Moisture Intrusion at Window Sills
Moisture absorbed into brickwork and plasterwork causes them to expand slightly. When the brickwork and plasterwork dry they contract slightly. The water absorbed by the bricks and plasterwork usually causes a slight vertical crack at the edges of the internal window sills. The paint then starts to bubble along the vertical crack. This crack may continue around the length of the window sill before you notice it. What started out as a small vertical crack then becomes a horizontal crack along the bottom of the window sill on the interior face of the sill wall.
The cracks are usually not significant unless allowed to continue unabated.
Sometimes the moisture intrusion at sills are mistaken for rising damp! Water leaking in at the window sill may bypass the damp proof course (DPC) built in under the window sill as a water-resistant barrier. The moisture may then appear as bubbling paint or crazing cracking of plasterwork, or both, below the window, extending down to floor level.
On external face-brick walls, this may appear as efflorescence (a white powder).
Internally, this may appear as bubbling paint above the skirting or discolouration of the skirting itself.