A space blanket (Wikipedia) is an extremely small and lightweight survival blanket made from a heat reflecting material. These blankets are normally thin Mylar and coated with a material that reflects infrared waves. They are normally rain and wind proof. Every emergency and hiking kit should include some form of space blanket. Space blankets come in a few different styles including:

The traditional space blanket is very small, light, effective, and inexpensive (less than $10). Place at least one in your vehicle kit, hiking kit, and bugout bag.

The heavier space blanket is more durable and can be used as a tarp, shelter, or blanket. These are ideal for car emergency kits because they are versatile, reasonable small, and inexpensive (less than $20). I routinely use it under my tent to protect the floor.

A space blanket bivy is a space blanket sealed on three sides to form a sleeping bag. This style bivy traps and reflects your body heat very efficiently. There are different types of space blanket bivies. Click here for more details.

Space blankets can be used for a variety of functions including keeping yourself warm, creating a shelter, and distress signal. Some space blankets have two sides: one highly reflective (normally silver or gold) side, and one side that is normally red or green. With these blankets place the bright side towards you to retain heat. Place the reflective side outwards to reflect heat outwards in high sun environments such as deserts.

Recommendations

My Vehicle Kit contains a heavy space blanket, a lightweight space blanket, and a space blanket bivy. The heavy blanket can be used as a tarp or cover as well as a space blanket. If you are evacuating in your vehicle then you may have others with you. Having multiple blankets gives you more options.

My Hiking Kit includes a space blanket bivy. You would not go amiss by also including a lightweight space blanket. The lightweight blanket could be used as a cover, a signal, or as a blanket.

My Bugout Bag includes a heavy space blanket, space blanket bivy, and sleeping bag liner. The heavy blanket could be used as a blanket, tent footprint, tarp, cover, and signal. The bivy can be used for warmth, rain protection, etc. The sleeping bag liner can be used for warmth and help with condensation that would collect inside the bivy.

!!! IMPORTANT !!!

Many people buy a space blanket, put it in their kit, and never use it. One problem is that these thin blankets can deteriorate over time. A search and rescue worker told me how space blankets can fall apart along the folds if they have not been used for many years. With your space blanket or bivy be sure to unfold it every year or two and then repack it. Replace space blankets that have weak spots. Always make sure that your equipment is in good working order and be sure to periodically check your space blankets.