Well, the estimated weight of the insulation is about 2 1/2 pounds and the orbiter was designed for 100 missions. The weight, unless drastically increased by ice, does not seem significant enough to cause torque on the frame and the design parameters for missions are far beyond the actual 28 flights. Of course, the public statements by NASA could be seriously in error, but these variances indicate that the search for another cause will go on.
I suspect that 100-mission plan per orbiter is way out of line with reality. Since in a fleet of 5 (counting the replaced Challenger) Columbia had the most flight time with 28 missions, in a 22 yr service life...If they were REALLY planning on flying each orbiter 100 times, at this rate each orbiter would have ended it's service life at 100+ years of age! Ridiculous! I suspect the 100-mission profile was strictly for Congressional Funding purposes.