I always had about a 3-5 degree difference between the bottom shelf and the top shelf. After trying numerous things to correct the problem, I finally found the easiest and most effective way to maintain constant temps thruought the rack.
If you have a rack where the bottom shelf is always a lower temp than the rest, here is the solution.
This is 1/8 inch thick Class A rated insulation with a 3.0 R Value attached to aluminum tape. It is preglued to attach to the rack.
I have been using this and found that my temps on the bottom shelf are now within 0.4 degrees of the top shelf.
It is available in 12" width and is being sold by the foot.
The cost is $1.99 per foot.
Shipping will be $3.50 for the first 3 feet and an additional $1.00 per foot after.
Quantities are limited so act fast.
I can send pics of this stuff in use on my racks to anyone interested.