lost in the cold rain...

[Koda]

So the best method then would be to wear only your baselayer with your main insulation item (warmest) next to that, then wrap that aluminum space blanket around the outside like a burrito…. Silver side inside of course.

This makes sense to me, but might contradict what I have always heard that wearing clothes to bed adds warmth to the sleeping bag…. But my experience validates Potatoes theory that it takes longer to reach peak warmth that way, so what I do is wear only my baselayer but then stuff my other clothes inside the bag with me and let them find their spot, I sleep warmer that way than if I wear all my clothes to bed.

This may be a slight drift from the main topic at hand, but not really if this is true and looked at correctly then in a survival scenario without a down sleeping bag the same concept of layering applies…. a person would consider putting their main insulation item next to their base layer and avoid “layering” up with all they got before wrapping up in the space blanket… any clothing item left over can go between the main item and the space blanket? Would it be good to put an item over the space blanket to prevent conductive loss?

Eg: I’m stuck overnight in winter with only a down puffy, a softshell jacket and a baselayer and a space blanket. Rather than wearing the puffy over the jacket, Layer up in the following order 1)baselayer 2) puffy, the main insulation 3) wrap up in the space blanket…. Leave the softshell out… maybe put it over the space blanket if possible…

[raven]

Since the space blanket doesn't breathe, using it as an outer layer against your clothing or sleeping bag guarantees you'll get your insulation wet from condensation unless you are using a vapor barrier.

(Sorry I added a new variable to the conversation, but vb's are important.)

[Koda]

Since the space blanket doesn't breathe, using it as an outer layer against your clothing or sleeping bag guarantees you'll get your insulation wet from condensation unless you are using a vapor barrier.

(Sorry I added a new variable to the conversation, but vb's are important.)

Does this mean that emergency blankets should not be used?

[texasbb]

So instead of shining through your clothes, that radiative heat is reflected back at you and it warms you up somewhat. I don't see any reason why the amount of heat reflected back at you would depend on an air buffer between you and the reflective blanket, ...

I agree with your first sentence, but not your second. If the reflective surface is facing you, there must be an air space in between or there's no radiation to stop. However...

(Sorry for the late reply)

I don't see what you mean when you say "...or there's no radiation to stop." Where would it go? If your body is warm it is radiating...
Maybe you are saying that if you just have a reflective blanket against your skin and you're losing tons of heat to convection & conduction, then you will be so cold that you won't be radiating much heat? I'll agree with that but I think that you could also have a reflective blanket wrapped tightly against your skin, with insulation on the outside of that, and as long as you're warm the reflective layer is working just fine. I do think the "Omni-heat" products from Columbia work that way (I have an omni-heat jacket).

If you have reflective foil and hold it 1" away from a light bulb vs. directly on the surface, it should reflect the same percentage of light, yes?

For two objects to have a net radiative heat exchange, they must (a) be at different temperatures and (b) be able to "see" each other. (Two walls an inch apart see each other almost perfectly; two postage stamps 20 feet apart barely see each other at all. Engineers call this a view factor, but that's not the issue here.) How much heat is exchanged also depends on the surfaces' emissivities/reflectivities, which is where the shiny surface comes in.

If the space blanket, shiny side inward, is tight against you, conduction heat transfer dominates and the shiny side of the blanket quickly warms up to essentially the same temperature as you. Hence, no radiative transfer between you and the blanket [see (a) above] and the shiny surface is wasted. However, with the space blanket now warmed up, it is radiating heat to the cold surroundings. That radiation can be minimzed by turning the shiny side OUT, because surfaces that are good reflectors are also poor radiators.

That's the theory. In practice, of course, it's hard to get that space blanket tight against you everywhere, so there *is* an air gap here and there, so the shiny surface does do some good. But it might do more good if it's not wrapped on you but instead used to line your snow cave or whatever.

