Why emergency blankets look like giant sheets of foil; a thin metallic coating reflects radiant heat toward the body

This seemingly large sheet of foil is actually just a demonstration of physics in action. Emergency blankets, those crinkly foil sheets distributed following disasters, carried by hikers, and packed into survival kits, are nearly comically simple in their construction. But don't be fooled: the reflective foil is doing some unexpectedly complex things.

As explained in a NASA Spinoff report about the development of the technology, the reason why this doesn't generate heat is that it's not the blanket's job to do so. What the metallic coating on the blanket does is reflect the radiant heat towards your body, which reduces one mechanism for losing heat. In essence, the emergency blanket is a tiny boomerang that returns the heat radiating away from your body.

This leads to the explanation for the foil-like appearance. Behind the instantly recognizable silver shine, there is a metal coating, usually applied to a film of plastic, designed to reflect infrared radiation. While it might look like the world's largest sheet of kitchen foil, in actuality, the blanket's role is much more interesting: to prevent heat from being lost.


A leaky space station gave us a very weird camping accessory

The odd story about a product that was once intended to protect astronauts and now protects marathoners dates back to the 1950s when Mylar, a thin plastic film, was invented by DuPont. Engineers at NASA’s Marshall Space Flight Center then started to coat Mylar with a super-thin vaporized aluminum layer to help maintain the temperature of the spacecraft in the mid-1960s. However, the product started gaining popularity in 1973 when the heat shield of Skylab was ripped off upon launch and the temperatures inside climbed toward 130°F.

According to the same NASA account, engineers hurried to make an emergency parasol from the reflective material to reflect heat away from the space station and save the mission. Soon, a few years later, race organisers came up with the idea of doing the opposite to keep exhausted runners warm upon crossing the finish line, instead of obstructing the heat, in cold weather, and “space blankets” were invented.

Why shiny beats fluffy when it comes to body heat

This is where things go wrong for most folks: the emergency blanket isn't actually insulation like a puffer or a wool coat. Traditional forms of insulation function by trapping air pockets, which conduct heat poorly. Instead, the space blanket employs a completely separate mechanism: radiation. Our bodies lose heat continuously as invisible infrared radiation, which can either pass through a plain fabric or get absorbed by it.

On the other hand, the metallized surface reflects the radiation like a mirror. A research paper titled “Rescue Blankets-Transmission and Reflectivity of Electromagnetic Radiation,” published in 2020 in the journal Coatings, shows that measuring the interaction of these blankets with electromagnetic waves led the metallized surfaces to reflect the far-infrared heat from our skin onto us instead of letting it escape into the environment. That is the whole story packed into a material that can seem thinner than human hair.

The part you should keep in mind

That is where the space blanket falls a little bit short of glory and where millennials enthusiastic about survival hacks should take note. Heat reflection is only part of retaining body heat; you lose body heat also via convection (the wind removing heat from your skin) and via evaporation (moisture in your clothes and perspiration). A solid foil sheet is, in principle, both windproof and largely non-breathable, so it should offer some resistance to both. In practice, though, a lightweight, single-layer emergency blanket falls well short of that theoretical potential: it's thin, easily torn, doesn't seal at the edges the way a real garment does, and traps no insulating layer of still air against the skin.

As per a 2017 study titled “Measurements of rates of cooling of a manikin insulated with different mountain rescue casualty bags,” published in the journal Extreme Physiology & Medicine, which compared the performance of various rescue insulation materials under genuine cold air conditions, lightweight foil-based survival blankets lost heat almost three times faster than bulky windproof casualty bags used by mountain rescue squads.

Translation: That lightweight blanket in your emergency kit is really helpful when trying to stay warm in case of a short setback after finishing a race or when stuck in a broken-down vehicle, but it is not a replacement for appropriate, windproof layering if you are stuck overnight on a mountain. Indeed, you should still carry one; you just have to adjust your expectations. It is cheap, compactable enough to fit into small spaces, doubles up as a windbreaker, ground cover, or even a signal fire for search teams due to its reflective surface.

But do not mistake this item for a sleeping bag substitute or anything else that keeps you warm in extreme or prolonged cold. NASA developed the underlying material to protect Skylab, a space station whose program cost was commonly put at around $2.2 billion, from overheating after its sun shield was torn away at launch; the same reflective principle now helps keep a runner from getting chilled at the finish line of a marathon.