Plaster is not made to "absorb moisture". I am guessing you mistyped and meant "repaint" the outside of the building. Obviously whatever moisture barrier you had is no longer effective. You should paint the outside of the building and then repair any of the larger cracks and especially any that appear to have mold in them. Mold can cause serious health problems in certain people and is generally considered not very healthy. It can be quite expensive to have a room replastered.