Flexwatt temp problems

I have 5 strips of 11 inch heat tape currently on my rack. all of them around 12 inches long. i have a herpstat controlling the temps and have it set at 87.5 degrees and on the different levels off my rack i have these temps from top to bottom. 85.1, 89.3, 83.7, 85.8 and 81.7. the probe for the herpstat is on the second level as it is normally at the highest temp. i cant figure out how to get the temps to stay close together, i wouldn't mine a +-1 degree but when my high is 89 and my low is 81 I can't help but to think there is something wrong.

Any idea how to correct this?

How tall is your rack? Heat rises so the highest racks should be the warmest. The best thing to do on the lowest shelf is insulate the bottom of it. That will help out a lot on the bottom rack. You can encose the sides of the rack and that might help a little more. Do you have any pics of your rack system?

where is the probe on top or under flexwatt? Are you using aluminum tape? (if so replace al tape on the probe with eletrical tape al. can alter the reading.) What is the ambient room temp it has a huge impact on racks and tubs cool very quickly if the room is much below 78-9ºF

im going to try to insulate it and see how that goes.