I have an old heritage house that I am insulating in stages. I found strands of wire running through the ceiling joists. Is there anything I should be concerned about?

From your description these wires are most likely older electrical service wires called Knob and Tube wiring. They can be identified by ceramic knobs and tubes used as terminals and protection through joist holes. This type of wiring used until the 19/50”s is usually still reliable. However, if you find wires that have been spliced to newer “Loomex” wiring, the original wiring may not be as reliable and safe.

Be sure not to bury the Knob and Tube wiring in insulation. Although there is some debate about this subject, it is generally accepted that this type of wiring works best when the heat build up in the wiring can dissipate unrestricted to the surrounding air. As well, some insurance companies are now reluctant to underwrite new insurance policies on houses with Knob and Tube wiring even though the system may be safe.