What's the difference between an emergency bivy and a space blanket?

A space blanket is a flat sheet of metallized mylar you wrap around yourself, while an emergency bivy is sewn into a sleeping-bag shape you climb inside. Verified-purchase reviewers consistently note that the enclosed bivy traps more heat and blocks wind better, while flat blankets are cheaper, lighter, and more versatile for multi-use first-aid or signaling. Many buyers keep both.