I wonder whether one of the heat wraps that get distributed to runners at the end of a race would work well? Has anyone tried tucking one into a liner? Super lightweight, but their ability to keep in the heat is crazy. I suppose it could be a little noisy if you roll around.
No…and yes. Those mylar blankets are way too noisy (as noted elsewhere) and while they hold heat well, they also collect moisture so you wake up in the middle of the night dripping wet from your own condensation. However:
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Makes an interesting bivy…however it also is noisy as heck and rather hard to fold. There is a hack however. I put it into my front load washing machine on gentle, cold water with NO soap. Then hung to dry. It becomes quite soft and much, much less noisy, no more than any other nylon bag slithering around.
The additional hack, is you should carefully cut off the zipper and cut off the drawstring on the hood, they’re not needed, heavy, an annoyance and easier to stuff in its sack after. Additionally, use it upside down. The pictures always who the hood on the ground (leaving your head exposed), with the ‘hood’ part on top, as it covers your head and can be pulled up around your shoulders better.
That said, I don’t think it necessary. Unless you were travelling in October or later. Some of the albergues don’t heat from what I can tell…especially at the rural/small town places. Then paired with a light liner or an ultralight down blanket you’d be cozy.