Author Topic: survival bivvys  (Read 513 times)

landy87

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survival bivvys
« on: 15:46:40, 04/01/18 »
does anyone use a survival type bivvy wiith the silver heat reflective lining  for general use when it is cold?

im thinking my sleeping bag may need the extra boost  and the sol escape looks just the ticket being waterproof and breathable
only thing is it is reviewed as being on the small side is there an alternative?


https://www.youtube.com/watch?v=y5bNhqh2k-k

Annejacko

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Re: survival bivvys
« Reply #1 on: 16:00:34, 04/01/18 »
Are you using it in a tent or is the survival bivvy being used as a bivvy bag for weather protection outside instead of a tent?

When I've slept in a non breathable bivvy bag I've found condensation to be a problem.
If you're looking to increase warmth inside your bag in a tent a sleeping bag liner might be worth considering?
Enjoy every sandwich

NeilC

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Re: survival bivvys
« Reply #2 on: 16:05:47, 04/01/18 »
I've never done it but thought about it. 155g is pretty light. However I'm guessing that being what it is - an emergency item - the chances of it having Goretex levels of breathability are pretty low and I wonder how strong it is for repeated use. Could be wrong though.

I'm not convinced it would be much warmer than a standard bivvy (which isn't that warm from experience) when wrapped around a sleeping bag given a lot of the heat you lose in a sleeping bag is via conduction, convection and evaporation, and not radiation due to the relatively low temperature formed at the surface (brings back undergrad black body radiation lectures - The Stefan Boltzmann Law....shudder) of a half decent sleeping bag. Also reflectors are only any real use when there is an air (or vacuum) gap (ie something that allows radiation to pass rather than absorbing and remitting it) between the body radiating and the reflective surface. Once the the bag is pressed up against it, it doesn't have anything to reflect. It's the same reason people are wasting their time putting silvered foil under sleeping mats.

If it compresses your sleeping bag at all then it might actually reduce it's effectiveness. This could very easily happen with a down bag.

 I suspect it would be much warmer used inside the sleeping bag as there would be quite a few air spaces between body and bag. Inside the bag it would not only reflect better but act as a partial VBL which has additional thermal benefits since evaporative heatloss is significant. But it's gonna be less comfortable. Might not be that bad mind if you're wearing full thermals.

I've looked into sleeping bag boosting a fair bit and as far as I can work out, by far the most weight efficient method is a warmer bag to begin with since you're not wasting weight on additional face fabrics, seams etc. After that in the list is an additional bag, then thermals then a thermal liner.
« Last Edit: 20:54:32, 04/01/18 by NeilC »

landy87

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Re: survival bivvys
« Reply #3 on: 18:24:23, 04/01/18 »
i would use inside a tent without the inner so i would want a bivvy anyway

i have seen couple of reviews where a guy said he was fine in just his t-shirt at 29.F

durability may be an issue with it but it does have a zip so getting in and out shouldnt put much pressure on the seams

as for the loft concern wouldnt this be a problem that concerns all bivvy campers? ive used a lightweight (350g) bivvy that was snug and im not sure if it affected the temp but getting in and out was a pain as the sleeping bag would have to come partially out of the bivvy to get in and out  and during the night would end up creeping out of the bivvy too

landy87

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Re: survival bivvys
« Reply #4 on: 18:39:11, 04/01/18 »
so the effectiveness of the bag would only be noticed without the sleeping bag?

in that theory then wouldnt it be best to get in it starkers? :D

NeilC

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Re: survival bivvys
« Reply #5 on: 19:45:25, 04/01/18 »
so the effectiveness of the bag would only be noticed without the sleeping bag?

in that theory then wouldnt it be best to get in it starkers? :D

Even tight around a sleeping bag, the bivvy bag still operates as an windproof bag, sealing in air, reducing convection etc. That is worth something, potentially quite noticeable. But the value of its reflective properties would be very much reduced. If it were loose around the sleeping bag and crinkled then the air gaps would allow reflection.

Starkers would give you the best reflective effect as your skin temp is the hottest surface in comparison to the surface of thermals. Hotter surfaces radiate more. However I bet it still wouldn't be as warm as wearing clothing inside the bag. But the effect in terms of being noticeable would be potentially quite dramatic.

i would use inside a tent without the inner so i would want a bivvy anyway

i have seen couple of reviews where a guy said he was fine in just his t-shirt at 29.F

durability may be an issue with it but it does have a zip so getting in and out shouldnt put much pressure on the seams

as for the loft concern wouldnt this be a problem that concerns all bivvy campers? ive used a lightweight (350g) bivvy that was snug and im not sure if it affected the temp but getting in and out was a pain as the sleeping bag would have to come partially out of the bivvy to get in and out  and during the night would end up creeping out of the bivvy too

I've seen a review where the guy was too cold to continue in the low 30's wrapped up warm. I really cannot see it being especially warm at low temps in a t-shirt.

Overall a reflective bivvy is better than a non-reflective bivvy, all other things being equal but it's overrated when pushed up against things.
Yes loft concerns are universal but most bivvy bags are designed with that in mind as they're designed for use with sleeping bags but you're right, some of them are too tight and suffer because of it. If this one turns out to be as large as standard bivvies then it would be fine. But the video I watched made it look small on a little 5'8" guy.

EDIT: I've been looking up what proportion of bodyheat is lost via radiation and it turns out it's not a simple answer. It depends on the relative temps, air movement, perspiration, clothing etc. According to one medical paper (which is then quoted blindly all over the net) about 60% of heat is lost via radiation in patients but are they measuring surgery patients naked and lying still in a warm environment with little air movement? Looks like clothing reduces radiative losses quite a lot so wrapping in clothing with wind whistling around would be a different matter. But it could be higher than I thought, in which case the reflective nature could be more valuable - if the conditions allow reflection (air gap). If you get one, do report back how well it works - I'd be well interested.
« Last Edit: 21:23:04, 04/01/18 by NeilC »

landy87

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Re: survival bivvys
« Reply #6 on: 11:24:49, 05/01/18 »
@ neilC

this guy suggest also that it would be better inside the sleeping bag and that it added upto 15F worth of extra warmth also stating that it is too small too put a bag inside the bivvy

https://www.youtube.com/watch?v=azEwEstvYic