Question of the Day- which is better?

[Matt Dirksen]

If the reflective side is facing outside, then the surface will only reflect any radiant energy that may aid in warming you up. In other words: the shiny surfaced blankets don't help, unless you toss it on the outside of your tent in order to reduce the heat gain if your tent in the summer.

Packaging bubble wrap would be a much better material to use….

[Colin Krusor]

Jerry, radiant barriers don't need an air space. They need a thermal insulator, and preferably an IR transmissive thermal insulator, at the IR reflective surface (the aluminum side). A lack of an IR transmissive thermal insulator is the reason that putting a space blanket against your skin, aluminum side in, does nothing to reduce radiant heat loss. Multilayer radiant barriers are very important to controlling the temperatures of satellite components, and space is a vacuum so there is no air space against those radiant barriers. Multilayer radiant barriers on spacecraft have fabric or scrim between the layers as a thermal insulator, to prevent heating of each layer by solid conduction from other layers ("thermal bridging"). An emergency blanket on the outside of a sleeping bag with the aluminum surface facing in would be in the same situation. It would have lightweight fabric against the reflective surface.

What would a space blanket on the outside of a sleeping bag be doing if the aluminum layer is facing out? If air temperature outside is 30F, what is the temperature of a space blanket on the outside of a sleeping bag? 33F? Freezing of moisture INSIDE the insulation of a sleeping bag is a problem for winter camping, and the outside of a sleeping bag is even colder. It wouldn't be doing a very good job as insulation if it was warm on the outside.

Any heat absorbed by a layer of cold plastic film exposed to cold air will be lost by convective cooling. The cold air will carry it away. The fraction lost to the environment by radiation will be vanishingly small, so you're not accomplishing anything by having the aluminized layer on the outside. But you are putting the mylar layer facing in, where it can absorb radiant energy from the deeper layers of the sleeping bag, which are warm. The space blanket then loses that energy to the outside air by convection.

If the aluminum side is facing in, the radiation from within the sleeping bag is reflected back toward the inner layers of the sleeping bag. Very little gets absorbed by the space blanket, so it can't be lost to the outside air.

[Jerry Adams]

"If the reflective side is facing outside, then the surface will only reflect any radiant energy that may aid in warming you up"

One of the four causes of heat loss is radiant heat loss

Regular sleeping bag – it will radiate heat away making you colder inside

Aluminum covered sleeping bag – you don't have that heat loss

It's not that a radiant barrier reflects heat, it's that it prevents radiation

Again, I'm not positive about this but I think this is correct

[Jerry Adams]

"Jerry, radiant barriers don't need an air space. They need a thermal insulator, and preferably an IR transmissive thermal insulator"

All insulation for backpacking uses air. You can have a vacuum bottle or argon in a window but those aren't applicable to backpacking.

Anything like nylon or polyester will absorb IR if even a thin layer

An air space is the only thermal insulator relevant to backpacking that transmits IR

"Multilayer radiant barriers on spacecraft have fabric or scrim between the layers as a thermal insulator"

They have vacuum in between radiant layers, which has no thermal conduction. Any heat transfer is just at edges or wherever.

Because they are in vacuum it totally changes things. This is interesting, helps understand things, but isn't directly applicable.

In a house, you can suspend a radiant barrier and have an air space so that works.

A neo-air mattress does have suspended radiant barriers with air space between so that works.

In a sleeping bag, there's no way to suspend radiant barriers. If you had insulation in between to push out the radiant barrier, that insulation would absorb IR so it wouldn't work very good.

"What would a space blanket on the outside of a sleeping bag be doing if the aluminum layer is facing out? If air temperature outside is 30F, what is the temperature of a space blanket on the outside of a sleeping bag? 33F?"

According to wikipedia, the insulation value of a horizontal surface is 0.11 m2K/W. Human body emits 50 W/m2. So, temperature between surface and ambient is 5.5 degree C = 10 degree F.

If you're under a clear sky, then the surface of a regular sleeping bag will emit heat so it will be at the same temperature as ambient. If it was aluminum covered and you didn't have that heat loss, it would be 10 degrees warmer than ambient.

[Colin Krusor]

That computation doesn't provide any information at all about the temperature of the outside of a sleeping bag. It would be very elegant if it did.

Our disagreement on a few points has made this a thought-provoking discussion, but I assume we can agree that cooling of the surface of a sleeping bag is dominated by convection, not radiation. Any heat not lost from the surface as radiation, due to low emissivity, will be lost to the cold air (convective cooling rates increase with increasing temperature gradients even when the bulk movement of air is unchanging). The best way to reduce heat loss from the surface, in my opinion, is to reduce the amount of heat that gets there in the first place by facing the reflective surface inward. I grant that the solid materials of the insulating fibers in a sleeping bag (keratin or polyester) are not very IR transmissive, but the air between them is.

