reflects 90% body heat

[moonshot]

We've all seen the advertisements for the mylar-lined emergency bivvy that reflects 90% of your body heat to help keep you warm. Makes sense, sort of, but what about when you're using it as an emergency sleeping bag when it's so cold that you need to keep your pants, shirt, fleece, and winter jacket on (i.e. survival situation, below freezing, you have no better sleeping bag, no fire, etc)?

Will the reflective property of mylar do any good if you are wearing all these layers of clothes? How much heat will escape these clothes to be reflected back?

I'm asking because I am working on several options for an emergency "stay warmer" shelter setup for my EDC kit, but it involves relying on a small mylar bivvy sack and a GI parka with candle lantern (would prefer something like a HPG Mountain serape, but that's not in my budget). The reflective bivvy is intended more to keep me dry and out of the wind or rain rather than keeping me warm, but any added warmth will help.

My idea is based on this...

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

[voiceofreason]

You're best bet is what you have for a lightweight/compact setup. Some kind of structure or setup that can shelter your head/neck/face as well would be helpful using the same type of materials for cover, but something that can create a structure to shelter the head.

[RetroRevolver77]

They're junk. I bought one of those high price sub zero compact sleeping bag systems that had a liner like what you're talking about- basically fits into a coffee can type bag after squishing it down. Nearly froze to death in 20 degree weather with it. Get a real sleeping bag system.

[1168]

Better than nothing in an emergency. But it will tear, and mylar isn’t breathable, so you will wake up damp. I’ve had to use one in an emergency for a few days, and I didn’t die, but they are very sub-optimal.

[moonshot]

They're junk. I bought one of those high price sub zero compact sleeping bag systems that had a liner like what you're talking about- basically fits into a coffee can type bag after squishing it down. Nearly froze to death in 20 degree weather with it. Get a real sleeping bag system.

Better than nothing in an emergency. But it will tear, and mylar isn’t breathable, so you will wake up damp. I’ve had to use one in an emergency for a few days, and I didn’t die, but they are very sub-optimal.

I am aware of this, and agree completely. I have a USGI MSS in my car, and it’s kept me warm and dry in some nasty weather, but it also weighs 10 pounds and adds considerable bulk. My goal was to have a backup system in my EDC bag (in case I cannot get to my MSS bag) that will at least keep me dry and warm enough to survive and get some sleep if I have to spend a night or two without shelter.

The cheap mylar-lined bivvy should keep me dry (picture a downpour and sitting under a tree with no tarp). Again - sitting in it with it pulled up around my waist, perhaps up over my shoulders, but not laying down in it and using it like a sleeping bag. In this way, I hope to also be able to use my beeswax candle lantern to generate enough heat (about 1900 btu’s, but I can’t confirm this) to raise the inside temp enough to make a difference.

Again, it’s to allow me to hunker down somewhere for a night or two, keeping wind and rain off me, and hopefully creating enough heat to let me get some rest (while still wearing whatever I had on when the S HTF).

I don’t know if this idea will work. The poncho and liner with lantern demonstrated in the link I attached in my OP seemed to work (if you can trust Youtube, and I don’t), but that setup is more expensive, bulkier, and doesn’t seem as versatile.

I can test my idea in late fall or over the winter, but I was hoping for some feedback before then.

[voiceofreason]

Better than nothing in an emergency. But it will tear, and mylar isn’t breathable, so you will wake up damp. I’ve had to use one in an emergency for a few days, and I didn’t die, but they are very sub-optimal.

Lack of breathability is a huge issue... as is the fact they don't really keep you warm. Work as a wind protection layer, but that's about it. Too much heat escapes and without breathability the water vapor builds up. Like a mini-tent.

Still better than nothing in an emergency.

[flenna]

I keep the SOL escape bivvy in my get home bag. It is breathable and works as advertised. I have used it as a lightweight sleeping bag down to 50 degrees.

https://m.youtube.com/watch?v=B7bw_PYI_W8

[jsbhike]

From what I have read, the best way to use the Mylar blanket is by using it as a lean to and keeping a small fire out in front of it.

I haven't used the Mylar blanket like that, but the theory is sound based on my experience from sleeping between a fire and a cliff face.

[Jammer Six]

It's hard to imagine a scenario where I'd be outside without a fire.

[NWPilgrim]

Having spent some time in the woods overnight without shelter I found here in the PNW the things that chill you the fastest is rain and wind. Anything that protects you from wet and wind is good. The emergency nubby bags can be sliced open for more room and/or ventilation.

It is just plastic so not fire friendly. If no fire then I keep it wrapped around me and try to air it out occasionally to prevent too much condensation. If I have a fire then the Mylar goes behind me as a wind break and reflector.

No way will it replace a proper tent or sleeping bag. But it can make the difference between bone chilling death in a few hours or staying mostly dry, and warm enough to see the sun tomorrow.

If you can add a fire to your situation then you might even be somewhat comfortable. Your emergency candle is a decent last resort.

Mylar = wind and wet protection, and even then not great but sufficient. Mylar has zero insulation value.