Look at my Past Threads. Ive had this problem for months.. I finally have it set since maybe mid Dec with two pieces of 1/2 inch 2'x4' insulation foam board in between. I tried wood like you without success. That would work for a few hours and then by 24-48 hrs later the above enclosure floor would heat up to 97-98. The kind of insulation sheets used for basement walls or under vinyl siding for a house is what I went with. They are already cut to 4x2. I then bought a dowel from lowes and cut 1" pieces to replace the metal pegs (same diameter) that hold the two Enclosures together. I had to cut 2-3 times getting the correct length but i got it. My top enclosure RHP I moved to the cool side now. It runs very seldom and is set at 82 which keeps ambient cool side 78-80. The Hot hide is completely heated by the lower enclosures RHP thats on the Hot side to 91/92.7. However when I measure with the laser it is never above 90.7. My probe is under the Paper substrate so I know its A-OK.. This same problem drove me nuts and everyone else as I couldn't stop talking about it hahahahaha. Thank god I solved it.
Good Luck.
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