In the old days, every caver I knew carried a space blanket. Many still do. They come vacuum-packed in little squares that look like tea bricks, promising to help prevent hypothermia. They were originally marketed as NASA technology – because, in fact, they were. The original Mylar “space blanket” came out of the 1960s U.S. space program, meant to keep satellites and astronauts thermally stable in the vacuum of orbit. They reflected infrared radiation superbly, but only because they were surrounded by nothing. In a cave, you’re surrounded by something – cold, wet air  – and that’s a very different problem.

A current ad says:

Reflects up to 90% of body heat to help prevent hypothermia and maintain core temperature during emergencies.

Another says:

Losing body heat in emergencies can quickly lead to hypothermia. Without understanding the science behind thermal protection, you might not use these tools effectively when they’re needed most.

This is your standard science-washing, techno-fetish, physics-theater advertising. Feynman called it cargo-cult science. But even Google Gemini told me:

For a vapor barrier in an emergency, a space blanket is better than a trash bag because it reflects radiant heat.

Premise true, conclusion false – non sequitur, an ancient Roman would say. It seems likely that advertising narratives played a quiet role in Google’s large-model-based AI system’s space-blanket blunder. That’s a topic for another time I guess.

Back to cold reality. At 50 degrees and 100 percent humidity, radiative heat loss is trivial. Radiative transfer at 10–15°C delta T is a rounding error compared to where your heat really goes. What matters is conduction to damp rock and convection to moving air, both of which a shiny sheet does little to stop. Worse, a rectangle of Mylar always leaks. Every fold and corner is a draft path, and soon you’re sitting in a crinkling echo chamber, shivering and reflecting nothing but a bad decision.

The contractor-size trash bag, meanwhile, is an unsung hero of survival. Poke a face hole near the bottom, pull it over your head, and you have a passable poncho that sheds drips instead of channeling them down your collar. You can sit on it, wrap your feet, and trap a pocket of warm, humid air that actually slows heat loss. Let a little moisture vent, and it works for an hour or two without turning clammy.

The space blanket survives mostly on myth – the glamour of NASA and the gleam of techno-hype. The homely trash bag simply works – better than anything near its size will. One was made for space, the other for the world we actually live in. Losing body heat in emergencies can quickly lead to hypothermia. Without getting the basics of thermal protection, you might not pick the best tool.

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For those who need the physics, read on…

Heat leaves the body by three routes: conduction, convection, and radiation. Conduction is direct contact – warm skin meeting cool air, damp limestone, and mud. Convection is moving air stealing that warmed boundary layer and replacing it with fresh, cold air. Warm air is lighter. You warm the nearby air. It rises and sucks in fresh, cooler air. A circulating current arises.

At 50°F in a cave, conduction and convection dominate. The air is dense and wet, which means excellent thermal coupling. Each tiny current of air wicks warmth away far more effectively than dry air would. A trash bag stops that process cold. By trapping a thin layer of air and sealing it from drafts, it cuts off convective loss almost completely. It also slows conductive loss because still air, though not as good as down, is a decent insulator.

Radiation is infrared energy emitted from your skin and clothing into the surroundings. It follows the Stefan–Boltzmann law: power scales with the fourth power of absolute temperature. That sounds dramatic until you run the numbers. Between an 85°F body (skin temperature, roughly, 300° Kelvin) and a 50°F cave wall, the difference is about 27 Kelvin.

q = σ T⁴ A where q is heat transfer per unit time, T is temperature difference in Kelvins, and A is the radiative area.

Plug it in, and the radiative loss is about one percent of total heat loss – barely measurable compared to what you’re losing through air movement. The “90 percent reflection” claim is technically true in a vacuum, but in a cave or in Yosemite it’s a rounding error dressed up as science.

So the short version: the shiny Mylar sheet reflects an irrelevant component of heat loss while ignoring the main ones. The humble and opaque trash bag attacks the real physics of staying warm. It doesn’t sparkle, it gets the job done.