I have been dealing with a wall that is suffering from some penetration dampness. The wall was built circa 1910 near Cardiff. The outer skin is brick and the original mortar is a very dark back colour, I'm not sure if it is cement or lime-based. The stone window sill above the affected wall had a crack in the centre I patched this thinking it was the source of the water. There were also two hairline cracks in the render beneath the cill and above the affected wall. I insulated inside with Celotex plasterboard with a built-in moisture barrier. I am now getting some penetration damp at the very edge of where the board finishes. I have removed the tyrolean and render to expose the bricks. They are in very bad shape and feel a little damp, some of the mortar joints are crumbly to touch. My current thinking is, install a Catnic lintel, rebuild the brick courses up to the cill, and then install a new cill. Before rendering and repairing the tyrolean. I'm wondering if the hairline cracks in the render of circa 1mm and the poor brickwork are enough to have caused this amount of moisture or if something else must be going on.