I have encountered a number of people carrying The Emergency Mylar Blankets or sleeping/bivy bags in their kits or vehicles. In talking with them, many do not understand their limitations.
Emergency Mylar blankets can be useful for keeping warm under the right conditions, if used correctly. Their primary purpose is to keep you warm under limited conditions, not get you warm. Now there are many brands of emergency Mylar blankets on the market and they are not created equal. They range from very cheap Chinese ones to a good quality bivy sack made by SOL which is a division of Adventure Medical. This one is reusable and breathes to help keep you dry. I have seen these Mylar blankets sell from less than 1 dollar to 40 dollars each. The size, thickness and quality of the blankets vary greatly.
Now these blankets work by reflecting a portion of your body heat back into your body. Depending on the brand, they claim from 70 to 90 percent. Therefore, you must have sufficient body heat to begin with, for them to be effective. The colder you become before using the blanket, the less effective it will be. Also you need a large enough blanket or bivy bag to cover you completely. Many of the less expensive one will not cover your whole body. When using the blanket, you should squat down and wrap it around you. If you choose to lay down make sure that there is insulation between you and the ground.
Mylar blankets are ineffective in the case of hypothermia. In the case of hypothermia, external sources of heat are required for survival. You need to have an outside source of heat to bring their body temperature up.
In an emergency situation, use the blankets before you lose body heat. The warmer you are before you use the blanket the better. Exercise is one way to warm your body up prior to using the blanket. However, don’t let yourself start to sweat. Dry clothing is essential for maintaining body warmth. Do all that you can to keep dry or to change into dry clothes. Keep yourself hydrated, but do not eat snow, it will lower your body temperature. Try the blanket out in various conditions and for various lengths of time before needing one in an emergency.
Other uses for Emergency Mylar Blankets.
Your Emergency blanket can be used to make a shelter from the elements such as sun, rain, wind or snow, They can be used as a ground cover. If you are lost, you can use the reflection to signal for help. They can be used to collect water. If you have a fire, they can be used to reflect heat. Emergency Mylar Blankets are a tool and have many uses, but like most tools, you have to understand their limitations and work within them.