One of the biggest questions we are asked is when retrofitting an older house, how to be careful with knob and tube wiring. Its exposure in the attic and walls creates concern. The use of knob and tube goes back a hundred years. There is a good chance that the insulation on the wiring could create an issue.
Knob and Tube Wiring in Old Houses
Houses built before the mid 20th century used a type of knob and tube wiring. Porcelain knobs and tubes were used for and older system of electrical wiring. These knobs and tubes are drilled into the rafters or attic floors where electrical wiring passes through. Parallel strands of wire run in the rafters or attic floor. These wires are at least 3 inches apart from each other to keep them from overheating. If this type of electrical wiring is insulated the circulating air would be cut off and increase the chances of these wires overheating. Another issue is moisture. If the wire’s insulation acquires moisture, a conduction path between the two wires could happen for a potential fire hazard.
Though most old know and tube wiring in homes have probably been redone or modified due to greater electrical needs or due to deterioration, there are still a large number of older homes which have intact knob and wire system. These are the homes that are consuming a lot of energy to heat and cool their homes. How does one insulate these homes?
Insulating Knob and Wire System
Note that in 1987 the US National Electrical Code has decreed that homes with knob and tube wiring system are not permitted to insulate unless the system passed the stringent requirements of the US NEC.
Any type of insulation is possible if the existing knob and tube wiring is protected by a box large enough to provide 3 inches clearance around the conductors. However this is not really feasible because the system has no grounding conductor which means it does not have a shock protection built in the system. Majority of old knob and wire installation deteriorate and become brittle anyway.
The best thing to do is to have a qualified electrician look over the old installation and replace it if necessary with plastic-sheathed cable. This is the type of electrical wiring system that is compatible with all types of insulation, blown-in insulation included.
Once the wiring has been upgraded, check the attic for holes drilled for the new wiring. Caulk them first before proceeding with the application of the new insulation.