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  1. #1
    Senior Member TeeDee's Avatar
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    Reflective Barriers

    Reflective Barriers

    There is persistent myth I see being quoted sometimes on the forums.

    To wit:

    Reflective surfaces, e.g., a Space Blanket, SB, need an air gap to work or be effective in containing heat.

    This myth is totally, 100% percent false.

    For those that want further details read-on.

    Background

    Infrared, IR, energy is generated throughout the volume of a warm body. For all practical purposes, almost all of the IR energy is then re-absorbed within the body itself. The IR energy that escapes the body is generated within a very thin layer on the surface of the body.

    If the IR energy generated near the surface is not re-absorbed by the body itself, it escapes the body. This is the IR energy seen using special film or special optical devices.

    Note that for the IR energy to escape the body is not dependent on other bodies which may be in contact with the surface of the body. This is true whether the other body in contact is air, skin, fur, nylon, etc.

    Once the IR energy has exited the body it has escaped.

    I repeat, this is not dependent on what is in contact with the surface of the body except in very special cases for which the optical properties of the two bodies cause the IR energy to be reflected back into the body. Special coatings on lenses which reduce or eliminate internal reflections at the air/lens boundary are cases in point of such special optical properties. In that case the attempt is to enhance escape of the energy rather than contain it.

    Once the IR energy has escaped the emitting body, it will be absorbed or reflected by the air or another body upon which the IR energy impinges. The absorbing or reflecting body may be separated from the emitting body by a gap or it may be in contact with the emitting body. If the second body has a reflecting surface in contact with the emitting body, then the IR energy is reflected back into the emitting body and re-absorbed therein.

    Note that it is irrelevant if the reflective body is in contact with the emitting body or separated from the emitting body, the reflective properties are unaffected and the IR energy is reflected.

    It is also irrelevant to the IR emission of energy if another body is in contact with the emitting body. The contact of a second body is important and necessary for the pysical conduction of heat through thermal excitation of the atomic and molecular structures of the two bodies, but not the IR generation and emission. The IR emission is only affected by the more rapid cooling of the emitting body by the more effective cooling by heat conduction.

    Consider, for example, the glass Vacuum Thermos bottle used for many years to keep a liquid hot, such as your favorite coffee. The glass container is a double walled container with the space between the walls evacuated to a near vacuum. The vacuum reduces the convective transfer of heat. However, the radiative transfer, i.e., transfer via IR energy, is unaffected by the near vacuum. Thus, such containers also have a highly reflective coating on the inside surface of the glass bottle to reflect the IR energy back into the liquid (or back out of the bottle in the case of a cold liquid within the bottle). Thus, the bottle greatly reduces both the conductive transfer and the radiative transfer of heat and the coffee is kept hot for a much longer period of time.

    If the reflective coating on the inside surface is not accomplished, the liquid inside would lose heat rapidly through radiative transfer, i.e., IR energy radiation, through the glass and near vacuum to the outside environment. Experience has proven this to be true, hence the reflective coating. That coating is not done for aesthetic purposes, it is too costly for that.

    You will note that it is not necessary for the reflective coating to have an "air gap" between the hot liquid and the reflective surface, to be effective in reflecting the IR energy back into the hot liquid.

    Thus, an air gap is entirely irrelevant for the proper functioning of the reflective properties of a Space Blanket.

    This is the reason that the SB is so effective within the Hennessy SS, not only as a vapor barrier, but also in containing the occupant's heat. It is equally effective as a radiative barrier in any hammock, not just the Hennessy SS.

  2. #2
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    Most excellent! That means a torso sized piece of space blanket should be in the kit of everyone who uses an UQ. Knowing the limits of your particular system should allow you to deploy the space blanket between the hammock and UQ in marginal conditions. Since the sweating response is markedly reduced in even a slightly chilled person little condensation should occur for most.

  3. #3
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    I agree with your general principle. However, the distance is important. Their is a maximum allowable distance. I.e. if the reflective barrier is to far away (or even reflecting the heat at the inappropriate angle) the heat will be lost through diffusion. What needs to be considered is "Input - Loss of Storage (Air between layers) = Output

  4. #4
    Senior Member Ramblinrev's Avatar
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    The blanket is most effective when used directly next to the skin - if required material blankets may be wrapped on the outside of the Emergency Blanket.
    From: http://www.traditionalmountaineering...ceBlankets.htm
    I may be slow... But I sure am gimpy.

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  5. #5
    Senior Member fin's Avatar
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    From another reputable source:

    A radiant heat barrier works by reflecting radiant heat back toward the source. It does not reflect conducted heat, nor can it reflect heat within a solid object. In a vacuum, such as outer space, radiant heat barriers are very effective. In this air-free environment there can be no conduction (except through solid objects) so all heat transmission is by means of radiation. A radiant heat barrier on the outside of a space ship or satellite proves a very efficient insulator by reflecting back up to 95% of the radiant heat which strikes it. For this same reason, thermos bottles (ie. Dewars flasks) are also coated internally with aluminum radiant heat barriers.

    However, once our space ship returns to the atmosphere of earth, it's radiant heat barrier becomes considerably less efficient. This is because the source of the heat is no longer completely radiant in nature. The warm air is also transferring heat to the ship's skin through conduction. How much good the radiant heat barrier is now doing depend on several factors. If the ship is parked on the runway in the bright sun, the percentage of radiant heat to conductive heat would be very high and the barrier would still be quite helpful. Once the ship is moved into a hot hanger, the conductive heat of the surrounding air become much more dominate and the barrier contributes little.

    One example of the misuse of radiant heat barriers can be seen in the common practice of laminating foil sheets between pieces of standard foam insulation. In this case, all heat reaching the foil barrier is conductive and passes straight through making the barrier useless. Radiant barriers reflect infrared most effectively back into a vacuum. As the density of the material in contact with the barrier increases the effectiveness decreases. A barrier which would be highly efficient in space would be totally ineffective if sandwiched between insulating foam.


