The ImReadyCo Emergency Sleeping Bag is a sealed, person-sized bag you climb all the way inside. The outside is a windproof, water-resistant shell, and the inside is lined with a reflective layer that bounces your own body heat back toward you.

Because you're inside it instead of under it, it traps a pocket of warm air around your whole body. A flat foil blanket can't do that, since it slides off and lets the wind in at the edges.

It needs no power, no fuel, no fire, and no setup. You just pull it out of the sack and climb in.

At home, it works with your blankets, not instead of them. Climb in, then pile your blankets on top. The bag seals the gaps and sends your heat back to you, and the blankets add the thickness.

To see why that matters, it helps to know the four ways your body loses heat:

Radiation. Your body gives off heat all the time, like a warm stove. In a cold room, that heat just leaves. The reflective lining sends part of it back to you instead of letting it escape.

Wind. Moving air pulls heat off your skin fast, whether it's a winter breeze outside or a cold draft under the door at home. The sealed shell blocks it, so the warm air around you stays put.

Wet. Rain, snow, or damp clothes pull heat out of you quickly. The shell sheds rain and splashes, so you stay drier.

The ground. Cold floors and frozen ground drain heat right out of your body. This is the one the bag can't stop on its own. Put something between you and the cold surface, like a couch cushion, a folded blanket, some cardboard, or your car seat.

It handles three of the four. For the fourth, all you need is something to sit or lie on.