Those mylar emergency blankets are just plastic with a reflective metallic surface.

The temps here have dipped lower than I'm used to, I haven't been getting off work until after 9 p.m., and the car heater takes a while to produce heat. I've got my coat and a knitted hat. I could still feel heat leaking through the hat. When I got home, I cut out a circle about a foot across from a blanket and put it on my head under the hat.

WHAT A DIFFERENCE!

Then I had to work in my friend's cold garage for several hours on a project, so I wore my two-layer hat. When I got home and took it off, the inside of the mylar was coated with moisture.

So, what is one of those mylar sleeping bags going to do? First it warms me, then it makes me sweat, then I die of hypothermia? Or should they just be used for wind deflection with plenty of ventilation?

Any real-time experience out there?

How about a perforated emergency blanket, aka WallyWorld Goretex? <img src="/images/graemlins/smile.gif" alt="" />

Sue

If there was a simple answer to your question... I have used vapor barriers (VB|) inside sleeping bags for many years - only in very cold weather. Insensible persperation is what keeps our skin flexible and that is where the moisture you found came from. When the humidity gets high enough, the insensible persperation slows to nil or stops. Inside of insulation (sleeping bag), a properly used VB liner adds the equivalent of a few degrees to the bags rating - there is no question about that. Perhaps more importantly, it VASTLY reduces the amount of water that migrates into the bag's insulation (which condenses and freezes). On extended trips in interior Alaska winters, that kept my down bag functioning for weeks without extensive down time for exposing the bag so the ice crystals can (slowly) sublime away - important when I was packing up and moving every day.

But they are unpleasant to use until you get used to them and they really don't slow the persperation down that much if they are not sealed up properly (I have drawstring closures on the ones I made - like a people stuff sack 0 head and upper chest out, of course).

I have used an emergency mylar space blanket inside kids bags for them and it worked OK, but I'm not sure it was that much of a real improvement. I think you have to try this out for yourself and see what works for you and your body's particular metabolism and so forth. It may be good enough for you or you may want to find some other solution.

Now, if you're soaked and you wrap one around you to cut the wind and reduce evaporative cooling, they work pretty good. They have their uses. I'm comfortable enough with my experiences in general to believe that it is worth the little bit of space and weight to carry a couple of these, but they are just part of what I routinely carry for ready gear.

HTH

Tom

I'm curious- because I haven't tried it, but I did spend I few minutes thinking about it- surely if you were using a mylar blanket in conjunction with a sleeping bag you'd want the mylar on the outside of the bag?

The mylar, being impermeable, would protect the sleeping bag from water (either rain, or drawing up from the ground you were lying on), and also protect against wind blowing through the sleeping bag and wicking away heat. Meanwhile, with a layer of insulation (the sleeping bag) between you and the mylar you would allow air to circulate around your body, and you would keep your (hot, sweaty, evaporating) body away from the (cold) outside, thus preventing condensation. The heat reflecting properties of the mylar are a bonus, but the wind- and waterproof properties are what's important.

Again, I haven't tried this, so I could be talking through my hat...

A

Training in the Canadian winter, you learn a few things... Your sweat will condensate and freeze inside the last layer of material. If that's a sleeping bag, those ice cristals can melt and make it wet and miserable. If it's your mylar, you can just shake it off.

I agree the mylar can make you sweat more & make your clothes / sleeping bag more humid. But due to their weight & bulk they are invaluable in many situations.

Our team uses the Mylar blankets as a vapor/wind barrier. We create a patient package, by first laying down a Mylar blanket into a rescue basket and placing 2-3-wool blend disaster blankets on top. Onto this package the patient is placed (often with heat packs and any other medical necessities – BP cuff, pressurized IV, etc.) and as tightly as reasonable (body hugging) we fold one side over the patient and then the other side, sometimes duct taping the seam. Depending upon the weather, we may place another Mylar blanket over the top of the patient.

While they can be used many times, we consider the wool blend disaster blankets to be disposable, in the sense once we complete our portion of the rescue, we rarely ever get the blankets back from the hospital.

For personal use, I would have the insulating layer of clothing closest to the body, covered with the Mylar blanket.

Pete

Pete,

That makes very good sense to me, and I hadn't seriously thought about that way of using the mylar. I think that for short duration - up to roughly overnight in below-freezing temps - that would be the best way to go.

For longer duration, it seems to me that two mylar sheets might be better - mylar, insulation, mylar, and roll up in the package so there is mylar protecting the insulation from both exterior factors (wind and rain/snow) and interior factors (primarily insensible persperation). Hmmm. I might give that a try. Maybe my Tyvek ground cloth (outside layer), a poncho liner, and a mylar sheet (inside layer), since I carry all three of those anyway.

Regards,

Tom

Susan,

You don't necessarily die of hypothermia if you are damp. It is OK to be warm & damp, it's just not particularly comfortable. Hypothermia comes from being damp/wet & exposed.

The mylar survival blankets were never meant to compete with a nice fluffy down sleeping bag tucked inside a breathable bivy sack. The mylar survival units are just that: for survival, not comfort. You sacrifice the comfort for compactness, light weight, and easy availability.

FYI the ThermoLite bivy sack from Adventure Medical has a mesh vent at the foot end. You adjust the opening at the head end to provide some airflow through to the foot vent. Or you can seal it up tight.

Many have found that wrapping their heads with aluminum foil not only acts as a vapor barrier, but it also protects them from the effects of evil secret government agencies. <img src="/images/graemlins/grin.gif" alt="" />

I have a fleece jacket from Cabelas that has a "windshear" layer sandwiched between two layers of fleece. Even though it is supposed to breath well, I found that it was too hot for moderate temperatures (50-60F) and would make me sweat too much. Now that it is colder out I find it does a great job at 30-40F.

I've always assumed that the idea is that under survival conditions one would cover themselves with the vapor barrier enduring any uncomfortable persperation, and wait for help to arrive. They shouldn't remove the barrier until alternative insulation or methods of warming are available - most likely after rescue.

I've been reading a lot about the supposed vapor-permiable materials and have come to the conclusion that they only work when someone is nearly comatose (not at all active). Any activity at all will result in excessive sweat accumulation. Right now I think the best bet for rain protection is a waterproof layer (plastic, coated nylon, ...) combined with very effective venting. Eureka's single-wall tents use coated nylon with lots of venting. Gore-Tex coats come with tons of venting holes & zippers. I tend to think waterproof boots rely on foot movement to pump out humid air. Someday I might try to design a poncho that has some kind of belt to limit the wafting from wind.

So, can I get hot sauce with that trauma burrito? <img src="/images/graemlins/smile.gif" alt="" />

But you can't put Mylar in the microwave!