Would foam core poster board in craft stores give extra help in holding heat in?

Re. cracked glass, yes, the instruction booklet that came with our ZooMed UTH emphasized that. I don't know what temperature it takes for the glass to crack, but whatever the UTH mat itself is set to, the more substrate above it, the less heat that would reach the snake. I'm glad we have an inch of bark in our tank because I spilled a bit of water out of the water dish a couple of days ago. It didn't hit the glass directly.