# Best way to use “space” blanket to boost heat when bivy camping

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
• 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. Nov 7, 2018 at 8:15
• @ChrisH - Yes, i forgot to mention i'd be clothed, now edited. Also added: bag liner layers. Thanks. 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??? Nov 8, 2018 at 13:17
• A more recent question got more answers outdoors.stackexchange.com/questions/21287/… 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 Oct 20, 2020 at 11:19