sentences:  To reduce weight and blanket thickness, the internal layers are made very thin, but they must be opaque to thermal radiation.Since they don't need much structural strength, these internal layers are usually made of very thin plastic, about 6 μm (1/4 mil) thick, such as Mylar or Kapton, coated on one, or both sides with a thin layer of metal, typically silver or aluminium.For compactness, the layers are spaced as close to each other as possible, though without touching, since there should be little or no thermal conduction between the layers.