The flexwatt is belly heat wired all from one side. I used 4" and placed it across the back of the shelf about 4-6 inches from the very back of the shelf. The top 3 shelves have one plug, the next 3 have one plug, then the next 2, and finally the last 2. I went with this in case one of the flexwatt strips goes bad I won't have to take all 10 off. Lol. The hardboard is shiny on both sides. If you can only find hardboard with 1 smooth side, I would put the smooth side inside the tub. I feel like the rough side would hold moisture and you may run into mold problems. Hope this helps!
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