Putting UTH on the sides isn't mentioned because it's not efficient: heat rises, so instead of going up & INTO the cage as it does when properly installed under
the tank, most of the heat is a "miss". You're better off insulating the sides, back & bottom, leaving breathing space as required for the UTH.
Damp towels help a little, but like heating the side walls, it's not the most effective way to raise humidity. Much of the desired humidity comes from within...from
the right substrate that holds moisture, from using a "humid hide", and from a larger water bowl, especially if it's sitting partly over the UTH (for evaporation).
The reason that the plastic cages "hold humidity" is that they have very little air exchange...many have tiny vent holes, but some only have what little air gets in
around the doors. Point being...many using a glass tank find something solid* to block most of the screen top to retain humidity. (*anything that humidity cannot
escape thru....plexi-glass cut to fit, foil or even Saran wrap...many things can work.)
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