I used Insul-Bright as well, and you can't beat the cost ($4 for a yard, on sale, enough for 4-5 machines) and the ease of installation (no itch factor, no weird smells, very robust)
Boiler temperature is, by any stretch of imagination, a very low temperature for the mylar and polyester in Insulbright, and well below the operational temperature of the lower temperature component
(polyester), softening around 200C and melting at 250C. Boilers at ~120C are not an issue. The best part is that a boiler, thanks to the design of the beast (pressurestat), can't go above dangerous temperatures anyway
for more details, see also
http://coffeesnobs.com.au/YaBB.pl?num=1255582897Sometimes I look at the materials used to insulate boilers used on this forum, and I scratch my head and wonder why people would want to spend that amount of money for such a low temperature (try working with ceramic kilns, for reference

)
I cut a strip 2-3" higher than my boiler and twice as long as the circumference. Then I slowly wrapped it around the boiler, pulling it thru an "opening" in the pipes and wiring, wrapping it around the boiler twice. My duty cycle is much better, and the machine gets warm faster and stays warm longer between uses.
Just make sure you do not wrap any wiring or the pressurestat, as the increased temperature won't prolong their life. And make sure you do not block the air flow in the machine, especially in the lower part, around the electronics (the lower part is the cooler, but only if convective air motion keeps drawing fresh air from the bottom)