We wanted to test a mylar sleeping bag to see if they really work. We found that they do have waterproof, wind breaking, heating properties.
In 35 degree weather with a moderate breeze and light rain I slid into an emergency mylar bivvy/sleeping bag and went into the back yard. I wore shorts, a t-shirt and flip flops.
I started out warm and dry and stood still within the 7 foot bivvy. I pulled it over my head and synched it up around my neck with my hands.
Here's what I found:
I went inside the garage to inspect the bivvy. It was torn in 3 places. I hadn't gone anywhere. I had only knelt and sat. The material had failed.
These emergency coverings are a good backup for normal insulated clothing. They are waterproof and windproof as long as you don't break out of them.
They work great when you can use their radiant properties. But they don't work at all as insulation. That means sitting or laying down without insulated clothing is not really an option because the ground will suck the heat out of your body.
We still believe these belong in a go-bag because the price and weight to function ratio is still worth it. We also recommend having insulating clothing in your go-bag.