How Does a Thermal Blanket Work?

The Basics

A thermal blanket, also known as an emergency blanket, Mylar blanket and space blanket, is a thin sheet of plastic with an extremely reflective coating. It is often used to treat hypothermia, but can also be used to make sleeping bags warmer, to construct homemade solar heating ovens and for other projects as well. It is either silver or gold in color, and is usually big enough to wrap an adult in. When it is folded up it is extremely compact, and it weighs very little, which makes it useful for emergency kits.

People lose heat through thermal radiation. All objects radiate infrared energy. The warmer the object, the more energy is radiated, cooling the object. A thermal blanket is more than 80 percent reflective. That means that more than 80 percent of the thermal energy that reaches it is deflected back towards its source. When someone is wrapped in a thermal blanket, his own reflected infrared heat is reflected back towards him, warming him up more quickly.

People also lose heat through evaporation and convection. When sweat or other liquid on the body evaporates into the air, it takes some body heat with it. That is why people perspire--as a way to keep cool. Convection--the movement of air currents--can speed up evaporation, cooling the body much more quickly. If someone gets wet in an extremely cold environment, evaporation can become a big problem, causing hypothermia or even death. A thermal blanket forms a barrier between the user and the surrounding air, preventing moisture from escaping and carrying heat away.

The biggest weakness the thermal blanket has is in stopping conduction. When a warmer object comes in contact with a colder object, heat slowly flows from the warmer object to the colder one in a process called conduction. The best way to stop conduction is with a thick insulating layer, such as a winter jacket, which can slow the flow of heat. Because a Mylar blanket is so thin, it only has a limited ability to prevent heat conduction. Because of its other useful thermal properties, however, it can still help keep someone warm in an emergency situation.