I use them in my makeshift shelving unit greenhouse in the basement and spill water on them all the time--no problems at all. If you set them up in the basement, too, just don't have your basement flood during a storm and run down to unplug the submerged surge protector while standing in five inches of water.
I also learned the hard way that I didn't need heat on every shelf and not to keep the bun rack cover zipped up at all times. No, I didn't take a hint from the condensation covering the inside. Nor did I realize the thermometer I stuck in there reading over 80 degrees was trying to tell me something whenever I unzipped the unit to check on seedlings that were turning yellow--also trying to tell me something--even though their trays were dry (all the water was evaporating and condensing on the inside of the plastic cover!).
So now I turn one large mat on under the lowest shelf of seedlings that needs heat (pansies for example don't) and zip it up. The zipper on the plastic cover came loose from the plastic on one side, so there's a natural air hole that prevents vapor and too much heat "naturally" now. Heat rises, so it's always warm enough top to bottom, but still warmest at the top. I put the warmth-loving plants closer to the top and everything else in the middle and bottom shelves.
