Popcorn ceiling peeling off in a bathroom

I live in a house built in 1980 in CT, which I bought in 2011. I have a kind of unique situation in my downstairs bathroom: it has popcorn ceiling on a heated ceiling. The whole first floor has it, but the bathroom is the only one peeling. I installed a fan in the basement that sucks a considerable part of the humidity by using an automatic switch, but still, walls and mirror get foggy when hot showers are taken, and of course, the ceiling gets most of it, so it peeled again after I installed the fan and repainted it.

I thought about replacing the drywall with cement board (and possibly ceramic tiles), but I don't really know if I can do that, considering the heating system is somewhere over there. I considered leaving the drywall in place (covering it with the cement board), but I wonder if the heating will still work. One option could be to install a baseboard heater and go for the drywall replacement, but a solution without that would be better.

When I painted it, I scraped only the parts that were loose, and applied a primer specific for that, after getting out the dust. It's peeling less than before, but still doing it, especially right over the shower (where the upstairs bathtub apparently leaked on and was repaired) and around a drywall tape in the middle of the ceiling. Am I missing any possibilities here? What would you suggest?