1/1.5 brick thick Flemish bond, Lime mortar, originally weather struck and cut/flush pointed. 

The brickwork and lime mortar allow the property to breathe, moisture from precipitation etc. being absorbed and released. The chimneys would be open and assist the breathing process.  The internal walls would be lime plastered to allow the property to naturally breathe.  Air bricks also assisted the ventilation process.  Single glazed original sliding sash windows additionally assisted the breathing process.  The properties ability to breath should not be interfered with unless planned by a professional.