Question: Best substrate to keep cool?

[jbherpin]

I use 50/50 soil/moss blend in 4 tanks, a soil only in 2, a soil/small river pebble mix in 1, and moss only in 1. Which of these is the best for allowing critters to cool down if needed? I have hot summer peaks here in OH, and it is vital to provide the most adequate measures to cooling as possible.

Thanks in advance!

-jbherpin-