Heating question

I am firing up a rack to house my new Honduran Milk snakes. My question is since I using aspen bedding should I read the temp with my temp gun on the surface of the aspen or on the floor of the tub

I am getting 85 degrees at the top of the aspen but as high as 98 on tub surface

Please advise.

Thanks

Always the surface they can touch.
They do burrow....

Thanks. I thought that was the case but wanted to double check. Appreciate it.

What pit said, you want to take the temp right above the surface of the tub not above the substrate.