What type of insulation is blown in? I haven't come across any fifty year old homes where the insulation was blown in at the time of construction but rather the blown in (loose fill) insulation was added over an existing insulation, e.g. over fiberglass batts were that initially installed. My opinion may depend on if you would benefit from a vapor barrier if you do not have one, and does the existing have any real value or would you be better off removing all of it and starting with new insulation. Also, depending on the type of blown in, e.g. cellulose, my personal preference would be to remove it.
BEFORE you add more insulation, you may want to move some around and do some air sealing. Look for any penetration into the ceiling; pipes, ducts, elect wires, etc. Caulk and/or spray foam any gaps, and seal any electrical boxes that penetrate the ceiling. If you have canned lights protruding into the ceiling, get code-compliant boxes around them. Then, blow in as much cellulose as your sheet rock can support, and you have room for in the attic. (You need ventilation over any lid insulation.) Unless you are in a super cold location, avoid the vapor barrier.
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