[texasbb]

-Jumping under your down quilt in just your underwear or a base layer, you will reach thermal equilibrium much more quickly and it might be more comfortable for the first hour or two compared to having lots of clothes on, but the final temperature against your skin will not be quite as warm for the rest of the night.

That's an untested theory and I need to do some trial runs in my backyard... I will not be stubborn about defending it if anyone disagrees

I agree with that, but in my experience the difference lasts mere minutes, not an hour or two.

[texasbb]

This may be a slight drift from the main topic at hand, but not really if this is true and looked at correctly then in a survival scenario without a down sleeping bag the same concept of layering applies…. a person would consider putting their main insulation item next to their base layer and avoid “layering” up with all they got before wrapping up in the space blanket… any clothing item left over can go between the main item and the space blanket? Would it be good to put an item over the space blanket to prevent conductive loss?

If you smash that space blanket in between other layers, it will do nothing for you except maybe act as a vapor barrier, for good or bad. In fact, if you've got that many insulation layers, the space blanket isn't going to do much for you no matter where you put it. Maybe it can keep rain off you.

[potato]

For two objects to have a net radiative heat exchange, they must (a) be at different temperatures and (b) be able to "see" each other. (Two walls an inch apart see each other almost perfectly; two postage stamps 20 feet apart barely see each other at all. Engineers call this a view factor, but that's not the issue here.) How much heat is exchanged also depends on the surfaces' emissivities/reflectivities, which is where the shiny surface comes in.

If the space blanket, shiny side inward, is tight against you, conduction heat transfer dominates and the shiny side of the blanket quickly warms up to essentially the same temperature as you. Hence, no radiative transfer between you and the blanket [see (a) above] and the shiny surface is wasted.

I guess I'm still missing part of the picture. I see what you're saying about there being no net radiative heat exchange when the objects are the same temperature. What I don't understand is why this matters . Are you saying that when the blanket is a different temperature from your body and is therefore undergoing a net radiative heat transfer, it reflects a higher percentage of your radiated heat, and once it's been heated up to the same temperature it becomes transparent to the radiation leaving your body? So if you were to watch someone put on an emergency blanket with an IR camera, tight against their skin, they would be invisible to the camera at first but their radiation would quickly shine straight through the reflective blanket as it heated up?
I wasn't thinking it really mattered what happened to the temperature of the blanket; I figured its reflectivity was a property of the material that wouldn't vary that dramatically with temperature.

Sorry if I'm beating this to death, I just want to understand.

Edit:
Wait. Yeah, if someone warms up a reflective blanket (shiny side in) then of course you will see it with an IR camera.
Actually this page is helping me understand the concept I think http://en.wikipedia.org/wiki/Multi-layer_insulation

[raven]

Koda,

Use a space blanket as small tarp. You gain reflectance on a clear night or if you have a fire; you gain some rain protection if there is rain. I think their best use is as a roof when sitting up all night because a small roof will do, if you are alone.

I gave up on carrying them years ago, because every time I thought I ought to have one along other alternatives seemed wiser.

[Koda]

You cant use a space blanket as a tarp.

Raven, what are the other alternatives?


At first i thought we proved space blankets effective because they really do reflect your heat back at you.... but now ate we saying they dont work because they are not breathable. Isnt it better to be warm and wet than cold and dry? Isnt this why they are called "emergency" blankets? Isnt this about 'survival' not comfort? They are barely 2 ounces.... seems like worth their weight.

[texasbb]

Think of it this way, potato: you don't radiate through things, you radiate to things. If the space blanket is touching you, it's basically part of you. Your skin (or shirt or whatever) is no longer the radiator, the back side of the space blanket is. That's why it can help to turn the shiny side out if the blanket really is tight against you--it makes the you+blanket combo an inefficient radiator, so you keep more of your heat.

But again, the theory breaks down, at least partly, because you often can't really get that blanket tight against you. If you've just got it draped over your shoulders, it will probably do a fair bit of good. If you've got it tightly sandwiched between other layers, not so much.