Mould can grow due to moisture - it doesn't necessarily have to be a leak. If there was no leak, I would suspect that you have condensation on the back side of the drywall, which is causing the mould. The condensation could be caused by lack of vapour ######er or a vapour ######er which isn't sealed properly or has holes in it. If you have determined that the vapour ######er (polyethylene sheet), behind the gypsum board is the problem, make sure you fix it before you replace the gypsum board in that location. Also, make sure you have sufficient insulation behind the vapour ######er (on the cold side).
Another place that the moisture could be coming from, is through the floor slab - at the perimeter where it meets the wall. It could just be moist migrating from the soil below, or you could have water coming in at that location. This could be a result of having a high water table around your house (unlikely), or maybe the weeping tile around the house is plugged and you have rain water or melting snow, which isn't draining properly around your house.
So, if you have mould, make sure you determine the cause before you do any repairs.
Oh - to kill mould, bleach is very effective.
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