Moisture makes mush of basement shower

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Q: We have a spare bedroom and bath in the basement of our house. The stall shower had been lined with wall board with a vinyl or Formica-type surface. After more than 20 years of infrequent use in a damp basement, the surface began to blister. We had a local handyman replace the shower enclosure with similar new paneling in 2005. The stuff he used is called melamine-faced panels, "ideal for tubs, showers and kitchens." I believe he bought it at Home Depot. After only one year, the bottom 4 to 5 inches above the shower pan began to discolor, then essentially turned to mush -- and the shower is no longer usable. The backing on the board is just pressed wood. Fortunately, we have two showers in the upper floors, so it's not a disaster, but if we do it again, obviously we don't want to use the same stuff. What type of paneling should be used, and do you have any guesses as to why the stuff we used disintegrated? We can hardly go after the handyman because the stuff he used says it's ideal...