Basically, a Survival Blanket is a sheet of foil which reflects most of a person's body heat. It's made from a material developed by NASA in 1964, which is why it is often known as a ‘space blanket’.
Emergency Blanket might be a better name.
Most Survival Blankets are made from Kelvalite, which is a material made from plastic, coated with a metallic reflecting agent. It looks a bit like aluminium tin foil. You can also get gold coloured blankets, which are popular with photographers as a makeshift reflector because the gold colour improves skin tones.
Whatever it is made from, its ability to reflect most of a persons radiated body heat is why it is such an invaluable survival aid.
Retaining Body Heat
The body loses heat by radiation, convection, conduction, evaporation and respiration.
A space blanket will reflect between 80% and 95% of radiated body heat, will help cut down on heat loss by convection (wind chill), and can cut down on heat loss by evaporation (sweat) if you ensure there are no gaps where moisture can escape.
Although it will help a little against heat loss by conduction, this can be fairly minimal. The obvious place for heat loss by conduction is the ground and you would be much better placing the patient on a camping mat or sitmat.
Extremely Compact & Lightweight Survival Aid
It's compact size when folded, light weight, and thermal qualities have made the space blanket a virtual 'must have' in a survival kit. Water proof and windproof as well, they really could save someone's life - indeed have on many occasions.
They can be a very important part of your survival kit, especially if it's likely to be cold.
Remember, that even in the summer a person could get hypothermia after prolonged exposure in the water.
When folded they take up less space than a 2 oz tobacco tin and weigh about 60 grams.
And, costing only about £2, a survival blanket really should be part of your essential survival equipment.