[Jerry Adams]

Yes, good discussion : )

The Stefan-Boltzman equation tells how much power is emitted from a surface

But I wonder how that translates to temperature difference. For example, will the outer surface temperature go below ambient?

I don't think there are any products that use an aluminum surface except emergency blankets, so our discussion is just academic.

If I was in an emergency, I'de put the blanket on the outside, to keep out rain, and wind, and prevent warm air from drifting away (convection), and I think it would also minimize radiative heat loss : )

Someone should make a fabric for the outside of a sleeping bag, with an aluminized surface, and holes for water vapor to exit through. Like 5% or 10% of the surface would be holes, then it would be 90% or 95% effective at not radiating.

[Matt Dirksen]

"In a sleeping bag, there's no way to suspend radiant barriers. If you had insulation in between to push out the radiant barrier, that insulation would absorb IR so it wouldn't work very good."

And that perfectly describes the whole paradox. For the barrier to be at all helpful in this situation, Two things must take place:
1) It would need to face inward, however:
2)It could not touch the skin of the person whatsoever, or have any obstructing material between it and the skin, in order to adequately reflect energy back.

The second the shiny material touches the surface of the skin, it would no longer act as an emitter, and become more of a conductor. The R-value this product is negligible, so it would not aid in insulating. Thus, any material between it and the skin would not only severely cut down on the radiant heat energy being lost by the person, but would cut out any reflected energy from entering the system.

In my opinion, emergency blankets:
1) Could only be marginally useful when an actual blanket is not around.
2) Should not even be marketed as a "blanket", since a tent ground cover likely has a higher R-value than them.

On a semi-related note (and to illustrate the never ending confusion between emissivity and insulation:) here is the unfortunate story of a person who was so convinced of the "thermal qualities of insulating reflective paint", that he forced his builder to use the paint instead of insulation. He sued the HVAC Contractor thinking his homes heating and cooling problems were his fault.

http://www.greenbuildingadvisor.com/blogs/dept/musings/insulating-paint-salesman-tripped-his-own-product

[Matt Dirksen]

"I don't think there are any products that use an aluminum surface except emergency blankets, so our discussion is just academic."

Perhaps we should ask Robert Caffin? I wonder if he would be interested in testing the emergency blankets?

http://www.backpackinglight.com/cgi-bin/backpackinglight/testing_thermal_insulation.html

Kelty used to sell sleeping bags in the 80's with a shiny interior surface. I don't think it lasted too long. Regardless of the lack of products in the outdoor world, there is much data on these types of materials in the building world:

http://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-search-problem

There are pretty good reasons why you DON'T see any building codes require it in their wall/envelope assemblies. These products have only been reasonably helpful in reducing solar heat gain, but not keeping heat from leaving us. But regardless of their performance, they can't come close to the performance of simply having more insulation.

[j lan]

Here is the only emergency blanket that I know of that actually provides insulation in a way similar to the neo-air.
http://www.blizzardsurvival.com/product.php/100/blizzard-survival-bag

[Jerry Adams]

That's interesting

385 grams = 13.5 ounces

They say it has 8 togs of thermal insulation value = 5 clo which would be good down to maybe 20 degrees F. That is very good compared to a regular sleeping bag.

Except tog/clo/R/m2K/W all are conduction measurements. Radiation doesn't directly convert to that unit. However, you could measure temperature gain with it and use the conduction unit of measure to produce the same temperature difference. It would depend on whether the sky was clear or cloudy or you were in a sheltered location.

They measured at Leed's University which has the best reputation

There was one diagram that showed 3 layers of aluminized material with air in between cells

The only problem is it doesn't pass water vapor, but you could use VBL

[Jerry Adams]

Looking at the Blizzard Survival site some more

Once again, they had a picture with the colored side out and the shiny side facing in

And you can get it in 3 colors – what's that about? – marketing???

But, they said that if you have two space blankets on top of each other that the 2nd blanket adds little extra warmth. That tells me that to be effective, the aluminized layer either has to be on the outside, or facing an air space. If you face the aluminized layer in, then it will be against whatever clothing you're wearing – maybe a big enough air space on average to be a little effective.

I'm beating this subject to death – I'll stop now : )

[j lan]

Here's my guesses on the three colors

Silver – emergency personnel use, possibly most effective?
Olive drab – military use to avoid being spotted
Orange – outdoor use to double as signaling device

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