Emergency blankets are made of a thin plastic film, usually polyester (often sold under the brand name Mylar), coated with an ultra-thin layer of aluminum. This combination creates a lightweight, compact sheet that reflects your body heat back toward you. The blankets weigh only a few ounces and fold down small enough to fit in a pocket, which is why they’re a staple in first aid kits, marathon finish lines, and backcountry packs.
The Two Layers That Make It Work
The base material is polyethylene terephthalate, or PET, the same type of plastic used in soda bottles and food packaging. PET is strong relative to its weight, tear-resistant enough for field use, and has a melting point of about 254°C (roughly 490°F), so it holds up well under normal outdoor conditions. On its own, though, PET film wouldn’t do much to keep you warm.
What gives an emergency blanket its distinctive mirror-like finish is an extremely thin coating of aluminum deposited onto the plastic through a process called physical vapor deposition. In a vacuum chamber, aluminum is heated until it evaporates. The PET film passes near this metal vapor, and the aluminum condenses onto the surface in a layer so thin it’s measured in fractions of a micron. The result is a sheet that looks and feels like metallic foil but is far lighter and more flexible than actual aluminum foil.
How It Keeps You Warm
Your body loses heat in four ways: radiation (infrared energy radiating off your skin), convection (wind carrying warmth away), evaporation (sweat or rain cooling you), and conduction (touching cold ground or surfaces). An emergency blanket primarily targets radiation and convection.
The aluminum surface reflects infrared radiation, which is the main form of heat your body emits. A single-layer emergency blanket with the metallic side facing your body blocks a median of about 82% of infrared transmission, based on laboratory measurements published in the journal Coatings. The range across different conditions falls between roughly 73% and 92%, depending on factors like how many clothing layers sit between you and the blanket. Stack three layers of the material and it blocks essentially 100% of infrared transmission.
The plastic film itself acts as a windbreak. By creating a barrier between your skin and moving air, it reduces convective heat loss and slows evaporation from wet clothing or skin. This combination of radiation reflection and wind protection is what makes such a thin, fragile-looking sheet surprisingly effective at preventing hypothermia.
NASA Origins
NASA first developed the metallized film technology in 1964. The original application was spacecraft insulation, not wilderness survival. Aluminized Kapton film, with foil thicknesses of 50 and 125 micrometers, was used on the Apollo Lunar Module to protect equipment and astronauts from extreme temperature swings in space. The consumer version, repackaged as the “space blanket,” followed soon after, using cheaper PET film instead of Kapton to bring the cost down to a few dollars per blanket.
Different Versions for Different Uses
The classic disposable emergency blanket is the thinnest and cheapest option. It’s essentially a single sheet of metallized PET, usually silver on one side and sometimes gold on the other. These are designed for one-time use. They tear easily, crinkle loudly in the wind, and don’t hold up well to repeated folding and unfolding.
Heavier-duty versions exist for more demanding situations. Emergency bivvies (bag-shaped blankets you climb inside) often use thicker PET film or reinforce the metallized layer with additional plastic to improve durability. Some are designed as reusable gear and may incorporate heat-sealed seams. Mountain rescue teams typically use multilayer wraps that combine a reflective vapor barrier with insulating outer layers, creating a more complete system than a single sheet can provide.
Limitations Worth Knowing
Emergency blankets have real drawbacks. The biggest is that they’re completely non-breathable. The same vapor barrier that blocks wind and reflects heat also traps moisture from sweat and respiration. Over time, condensation builds up inside the blanket, soaking your clothing. Wet clothing conducts heat away from your body far faster than dry clothing, so in prolonged use, the blanket can actually work against you if you don’t manage moisture.
They also provide zero insulation from the ground. Radiation reflection only helps when there’s an air gap between the blanket and your body. If you’re lying directly on cold ground with the blanket beneath you, your body weight compresses it flat, and heat escapes through conduction. Placing insulating material (a foam pad, a pile of dry leaves, a backpack) between you and the ground matters more than the blanket itself in that scenario.
Wind is another challenge. The material is so light that it flaps and tears in strong gusts. If you can’t secure the edges with rocks, your pack, or your own body weight, the blanket may become useless before it has a chance to warm you. The thin metallized coating can also scratch or flake off with rough handling, reducing its reflective effectiveness over time.
Beyond Warmth
The highly reflective surface serves a secondary purpose in survival situations: signaling. The mirror-like finish catches sunlight and can be visible from a significant distance, making it useful for attracting the attention of search aircraft or rescue teams. Some people carry emergency blankets specifically for this dual function, since few other items in a survival kit weigh so little while serving as both thermal protection and a visual signal. The gold side, when present, is slightly less reflective but can be oriented outward in hot conditions to reflect solar radiation away from you rather than trapping heat.