This report suggests NASA scientists may have left out the effects of cold on the foams hardness:

Cold and Foam Are Called a Dangerous Mix
By WILLIAM J. BROAD
New York Times, February 8, 2003
"The large chunk of foam insulation that flew off the shuttle Columbia's 15-story external fuel tank and hit its left wing during blastoff may have been stronger and more dangerous than NASA realized, an expert on synthetic materials has warned the federal board investigating the disaster.
"The expert, Russell Seitz, told the board that NASA might have overlooked the drastic changes in the qualities of such materials when their temperatures are lowered hundreds of degrees. Mr. Seitz made the observation on Wednesday in a letter, which he provided yesterday to The New York Times."
http://www.nytimes.com/2003/02/08/national/nationalspecial/08FOAM.html

Bob Clark