My neighbours have built a wall parallel to mine, small gap between filled with polystyrene. Unfortunately, before they put their roof on, we had heavy rain and it seems that lots of water has gone into the gap, the plaster on my wall is soaked through. They have now put their roof on so no more water can get in, but I'm wondering whether the water that is already there will need to be drained somehow, or if it will slowly evaporate through my wall. Is it likely to structurally damage my wall?