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A thin, heat-reflective sheet (aka “space” blanket) is a popular outdoors emergency item, having a number of uses. See adventure.howstuffworks.com/survival/gear/space-blanket and gotimegear.com/blogs/survival-gear/do-cheap-emergency-blankets-work.

If I’m bivy camping – that is, I’m clothed, inside a bag liner, inside a sleeping bag, on top of a sleeping pad, and inside a bivy sack – what then is the optimum location for a shiny survival blanket, for the purpose of keeping me the warmest? Assume two possible sizing situations:

  1. A narrow, or single-width, sheet that I can lie over or under (not both).
  2. A wide, or double-width, sheet that I can fold over so I can lie inside.

Ideally, you have tested, or know of tests involving, multiple orientations.

                                        ← 9
Bivy sack →    _____________________ 
              /                     \   ← 8
Sleep bag →  |   x x x x x x x x x   |
             | x                   x |  ← 7
Bag liner →  | x     _________     x |
             | x    (         )    x |  ← 6
             | x   (  Clothed  )   x |
             | x   (   Body    )   x |
             | x    (         )    x |  ← 5
Bag liner →  | x     (_______)     x |
             | x                   x |  ← 4
Sleep bag →  |   x x x x x x x x x   |
             |                       |  ← 3
Sleep pad →  | ===================== |
             |                       |  ← 2
Bivy sack →   \_____________________/
                                        ← 1
The ground  ~~~~~~~~~~~~~~~~~~~~~~~~~~~

If you understand the above schematic cross-section, then you can see that

  1. sizing situation one has nine possible orientations, numbered 1 through 9, and
  2. sizing situation two has five possible orientations: 1-9, 2-8, 3-8, 4-7, 5-6
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  • 1
    You've actually omitted a potential layer (assuming you don't sleep naked when it's that cold) under your clothes wrapped to keep your core warm. This can be the best place in other situations as it keeps it in place even if you move. For that matter, between any sleeping bag liner and the bag might be good.
    – Chris H
    Nov 7, 2018 at 8:15
  • @ChrisH - Yes, i forgot to mention i'd be clothed, now edited. Also added: bag liner layers. Thanks.
    – Martin F
    Nov 7, 2018 at 19:12
  • It's purely anecdotal, but I have camped with another ultralight backpacker who simply suspends a space sheet from the roof of his solo shelter (thus making it pseudo double-walled) and claims (via the NIST-certified thermometer he carries) that it keeps his shelter about 10*F warmer than it would be otherwise. The problem you'll have in a bivy sack is condensation. Rather than using a mylar sheet, vapor-barrier clothing normally used in winter may be a better choice???
    – topshot
    Nov 8, 2018 at 13:17
  • A more recent question got more answers outdoors.stackexchange.com/questions/21287/…
    – Martin F
    Dec 19, 2018 at 18:17
  • @topshot's comment makes sense on a clear night - in radiative terms the night sky is very cold, and common tents/tarps/sleeping bags etc. almost transparent to thermal infrared. Now I want to test my cheap silvered tarp
    – Chris H
    Oct 20, 2020 at 11:19

1 Answer 1

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Space blankets are good insulators. They prevent your body heat from escaping due to radiation and convection. They however do not prevent conduction based heat loss.

Given this, the best place to use a space blanket would be around your body. This would mean that the heat radiated from your body is reflected back and the warmth is retained. In most of the emergency situations this is what is done. However, in your situation, using the space blanket inside the sleeping bag wouldn't be a great idea. The reason being space blankets are non-breathable. Hence, you'll end up with a lot of condensation within your sleeping bag.

In your situation, using the space blanket around the sleeping bag, or around the bivvy sack itself would theoretically reflect back any heat escaping from the sleeping/bivvy bags (and hence is the best possible option for you). But this in itself would be pretty minimal.

Based on the same reasoning, using the space blanket as a single width sheet, under or over at any layer wouldn't add much of a protection as there's little heat to reflect.

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3
  • Your answer assumes normal usage, which is fine, however; what about the case where there is an unexpected cold snap or you are stuck out longer in a storm or other situation in which your equipment is insufficient and you need to do as much as possible to keep warm (ie: perspiration is minimal as you are cold). Not necessarily life-or-death cold (though maybe), but at least "I'm having a hard time falling asleep" cold. Is it correct to assume that you would go back to directly around your body even inside the bag?
    – Loduwijk
    Nov 7, 2018 at 17:00
  • @Aaron Yes, in a very cold, critical situation you could use it within the sleeping bag. The only reason to use it would be to retain any heat being lost by radiation. However, it's only reasonable to think that after you warm up a bit, you will start to perspire and then the blanket will act like a moisture barrier.
    – Ricketyship
    Nov 7, 2018 at 17:16
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    This makes me wonder now how groups of people who have died in the cold would have faired if they had 1 or 2 of these. For example, the famous class that got stranded on Mount Hood, which piled into an emergency snow shelter and only 2 of those who stayed in the shelter survived: I wonder if they had space blankets and laid 1 under the pile of people and 1 over, if they would have increased the number of survivors, or if they would have trapped enough moisture that they would have frozen worse. Different question, and it would be speculative. I assume it would have helped though. pondering
    – Loduwijk
    Nov 7, 2018 at 17:31

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