Think in terms of how you lose heat. The wind proof and water proof properties of the blanket do help significantly, but these blankets have no insulation. They do reduce heat loss. Every degree you keep could potentially be a life saver.

SOL Emergency Blanket

These new emergency blankets are good. I have used them as a wrap and as a shelter. As durable as the old "Space Blanket", with grommets.

With some insulation underneath, their utility goes way up. I used to use the space blankets in conjunction with wool blankets and poncho liners, even in sub freezing weather (sometimes down in the 20s F), and survived. Not as comfortable as a good sleeping bag, but it works.