Cardboard box raises temperature in grow box. What other materials can help me regula

I noticed that putting in a cardbard box to raise the hieght of my plants will raise temperature by a couple of degrees. Anyone else have this experience?

I was wondering if I could simply put in materials depending on the temp inside the box. If it gets too cold, just pop in some cardboard.

What other materials will raise temperature?

Something metallic would be better. Cardboard is a type of insulator and will hold not only the heat in, but moisture as well. Last thing you want is your cardboard to get damp and start growing fungus. Stainless steel works great.

GTD