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.
How do you prevent the moisture intrusion into window sills?
Inspect the sill at all windows!
- cracks and holes on plastered window sills
- that have cracks and holes between the sills and the window frames
- cracks and holes at the junction between the window sills and the walls of the residences.
- the bottom of plastered window sills that slope back toward the wall
- cracks or holes between the joints of face brick window sills
Once you have identified any or some of the defects on the windows sills you need to seal them. Your local hardware store will most likely stock the sealer you require. Furthermore, the personnel there should be able to advise you on the best sealers to use externally.
Before you proceed with the sealing, you will need to scrub down the sills, window frames and adjacent walls properly. Allow them to dry for a day or two. If there are holes in any joints or cracks you may have to fill these first. Your local hardware will also advise on the best filler to use on the outside of your home. In addition, when you are sealing the sills you should seal around the whole window as an added precaution.
Once you have sealed your windows you can proceed to repair cracks, damaged plasterwork and bubbling paintwork below the sills. When you repaint the wall, all that should be required is slight sanding at the crack or bubbling paintwork. At most, you may need to apply a filler before repainting of the wall.
Lastly, always remember to wipe the walls down to remove any dust or dirt (like hand marks) before you repaint.