Reflectix is a reflective insulation that looks like silver bubble wrap. It’s commonly used in attics as insulation. It reflects heat, and works best if there’s a small dead air space between it and whatever you’re insulating (you can use a small strip of reflectix to create that space). It’s lightweight, you can buy a 4′ x25′ roll of it for less than 50 bucks, and it’s easy to work with.
I use it in a lot of ways beyond its traditional use as a radiant heat barrier in attics. It’s great for small-scale insulation projects, covering frost-sensitive plants at night, covering a window when the sun’s blasting in.
Last winter, I threw some over the chicken coop when it got really cold at night, and I’m planning to use it to shade their coop when it gets hot this summer. While I’ve never used it this way, it might make a great parabolic reflector for solar uses, as long as it didn’t get too hot. I can see lots of backpacking uses for a length of this lightweight stuff (as a thermal barrier under a sleeping pad, as a reflector behind you when you’re sitting by a fire to reflect the heat back (though you should take care to anchor it well so it doesn’t get into the fire), or as a sunshade on a hot day.