Space blanket

An aluminized mylar sheet that can be wrapped around a person to provide a wind shield or retain heat. They can be packed very small and are quite light so there is little reason to avoid having one.

The standard mylar blanket is prone to tearing. If you want a tougher version, consider the aluminized tarp style. These are heavier, made with the same material commonly used to make inexpensive plastic tarps. They have a aluminized coating on one size and usually have grommets in the corners.

There are also space blankets that come in the form of a tube or mummy bag. These are better shaped to wrap a person in.