Knob and tube electrical wiring was an early method of electrical wiring in buildings and was commonly used in North America from about 1880 to the 1930s. While it was replaced with other electrical wiring methods decades ago, home with energized knob and tube electrical wiring still exist.
Knob and tube wiring consists of single-insulated copper conductors run within wall or ceiling cavities, passing through studs and ceiling/floor joists through protective porcelain insulating tubes. The wires are supported along their length by porcelain knob insulators that have been nailed in place.
Knob and tube wiring runs at a higher temperature than modern wiring materials and requires a minimum of three inches of air space around the length of the wire. This requirement adds an additional challenge to home owners wishing to add insulation in order to increase the comfort and energy efficiency of their older home.
If you have knob and tube electrical wiring in your home and plan to add insulation, first have a qualified electrical contractor inspect your wiring to ensure that it is safe prior to adding insulation. Ensure that all connections are enclosed in appropriate protective boxes, that the wiring insulation is intact and in good condition and that it has not been modified in any manner since its original installation (addition of non-metalic sheathed cable wiring circuits, for example).
The next step, prior to insulating attics or floors where knob and tube wiring is present, is to identify and seal air leakage points. This is an important step that will maximize the efficiency of the insulation, extend its life and assist in providing a comfortable and healthier living space.
Do not insulate wall cavities containing knob and tube wiring.
Knob and tube wiring in attics may be isolated by building a barricade around it with R-30 unfaced batts. Ensure that the batts are at least three inches away from the knob and tube wiring.
While the presence of the older knob an tube wiring does not, in itself, violate national electric codes (though some regions forbid it), the best solution for knob and tube wiring is to replace it, if economically possible.
I’m sure that there are electrical contractors and weatherization folks who could add much to this topic and I hope they will chime in. Home owners should always consult a professional electrical contractor prior to adding insulation in the presence of knob and tube electrical wiring.