    It seems that having a vacuum to reflect infrared radiation back into is the most effective way to use a radiant barrier. The closest thing to a vacuum that we can create in our hammocks is dead air space. Wouldn't that seem to suggest that having dead air or an air gap would be a good thing with a radiant barrier? I don't know - I'm not a physics major. Having an insulative layer outside of the radiant layer is the most effective insulation, and having a vacuum between the radiant layer and you appears to be the most efficient use of a radiant barrier.

    Reflective layers on their own are almost useless as insulators, but are good at doing exactly what you state - reflecting IR energy. That is why they need to be as close to you as possible, with as little between you and the reflective layer as possible.

    Quote Originally Posted by TeeDee View Post
    The IR emission is only affected by the more rapid cooling of the emitting body by the more effective cooling by heat conduction.
    Agreed. If your emitting body is also cooling through conduction quicker due to inadequate insulation, the radiant properties would lessen as the body cools.

    Quote Originally Posted by TeeDee View Post
    You will note that it is not necessary for the reflective coating to have an "air gap" between the hot liquid and the reflective surface, to be effective in reflecting the IR energy back into the hot liquid.

    Thus, an air gap is entirely irrelevant for the proper functioning of the reflective properties of a Space Blanket.
    What you really prove is that you need insulation on the outside of your reflective layer to prevent the loss of heat through conduction, not that an air gap is relevant or irrelevant. If you have a cheap plastic thermos, it loses heat faster than a good metal wall thermos with a vacuum between the reflective inner wall and the outer steel wall. Doesn't this imply an air gap or insulation outside the reflective material? This doesn't mean that it is required, just that insulation or an air gap with an outer insulative layer is effective in combination with reflective materials.

    Last edited by fin; 02-18-2009 at 22:10.

  6. #6
    MacEntyre's Avatar
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    /engineer mode on

    Radiant heat is not restricted to travel through air. Nor is a reflective layer effective only in air. As FFinn's source said, it radiates from solid objects, not through them.

    House insulation has a reflective layer with no air gap, on the side of the insulation batting facing the living space, which is immediately adjacent to the inside of the sheetrock wall. There is no designed air gap, only air gaps from construction flaws.

    The intensity of electromagnetic radiation is inversely proportional to the square of the distance from the source. The further from the source, the more radiant heat is lost permanently. Low intensity radiation is reflected, but doesn't travel as far. Therefore, the reflective surface should be as close to the source as possible, which would be immediately underneath the hammock, on top of all insulation.

    /engineer mode off
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  7. #7
    Senior Member Frawg's Avatar
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    TeeDee, I thought that was an excellent writeup. I would agree with your "100% false" claim if you reworded the 'myth' to replace "containing heat" with "containing radiant heat". That's pretty much all you discussed -- and appropriately so, since radiant heat containment is the point of the SB.

    I think the 'myth' arises because the term 'heat' gets used in ways that muddy the relative significance of the underlying physical mechanisms in various situations. In HVAC applications, for example, Reflectix calls for an air gap in their installation instructions . Clearly the gap does nothing about radiant heat loss, but it is significant for controlling convective and conductive heat loss. I think it's easy for folks to get confused about what parts are played by different components of the whole insulation system, and come away with some faulty generalizations.

    Re your optical coating observation:
    I repeat, this is not dependent on what is in contact with the surface of the body except in very special cases for which the optical properties of the two bodies cause the IR energy to be reflected back into the body. Special coatings on lenses which reduce or eliminate internal reflections at the air/lens boundary are cases in point of such special optical properties. In that case the attempt is to enhance escape of the energy rather than contain it.
    I don't see this as a special exception, when you consider the whole picture. Outside the spectral passband implemented by the coating, reflections are actually worsened. By controlling system impedance as a function of frequency, reflections are just shifted by design to occur outside the photographically interesting part of the spectrum. IAE, it's still a passive system and there is no amplification of energy -- just an increased attenuation of the photographically objectionable stuff. In this case, the attenuation is the result of incident energy being reflected back to the source. It's not unlike the design of stub tuned filters for waveguides or coax systems, IIRC.

    IMHO, of course... it's been a while since I last had to deal with Mr. Maxwell.

  8. #8
    Senior Member animalcontrol's Avatar
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    Quote Originally Posted by MacEntyre View Post
    Radiant heat is not restricted to travel through air. Nor is a reflective layer effective only in air. As FFinn's source said, it radiates from solid objects, not through them.

    House insulation has a reflective layer with no air gap, on the side of the insulation batting facing the living space, which is immediately adjacent to the inside of the sheetrock wall. There is no designed air gap, only air gaps from construction flaws.

    The intensity of electromagnetic radiation is inversely proportional to the square of the distance from the source. The further from the source, the more radiant heat is lost permanently. Low intensity radiation is reflected, but doesn't travel as far. Therefore, the reflective surface should be as close to the source as possible, which would be immediately underneath the hammock, on top of all insulation.

    /engineer mode off
    So to summarize in layman terms...
    hanger>hammock material>reflective vapor barrier>UQ (if UQ is used)
    right? Or did I miss the point (again)?
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    MacEntyre's Avatar
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    Quote Originally Posted by animalcontrol View Post
    So to summarize in layman terms...
    yep! that's it!

    Also, if the reflective vapor barrier is not right against the hammock, it is just a vapor barrier.
    - MacEntyre
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  10. #10
    all secure in sector 7 Shug's Avatar
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    I love all this Smart-People Talk .......... Thanks for the break down.
    Whooooo Buddy)))) All Secure in Sector Seven

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