>With all due respect, your experiment actually proves the opposite.
And with all due respect in return, I think you misunderstood my simple experiment. Your question was, how well does a space blanket conduct heat? And the answer is 'quite well', as the experiment showed. "Feel" had nothing to do with it (that's why I used a thermometer instead of my hand on the other side of the mylar). Your experiment also showed that the mylar space blanket conducts heat readily, since your ice cube melted, and thus we agree.
>Conclusion: a mylar space blanket conducts less heat and is actually a far better insulator than silnylon.
But that wasn't the question. It is irrelevant that silnylon is not a good insulator. A mylar space blanket is not a good contact insulator either, even if it is better than silnylon. Anybody who's slept in a hammock on a cold night on only a space blanket can attest to that. It's not theory: I've done that for several nights at temps ranging from +40F to +27F and a space blanket is simply insufficient. (I related that experience in my first post in this thread. I assume my observations are not unique.)
>Was this due to the properties of the mylar material itself? Was this due to the reflectivity of it, providing a radiant heat barrier thereby insulating my hand more?
The relevant property is the thermal conductivity of the mylar material. The reflectivity is not a factor because your hand and the mylar are in direct contact and and thus the two _surfaces_ are at the same temperature (no temperature differential, no net radiant heat loss). The materials behind the surfaces, the interiors of the cube and your hand, are not at equilibrium and so heat transfer is occurring through conduction.
>People spout off about clouds in the atmosphere or sleeping under a tree - both of which have nothing to do with the radiant heat given off by a warm, living body. No one has any actual evidence to back their statements.
On what grounds do you make that claim? I would really like to know why you consider this to be untrue and unproven. This is supported by both theory and actual observation. I suppose your statement is literally true because other objects don't have anything to do with the radiant heat given off by a warm living body (about 500 watts), but that's just sophistry. _Net_ radiant heat transfer is a function of temperature differential between two bodies, and water vapor in the atmosphere (clouds) or trees are much warmer than the cold of space. The sky radiant temperature in dry air at night can be _much_ colder (a difference of -40*C or more in arctic regions) than the ambient air temperature. Thus net radiant heat loss under clouds, trees or bushes is significantly lower than under open sky, since these are rarely colder than ambient air temp. (Google "sky radiant temperature".)
This sounds familiar, and I realize now this just came up a month ago in this thread. PJ posted the relevant equations (Stefan-Boltzmann Law of Radiation) a year or so ago in another thread, and I'm sure Richard Nisley could carry this on better than I in the formal domain if you need those equations explained.
Here's the basic point: net radiant heat loss is all about temperature differential. If you're wearing clothes, especially appropriate body wrappings for the cold outdoors including sleeping bags and pads, the _outside_ of your wrapped body is not going to be significantly warmer than the objects surrounding you (e.g., a hammock, tent, tarp, forest, etc.). Thus there is little opportunity for net radiant heat loss. (Space blanket use in an emergency situation with insufficient clothing is not relevant here.) Nobody is saying that there is _no_ radiant heat loss, but appropriate measures to prevent convective and conductive heat loss have the side effect of covering the body with a wrapping that has an essentially ambient surface temperature when exposed.
>But where is the science? Where is the proof? There's plenty of science out there for other applications of radiant heat loss...
The math (if not the science) is simple: the relevant part of the Stefan-Boltzmann Law of Radiation states that the net radiant heat transfer is a function of the difference of the fourth powers of the absolute temperature of the radiant bodies. Taking an example of a wrapped body that is 5*C warmer than its surroundings, the net radiant heat transfer is about 25 watts; not much.
The reason radiant heat loss is significant in other applications is that those generally involve large areas and/or high temperature differentials, e.g., houses. Nobody wraps houses in a foot-thick down sleeping bag, and although high-R thermal insulation is used to insulate a house the outside of the house still ends up being much warmer than the surroundings, which is generally open space, not a tent. Thus, radiant heat barriers are useful in reducing heat transfer to the siding. Although these radiant heat barriers do use a radiant-reflective surface, they also have some kind of 'air gap' (whether air, nitrogen, vacuum or whatever in the form of an open space, large bubbles or a foam) in order to allow some space for the internal body to radiate and the reflector to reflect, while not 'touching' to a significant degree which would allow heat loss through conduction. As I see it, the point of the 'air gap' is to allow the heat from the warm radiant body to radiate and then reflect from the colder radiant barrier, without signficant transfer of heat to the radiant barrier though conduction or convection. (That's why Reflectix does work somewhat as a hammock pad while a mylar space blanket does not.)
And now we're right back where we started. No air gap, heat loss through conduction, no significant reduction of heat transfer to the atmosphere. A cold night in a hammock is the result.
I don't think anybody has claimed that radiant heat loss isn't possible, it's just that it isn't significant with proper and sufficient insulation to prevent conductive and convective modes of heat loss. Further, radiant heat barriers like space blankets are also vapor barriers. Moisture accumulation is a significant problem on its own, possibly greater in its own way than radiant heat loss.
>Yes, this is a good idea and, yes, someone has marketed it... and has been doing it for many decades. That would be Stephenson Warmlite.
Warmlite vapor barriers are not used on their own without a sleeping bag or air mattress. 'Nuff said.
>I would recommend using a more insulative backing, like maybe a simple closed-cell foam pad (the blue ones from REI are pretty cheap). This would add R-value and 'support' the space blanket, while the fleece also protects the space blanket and prevents direct contact with your skin (assuming you're not completely clothed under bag). If your fully clothed, you might even skip the fleece.
And you might also skip the space blanket, since the foam pad will significantly insulate the body and it doesn't radiate much heat. However, if the space blanket is placed between the foam pad and the hammock then it might help a little. (That is actually hard to quantify without lab equipment.)
>The fact that it doesn't "breathe" is not an issue - i.e. no condensation problems here... IF you're using it only as a bottom layer. I have yet to hear of anyone complaining about or wishing for a "breathable" sleeping pad.
I complain. When ground sleeping there isn't quite as large an area of contact between the sleeping pad and the body as there is between the body and material in a hammock, which also limits air/moisture circulation. After a single night in a hammock with a space blanket I noticed a significant decrease in the loft of the down on the underside of my sleeping bag; I have never noticed this when ground sleeping. Subsequently, when using a layered pad similar to that originally proposed in this thread, I found that the space blanket actually had drops of water on it in the morning, sufficient that I felt it necessary to shake it off before folding it. So condensation, and resulting damage to the compressed loft, is a factor with non-breathing underlayers in a hammock.