RADBAG is a compact and lightweight bivy bag that you get into when you need protection from the elements. It was developed for adventure racing, where you need to be able to deploy a sleeping system in seconds, catch some trailside sleep and then get going again in seconds. Unlike foil-based survival bags and space blankets, it’s durable, so it won’t rip even when you clumsily climb into it with your shoes on, and it doesn’t make that annoying crinkly sound every time you move, irritating your team mates!
While not as warm, it’s lighter than even the lightest sleeping bag plus it’s much more wind- and water-resistant. You wouldn’t mind laying it down on damp or muddy ground, unlike your down bag.
So if it’s really cold and you are sleeping out for the whole night (e.g. on a hike or winter bikepacking trip), you’d put your sleeping bag inside it to keep the dew and windchill off you, and your sleeping bag clean.
It comes with its own stuffing bag sewn to the bivy, designed in such a way to allow very quick packing away. It also has a drawcord at the top to pull it in tight over your head, with a breathing slit for your face. Lastly, it features tags at the foot end for ease of hanging out to dry.