Do emergency mylar sleeping bags actually keep you warm?

Across specialist-community discussion on r/Ultralight and r/Survival, the consensus is yes, but with caveats: reflective mylar bivies can reflect a large fraction of radiated body heat and block wind and rain, but they don't insulate the way a real sleeping bag does. You still need a ground pad, debris pile, or insulation underneath, and reviewers consistently warn about condensation building up inside non-breathable models.