"To keep people and spacecraft safe from harmful radiation, NASA developed a method for depositing a thin metal coating on a material to make it highly reflective."
https://www.nasa.gov/offices/oct/40-years-of-nasa-spinoff/radiant-barrier
"It came to be known as “radiant barrier technology” and was used during the Apollo program to shield spacecraft and instruments from the harsh conditions in space. Since then, nearly every NASA mission has employed the shiny material to protect astronauts and equipment."
https://spinoff.nasa.gov/Spinoff2013/cg_6.html
"The U.S. Department of Energy says that radiant barriers can reduce cooling costs by 5-10 percent in a warm, sunny climate."
https://www.americover.com/blog/radiant-barriers-myths-facts-benefits/
"Radiant barriers are designed for hot climates - the hotter the attic, the more effective the barrier.
Never use a radiant barrier as insulation. They work in tandem with attic or wall insulation."
https://energyattic.com/is-a-radiant-barrier-worth-it/
"Radiant Barriers require a dead airspace!
In order for a radiant barrier to be effective, it requires a dead airspace on at least 1 side of the product. If you sandwich a radiant barrier between two solid materials, then heat will just conduct through it, rendering it ineffective. The ever popular stainless insulated cups that work so well to keep your drink hot or cold for extended periods, use an airspace or vaccuum between layers to keep radiant heat from penetrating through it. Radiant barriers work on this same principle when they have a dead air space on one side. This allows the radiant barrier to reflect 96% of the radiant heat away from it and only allow 4% through it, making the product a no brainer to use in many applications."
https://www.ecofoil.com/pages/how-does-radiant-barrier-work