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.
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.
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.
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.
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.
Emergency blankets, bags, and shelters keep you alive, not (necessarily) comfortable. Use them as shields/shelters with an insulator to maintain or heat source to bump temps as needed, and something to keep you off the ground. Some are better than others. Cost-effective experimentation:
They are not technically wrong from a scientific standpoint, yet I can see why real world users have deemed them “shit”.
Heat works in three ways:
Radiative- How these blankets “reflect”
Conductive- think cooking bacon in the pan
Convective- boiling eggs or noodles in water
In space suits (my shit) mylar is the main insulation because there is no air or ground to steal your heat. Only the sun or shadow. Here on earth though conductive andd convetive heat flow are exponetially larger than any radiative heat flow, especially at night. Now if you managed to roll yourself in the mylar like a burrito it might reflect some of your radiated heat (again not very much) but the main benefit would be it would block convective (airflow) heat loss. Just trapping the little poclets of air around your body and allowing them to heat up would be the “warmth” if any you would get from this device. A real sleeping bag would be SUBSTANtially more comfortable. But a smart person in a survival situation shouldnt deem these completely worthless.
There are several vids on the use of those mylar sheets. I think they are like a knife or gun to some extent. It’s how you use it and what other things are around you.
I saw Cody Lundin put one across a slanting rock face, then put clear mylar as a front face and a fire outside that. Supposedly the fire heated that living space.
It was an episode of him and Dave C.
this guy shows some closeups of the mylar after one nights use… He’s doing a “I got lost unprepared” type deal. https://www.youtube.com/watch?v=xdOCuGb4108 42:00 he shows the damage of his mylar sheet.
He also has a pretty cool jacket I have never seen before. You stuff it with leaves and brush as insulation. He claims it was really dry the next day so useful for tender.
There are many more vids. If you watch enough of those hiking vids all the most experienced people have the mindset that every piece of gear they travel with has to have two or more uses.
Not sure if you have seen those sleeping mats by thermarest but the Thru Hikers swear by them. Very light and also made to reflect heat internally. They make a yellow one and gray one. The gray is a little more robust I believe.
While a good sleeping bag might run you $350 and pack into a space the size not much larger than a softball I have heard people say the REI stuff is very good too at 1/3 the price and only slightly heavier.
I don’t know about covering up you IR signature, but I tried my idea out last winter and it did work. Somewhat.
Temperature was in the 20s, moderate to light wind. I removed my jacket to simulate getting caught without a coat, and found a somewhat sheltered spot in my yard. Used old cardboard and evergreen branches for a makeshift ground pad. Sat down with my knees up and lit my candle lantern.
It did work. The temperature did go up to a level that I felt would definately aid in surviving the night. If it was 20 below with strong winds, maybe not, but the concept is sound.
I did find a few problems though. The Emergency Bivvy I used was too narrow to allow for comfortable sitting with my knees drawn up. It’s designed to lay down in as if it were a sleeping bag, but that won’t work with my candle lantern. The heat from the candle melts the bivvy. I next tried the Escape Bivvy, which is stronger and breathable (being made of an almost Tyvek-like material), but it’s even narrower than the Emergency Bivvy.
I finally tried the Two Person Emergency Bivvy and this is just the right size to allow for any seated position while keeping enough space between the candle lantern and the bivvy material.
I’m now trying to put together something that will let me fall asleep but support the candle lantern to keep it from tipping over and at the same time support the bivvy and keep it from colapsing on top of the lantern and melting. All this while being small enough and light enough to fit within my GHB (511 Rush 12).
As it is, the two person bivvy and the candle lantern take up almost no room in the pack and are very light. I keep them, a dry pair of socks and an insect hood all together in the small mesh pocket of my pack. I think they are a sound addition to my pack for a situation where I need to spend the night outside, in the cold, and without my prefered MSS sleeping bag.
I was rock climbing in the North Cascades and when we got to the peak we found we couldn’t cross over the back wall to rappel down due to an unexpected ice wall. We were forced to rappel back down the face 12 pitches. It was getting late so we ended up hunkering down in a small open area rather than attempt to rappel in the dark. We flaked out our ropes to cover the ground, put on all of our clothing, put our legs into our backpacks and covered ourselves with the off brand space blanket I carried. The blanket cut the wind off us and kept us warmer than without. We did get condensation on the underside. It was definitely warmer using the Mylar blanket. It earned a place in my kit.
For what little it’s worth, I took a wilderness first aid class this last spring. When it came time for scenario training and using only what’s in your bag to treat “casualties”, everyone in the class seemed to pull out one of those Mylar emergency blankets to treat the “patient”; myself included. When it was my turn to be the patient, the main benefit I found from the Mylar was as a wind break/ sunscreen. You’re laying there starring up at the sky while the other students are working on you, and the stuff actually helped to cut down on the glare and the wind chill effect. Most of my actual warmth on what was an early spring day laying outside was in getting off the ground and the other materials the students were using to turn me into a burrito with.
I still have one or two of the Mylar sheets I got as freebies somewhere, but right after the class I went to an outdoor store and bought something a little more robust to carry. I never had high expectations about the stuff in the first place, but it was an interesting experiment to be able to see what the stuff was actually capable of.
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.
If you want to make a cheap, waterproof but breathable bivy for a couple bug out bags you can use a roll of Tyvex. Especially if you are making a few, it can be cheaper than getting premade. Then you don’t have to worry about the mylar getting holes as easily etc. This guy goes into it in this video https://youtu.be/o_UHoJwLiWs (skip about halfway through). There are some premade ones if you’re lazy (Google ‘Tyvex bivy’ or ‘tyvex sleeping bag cover’) but if you’re dropping $20 maybe you could get something nicer.