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.
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