Susan,

You don't necessarily die of hypothermia if you are damp. It is OK to be warm & damp, it's just not particularly comfortable. Hypothermia comes from being damp/wet & exposed.

The mylar survival blankets were never meant to compete with a nice fluffy down sleeping bag tucked inside a breathable bivy sack. The mylar survival units are just that: for survival, not comfort. You sacrifice the comfort for compactness, light weight, and easy availability.

FYI the ThermoLite bivy sack from Adventure Medical has a mesh vent at the foot end. You adjust the opening at the head end to provide some airflow through to the foot vent. Or you can seal it up tight.