They tend to be a combination of a highly reflective material, aluminized mylar usually, with a thin mesh in between the layers. The goal is to reflect as much light as possible (specifically those wavelengths emitted by objects at human-body temperature), and keep direct contact between elements to a minimum. [Wikipedia][1] has a great picture, shown below. 

[![enter image description here][2]][2]


  [1]: https://en.wikipedia.org/wiki/Multi-layer_insulation
  [2]: https://i.sstatic.net/fN9hu.jpg