of water.  But the volume of air flowing over the surface of the bivi bag is far more than 800 times higher than the volume of water flowing over the surface of the bivi bag -- and that volume gets very large when wind speed is high.

Don't quit your day job.

It's a Heat Transfer textbook that is needed, not a thermodynamics book.

The rate of heat transfer is not the ratio of densities, but the value of k, the thermal conductivity. Water has a k value of about 24. Thus the heat transfer eqn is k*A*dT/d.

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Despair is only for those who see the end beyond all doubt.
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RandyHiker
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PostSun Sep 17, 2017 3:41 pm 
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Steve wrote:
Don't quit your day job.

It's a Heat Transfer textbook that is needed, not a thermodynamics book.

The rate of heat transfer is not the ratio of densities, but the value of k, the thermal conductivity. Water has a k value of about 24. Thus the heat transfer eqn is k*A*dT/d.

What the K value of air then and is not the total heat transfer the sum of heat losses from conduction, convection and radiation?  In practical terms is not convection to the air the dominant mode of heat transfer in non immersion outdoor circumstances?
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Bernardo
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PostSun Sep 17, 2017 7:26 pm 
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If you are worried about this, why not add a light tarp to your kit?  This would keep the water off your bag in most cases.  There are some plastics that are strong and light that might be very useful in a pinch in the woods.
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texasbb
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texasbb
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PostMon Sep 18, 2017 7:26 am 
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Euler wrote:
I mentioned survival because someone else mentioned comfort. Being comfortable would be a bonus, but I don't expect it.

I mentioned comfort only to suggest that the effect of increased conduction due to the weight of water pushing the blizzard bag against skin is a minor effect that might affect comfort but not survival.  I still think it best to leave the gimmicky bags to Bear Grylls and bring some warm clothes and a way to keep them dry.
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DIYSteve
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PostMon Sep 18, 2017 8:02 am 
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texasbb wrote:
I still think it best to leave the gimmicky bags to Bear Grylls and bring some warm clothes and a way to keep them dry.

Agree. And the most weight-efficient means to keep dry in most settings is a tarp. Silnylon tarps are light and tough. If strong whipping winds render it impossible to erect a tarp, you can roll up in it like a burrito.

Also see my prior post re using pine or fir or hemlock boughs as insulation in an emergency. Boughs provide insulation under you, over you and stuffed inside your clothing. A CCF pad over a thick pad of boughs is as warm as the warmest/heaviest insulated pad.
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Euler
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Euler
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PostMon Sep 18, 2017 12:42 pm 
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Thanks for the replies so far. I thought about this a lot over the weekend.

Tarp: setting up a tarp looks more fiddly than a bivy, and, my hands don't work very well when cold. I have some arthritis - that started at a relatively early age. (I'm working on learning to keep my hands warm enough.)  So I value less fiddliness. I usually don't carry poles, so that would be another issue with the tarp. I'm not sure I could set up a tarp in high winds on the Muir snowfield. In conditions where I am going to roll up in the tarp like a burrito, wouldn't it be better to have a bivy? It looks like I might need specialized snow stakes to set up the tarp in deep snow - not sure if that means I need to either decide what kind of stakes to take in the morning, or always take snow stakes and non-snow stakes. Blizzard bag is 13 oz, light tarp like Hyperlite flat tarp (expensive, too) is 8 oz, stakes are maybe 2 oz if I get the lightest possible stakes.

Bivy: if the bivy were not touching my skin (some bivys seem like this) then that seems to create a nice little micro-environment in there that I might be able to warm up, at least more than I'd have with a tarp. On snow and high winds, I might be able to dig a cave, or even a small depression, and get in with the bivy. I still don't have clarity about conduction. OR Helium Bivy is 18 oz. There are significantly lighter bivs around but I am not sure any of them have a hoop or something to hold the bivy up and I don't have clarity about how important that is for warmth. If warmth were the same then I would take the lighter one since this is for emergency use.

With these pieces of gear, bivy beats blizzard bag + tarp on weight, blizzard bag + tarp probably beats bivy on volume. Then there is this: http://borahgear.com/cubenbivy.html

A slightly strange thing to me is that this isn't a topic that appears in blogs all over the place. I'm not the only person preparing for this kind of scenario.

texasbb, I think I came across some of your posts on backpackinglight about this kind of thing.
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DIYSteve
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DIYSteve
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PostMon Sep 18, 2017 1:38 pm 
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Euler wrote:
I'm not sure I could set up a tarp in high winds on the Muir snowfield.

Are you actually concerned about getting pinned down for a night on the Muir snowfield? If so, pack a shovel and dig a snow cave. Or dig a pit and cover it with a tarp. Both are much better shelter than a bivi bag on snow.

Euler wrote:
bivy beats blizzard bag + tarp on weight, blizzard bag + tarp probably beats bivy on volume.

If you're carrying a tarp, there's no need for a bivi bag. Get under the tarp wearing your puff and rain shell, and you'll likely stay warmer and drier than in a bivi bag.

I've spent a couple dozen rainy nights in a bivi sacks and got soaking wet each time, but I have stayed dry on rainy nights under a tarp.

Euler wrote:
A slightly strange thing to me is that this isn't a topic that appears in blogs all over the place. I'm not the only person preparing for this kind of scenario.

No, you're not the only guy preparing for emergency bivis. When old-timers get together it's common to swap stories of unplanned bivis. (I've talked alot about my unplanned bivis but don't recall ever postin about them.)
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RandyHiker
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PostMon Sep 18, 2017 2:00 pm 
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Personally I carry an extra large SilNylon poncho for summer day trips as it doubles as rain gear.   For an unplanned night out in the summer, I would stick my feet into my pack and tuck the poncho in around the pack and gather the edges of the poncho. 

In the winter, I favor a zdarsky tent  This works best shared with another person (or two)-- who also reduces heat loss due to fewer sides exposed to the cold environment.  Hard to find commercially made, but an easy sewing project.

Steve's advice to use boughs or other plant materials as insulation is good -- note that this can not only be used underneath -- but leaves and boughs may also be layered on top to reduce exposure to the cold.   Finding the thickest thicket of brush or trees is a good strategy for reducing wind exposure.

If you've never spent an unplanned night out, it's hard to know exactly how well your setup will do at keeping you dry and not so cold that you suffer cold injuries.    It's possible to do trial runs by trying out your gear in your backyard or some other low risk circumstances.     Gear like "blizzard bag" or "space blankets" are somewhat problematic however, since they are packaged as one time use items and never re-pack anywhere near as compactely and tend to be fragile and easily torn.   The later issue in particular has caused me to favor gear that is more general purpose.
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