I would totally replace the knob and tube regardless of cost. The safety of this old type of wiring is far too questionable to rely on. The science of electricity has developed considerably since this type of wiring was used. New codes and materials provide a level of safety that makes not upgrading the old system seem foolish. Insurance companies sometimes will not insure a house if they are aware that it has knob and tube. There are numerous reasons i could list but mostly to rid the home of a fire hazard.
As far as venting into the attic, I would replace the wiring first before running any ductwork which might prevent access to areas an electrician may need to reach to rewire the house. Hope this helps.

I agree with Electric Bill. Re-wire as much as you can. My first major re-wiring project was helping a friend replace knob and tube wiring in his old home.

First, knob-and-tube wiring was installed when the major electrical appliances were electrical light bulbs. Now they are the smallest energy users in the home. You do the math on whether knob and tube is enough to handle modern electrical needs. Like using two AA batteries to power huge ghetto blasters.

Second, the insulation on the knob and tube wire is shabby, even if it looks fine. Modern NM cable is double insulated: one layer around each wire, and one layer around the group of wires in the circuit.

Moreover, knob and tube uses cloth or perhaps rubber, which has proven over time to not be a good insulator, nor does it last long. They replaced cloth insulation with rubber, which turned out to be not much better. With the benefit of a century of experience, wire is now insulated with a vinyl plastic, which is said to last as long as the house.

At least replace it in the attic, especially if you plan to insulate over it. Insulation is supposed to be fire resistent by law, but don't tempt it. Run new wire. It is cheap and you will sleep better.

As for re-wiring the entier house, it is a major project, but the affordability will also depend entirely on the layout. It might be possible to run all circuits through the attic and drop them down easily. However, an electrician might find obstructions that prevent it. So it is almost impossible to tell how big a job it would be over the internet.

[This message has been edited by Lawrence (edited February 15, 2001).]

Any more opinions on a subject that comes up all of the time?

I would look at Article 324 in the 1999 and earlier NEC's, and Article 394 in the 2002 NEC.

I heard that K&T cannot be covered with any kind of thermal insulation but cannot remember where I got that infomation. I cannot afford a $100.00+ NEC manual.

NECH 2002:

www.necdirect.org

"394.12 Uses Not Permitted.
Concealed knob-and-tube wiring shall not be used in the following:

1-4 .........

(5) Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors

Commentary:

Concealed knob-and-tube wiring is designed for use in hollow spaces of walls, ceilings, and attics and utilizes the free air in such spaces for heat dissipation.

Weatherization of hollow spaces by blown-in, foamed-in, or rolled insulation prevents the dissipation of heat into the free air space.

This will result in higher conductor temperature, which could cause insulation breakdown and possible ignition of the insulation."

You will need to run your insulation at right angles to the joist so that the rolls lie across the top of the joist rather than between them. This will render the attic useless for any kind of storage. If you must have the use of the attic and you do not want to insulate the rafter spaces rather than the ceiling joist voids then you will need to replace the knob and tube wiring. The applicable code section on insulation is copied below.

394.12 Uses Not Permitted.
Concealed knob-and-tube wiring shall not be used in the following:
(5) Hollow spaces of walls, ceilings, and attics where such spaces are insulated by loose, rolled, or foamed-in-place insulating material that envelops the conductors

NEC Handbook © commentary: "Concealed knob-and-tube wiring is designed for use in hollow spaces of walls, ceilings, and attics and utilizes the free air in such spaces for heat dissipation. Weatherization of hollow spaces by blown-in, foamed-in, or rolled insulation prevents the dissipation of heat into the free air space. This will result in higher conductor temperature, which could cause insulation breakdown and possible ignition of the insulation."

I am going to disagree with the others though on the need to replace all of the wiring. If the knob and tube wiring is in fact in good condition and has not been subjected to overloading then you could leave the existing lighting supplied by knob & tube to save money. You could simply install what grounded outlets you actually need as completely new branch circuits. If you are trying to avoid damage to interior plaster that is in excellent condition then consider the installation of surface metal raceway in place of the baseboards or in place of the baseboard cap moldings. The larger size goes in place of the baseboard and will carry power and communications to any point along the baseboard that is thus replaced. The smaller size is mounted on top of the baseboard in place of the cap molding serving outlet boxes mounted on the wall above the baseboard. Done carefully either technique is unobtrusive.

You have to make a judgment as to how you will use electricity and what kind of changes you want to make in the lighting. If you want recessed lighting or other major changes then bite the financial bullet and have the whole house rewired. But if you are trying to retain a historic or period appearance then leave the lighting on the knob and tube. One thing that is often true of knob and tube ceiling outlets is that the boxes are often tiny or very shallow. This makes it difficult to use newer fixtures that are designed to be mounted on full sized boxes. Modern fixtures do not provide enough canopy space to make the connection to the building wiring. Fixtures that hug the ceiling or are labeled for 90°C should not be mounted on shallow boxes with older wiring because the bulbs will cook the wiring and make it dangerous.

If you have further questions please ask.
--
Tom

opion from the limited amount of K&T I have seen. The 2 conductors are separated by several inches so there is little chance of a short even if the conductors become bare. The ceramic knobs and tubes insulate the conductors well from the wood joists etc. If the circuits are protected by the proper size fuses they should not be a problem.
The only place I see a potential for problem is at the outlet be that a switch or light or recepticle whre the wiring is being changed.

There are other considerations such as insulation and bare wires in exposed areas like basement ceiling. They look like nice clothes lines when exposed in a basement.

[This message has been edited by joed (edited October 04, 2002).]