Whilst re-decorating a sash window I, perhaps unwisely, removed a wooden board below the window to find exposed mortar (see photo). The mortar is crumbling and flakey. The external walls are sandstone. This is a Victorian first floor flat in Glasgow, Scotland. What is the mortar substance? Is it advisable to repair this, if so how? Should I attempt to insulate the cavity, or just box it back up with a wooden board?
Originally asked by FlamingGoose on Stack Exchange (diy) (CC BY-SA 4.0).
