Physics behind gold+silver space blanket placement

Question

In the first aid and mountaneering, "space blanket" or "astro foil" is used for protection against cold enviroment or as first aid after burns. There are many different versions of the product. This question relates to thin alumium-based foil of sizes around 200cm*150cm, in which the subject is wrapped. The foil consists of two different sides of different colors, one gold and one silver.

As poited out by @DavidHammen,

the gold-colored side is Kapton with a very thin coating of aluminum rather than gold. The silver-colored side is Mylar with a very thin coating of aluminum rather than silver

Example of such foil on Slovenian mountaneering shop, example on firerescue.eu.

Different manufacturers and books about first-aid advise using the foil with gold side out to prevent hypotermia in cold enviroment, and gold side in for cooling. I am interested behind the physics behind this claims. Are the claims correct? To what effect do the placement matter? Are the rules for placement in cold outdoors environment the same during the night and day?

The intuitive arguments (the ones I usually heard on this topic) behind the claims usually base on different albedo values for gold and silver, together with some derivation of Stefan's law for gray bodies: $M_{gray} = (1 - \alpha)\sigma T^4$, where $\alpha$ is albedo or reflection coefficient and $\sigma$ is Stefan–Boltzmann constant. This intuitive approach comfirms the usage-guide, but does not take into account other things, which could potentially effect the heat-transfer.

Some of the factors, ignored in the "intuitive" approach:

• Radiation of the enviroment and the difference between the night and day usage. At night, the sky and the air is much colder than at day.
• Conduction - direct heat transfer from body to the space blanket and from humid outside air to the blanket, which could effect the foil temperature
• Presence of air/other isolation materials between the subject and the foil (or can we assume vacuum around the subject?)
• effects of the outside layer of the foil

Due to the big number of different scenarios of the usage, I would prioritize debate on the following scenario: Foil used to prevent hypothermia in cold outdoor environment, subject wearing thick (3cm-8cm) clothing, which provide good isolation, and wrapped in the foil. No additional shelter between the subject and the environment. What is the effectiveness of the foil, how much does the placement matter, difference between nigh and day?

Additionally, can the theory be extended outside of the real-life scenario? What happens with different albedo values and different temperatures of the enviroment?

Answer 1

I am interested behind the physics behind these claims. Are the claims correct?

There's some truth to them, at least from a physics perspective. I am not at all sure of the day/night difference in performance, so my partial answer describes sunny days only.

A simple rule of thumb is that a good reflector is a lousy emitter and a lousy absorber, and vice versa (a good emitter is a good absorber but is a lousy reflector). However, albedo (reflectivity) varies with frequency. A space blanket has different colored sides, and those different sides have different reflectivities at different frequencies. Sunlight is predominantly in the visible and near infrared ranges, while thermal radiation at human body temperatures is in the much lower frequency thermal infrared range.

Taking advantage of how the two materials behave in those two different frequency ranges is key to survival (in sunlit conditions). In the visible and near infrared ranges, the gold-colored side is not quite as good a reflector as the silver-colored side. This is why one would want the gold-colored side on the inside and the silver-colored side on the outside in hot, sunny conditions. Reflect that sunlight away! The reverse is true for cold sunny days. One would want the more reflective side facing toward the skin. Soak up as much of the as paltry sunlight as possible!