Water condenses there and may later freeze. The water will also dissolve salts and move them around in the wall and will accelerate corrosion of metal fittings within the wall. The process is shown in the diagram. The temperature gradient is drawn diagonally down through the wall section. Another process that modifies this simple picture is that water vapour and air do not generally move at the same rate through a porous wall.
Water is absorbed into the bricks, for example.
If the low temperature is just a short, overnight event, the bricks may well have the capacity to prevent condensation from the limited amount of air that can pass through in this period. Engineers have responded to this threat to the durability of the wall by inserting a barrier to air flow through the wall, usually a polyethylene foil. Other professionals in the building trade then make holes in the barrier to insert electrical cables, bolts, pipes and just nails. This has resulted in some spectacular failures of buildings that were designed with an attention to detail appropriate to constructing a jumbo jet but built in the more relaxed tradition of the building industry.