I have a 1920’s house in the mid atlantic region that is constructed of concrete blocks with stucco on the outside and plaster directly applied to most walls on the inside.  However, the north wall of the living room has a wall framed out inside leaving an 8″ wide open cavity between the structural block wall and the interior plaster.  This was part of the original house to allow for a decorative feature on the front wall.  It was also used as a pipe chase to run heating pipes to an upstairs radiator.  I am doing some other work in this area and thought about blowing loose fill mineral wool insulation into this 8″ deep cavity to reduce heat loss from the radiator pipes and hopefully make a modest improvement in living room comfort given that this is the north wall.  I have read articles that advise caution in insulating old masonry walls, but I have not found advice on this specific circumstance.  Any thoughts on my idea to use blown-in mineral wool in this setting?  I appreciate any input — thanks.