The absolute best material is the blue house foam board insulation found at Lowes and Home Depot. VERY cheap, nice and thick, does the job better than any other insulation.
Other good materials include cork board, thinner project foam board, Refectix, and felt. It all depends on how cool your room gets and how much heat you need to keep in the tank. For example, if you keep your room at 80 all year, you'd only need a thin insulating material because the air outside is close to what it should be inside.
But, if your temps drop or stay cooler then you need thicker material. I live in Denver CO, so I use the house foam on my tanks and it works perfectly.
Hope that helped and didn't confuse.
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