I'm no expert on tubs so if anyone says otherwise listen to them First off I'll recommend a better thermometer. I switched to an accurite from the zilla and there was a temp difference of 3 degrees. Accurites are pretty cheap at walmart but I found min online for $11 apiece.

My opinion is that insulating the tubs without extending the air holes to the insulation is going to cause a rise in humidity. Depending on where you live this may not be much but if you're in the south like me it could be a lot. I'm guessing you have a UTH rated for 20 gal aquariums, this is fine for a tub that size and perhaps edging toward the big side. Smaller tubs are an option but if it was me I'd just get another UTH set at a lower temp for the cool side. This way you only have two items to get (better thermometer & UTH) rather than three.

Good luck with the tubs, I'm a recent convert and will never go back to aquariums for my BPs.