PYREX is low thermal expansion borosilicate glass. pyrex is not - it's soda-lime. Also, if it was made before they stopped using borosilicate exclusively, if the density of the glass is 2.23 or close you more than likely have borosilicate glass. Soda-lime has a density of 2.52 for comparison sake.
The breakage problem is from heat traveling through the glass; no insulator will help. What does help is not creating a thermal gradient through the glass by having too high a temperature difference from one side to the other. One might empirically determine that. For example, if one is defrosting a windshield, assume the air inside a vehicle starts at the temperature outside and then limit the air temperature of the defroster to some value that is seen to be OK, most likely by testing using hot-air sources and ice-coated glass.
