Where knob-and-tube conductors pass through wood cross members in plastered partitions, conductors shall be protected by noncombustible, nonabsorbent, insulating tubes extending not less than _____ ...

I’m buying a house in an older neighborhood to set up as a rental unit. The house has the older style knob and tube wiring, which is visible in the attic. I’d like to add insulation to the attic to ...