Thermodynamics Principle:
Every closed system tends to get a uniform average temperature over time. Suppose you have a perfectly insulated box with a uniform ambient temp of 80F, now you add an UTH in one side at 98F, the first thing to warm up will be the wall, then the air will gradually reach those 98F until the box reach it's new equilibrium at 98.
Now picture the same experiment but let's add an insulation panel in the middle, the box will get to the equilibrium but it will take more time to reach it. How much more time? depends on how much insulation the material provides. Now this is just theory and perfect systems like this don't even exist because there are always loss off energy/heat.

Substrate is just an insulation layer between your heat source and your enclosed system, the thicker the layer the more time it will take to change its temperature. But eventually it will get to the same temp, than your UTH, maybe a couple degrees less because of energy loss.

A solution to your problem could be if you pre warm the substrate no matter how thick and then just set your stat about 4 or 5 degrees higher than what you want the enclosure to be.


Sent from my iPhone using Tapatalk