Mylar is only 90% reflective(avg) over 300-700 nm wavelengths.
As I recall NASA uses carbon composites for heat shields. Reflective Mylar is polyester and aluminum.
A simple NASA technology that protected Apollo and Skylab is still coming to the rescue in space and on Earth.
NASA has used the same thin, shining insulation material on virtually all manned and unmanned missions. A memorable moment in the insulation's history was seeing its shiny swath around the base of the Apollo lunar landing vehicles.
In 1973 a parasol-type sunshield made of the material helped to save Skylab after the spacecraft lost a heat shield during launch and began to overheat. This dramatic rescue also provided the first possibility of a habitat for astronauts to live and work in space.
Image at right: Teams at NASA's at Marshall Space Flight Center and elsewhere constructed a reflective, parasol-like sunshield to protect Skylab. A second sunshield (pictured here) was installed during the Skylab-3 mission. (Image credit: NASA)
Today, the Hubble Space Telescope and Mars rovers can thank the shiny material for protecting them as they go about business in space.
The silvery insulation is a heavyweight on benefits and a flyweight on mass in order not to weigh down spacecrafts while protecting them. The material is made of a strong, plastic, vacuum-metallized film with an efficient, infrared-reflective coating of aluminum applied as a vapor.
The same properties that are critical in space have proved to be life-saving on Earth. NASA's insulation technology has been widely used to create slim thermal blankets. Perhaps no one can appreciate these benefits more than victims of the earthquake that razed Pakistan, Afghanistan and India in October 2005.
When relief poured into the region, aid included supplies of silver thermal blankets that doubled as ground covers during the day and blankets at night. Two people could also wrap up in the large blankets and share body heat as well as the warmth of the blanket itself.
Image at right: Reflective emergency blankets were used in Pakistan in 2005 after earthquakes devastated the region. (Image credit: Mercy Corps)
Marathon runners can also thank the NASA insulation for making their lives more comfortable. After running long distances in chill temperatures, runners may shiver from more than joy once they cross the finish line and begin a rapid cool-down. As hordes cross the finish line, it may take runners more than 20 minutes to climb into warmer clothes, opening the door to hypothermia. Today many marathons use "space blankets" to ward off the post-race chill. The silver-backed crowds of runners have become a well-known symbol of having finished a race.
Image at right: Marathon runners use reflective blankets to help stabilize body temperatures after a race. (Image credit: AFM, Inc.)
Operating rooms have also used space technology to improve conditions for patients and staff. Insulated bed sheets are used to warm pre- and post-operational patients; for hospital staff members working in chilled environments; and as surgical drapes that give surgeons access to specific areas of the body while keeping the patient warm.
Those who enjoy the outdoors will be glad to know that NASA's insulation technology is an integral part of life-saving gear such as emergency bivvies and rescue blankets. It also provides the simpler comfort of insulated gloves and vests that are supple and flexible, even in extreme cold.
And for those who prefer not to answer the call of the wild, there are still benefits to be found in the space program's legacy. Insulated vests are handy for running in the Iditarod, but also for sedentary hobbies that allow the body to lose heat -- like sewing.
http://www.nasa.gov/vision/earth/technologies/silver_insulation.html
But to keep it safe from IR and cold! 
