Shuttle Foam

With all the speculation over the foam used on the space shuttle I decided to look up the technical specs on the foam. This is from NASA's Thermal Protection System.PDF

Some interesting facts:
The housing is actually two layers, it the outer layer comprised of "low-density, closed-cell foam" and a "denser composite material called ablator, made of silicone resins and cork. An ablator is a material that dissipates heat by eroding."
The foam is sprayed on the tank.
"The closed-cell foam used on the tank was developed to keep the propellants that fuel the Shuttle's three Main Engines at optimum temperature."
"The foam insulation must also be durable enough to endure a 180-day stay at the launch pad, withstand temperatures up to 115 degrees Fahrenheit, humidity as high as 100 percent, and resist sand, salt, fog, rain, solar radiation and even fungus."
"Though the foam insulation on the majority of the tank is only 1-inch thick, it adds 4,823 pounds to the tank's weight. Insulation on the liquid hydrogen tank is somewhat thicker -- between 1.5 to 2 inches thick. Though the foam's density varies with the type, an average density is about 2.4 pounds per cubic foot."

So I hope this helps answer some of the questions people might have about why the foam is used and exactly what the stuff is made of.