Improper insulating tied to fires
Home Fix: Dwight Barnett answers home-improvement questions. This week's topic clarifies why it is generally not safe to insulate over knob-and-tube wiring.
Scripps Howard News Service
Q: You have written that it is not possible to insulate over older knob-and-tube wiring in walls and attics. Many states have adopted amendments to NEC 324.4 to allow for exactly this practice. The amendments do require a licensed electrician to inspect the wiring, repair defects and provide over-current protection before insulation can be installed.
A: The misconception that costly rewiring is necessary before insulating prevents many homeowners from making these important upgrades to their homes. People should know that they can avoid the thousands of dollars associated with rewiring and spend a couple hundred to instead have the wires inspected.
Of course, in some limited cases, the wiring will not pass the inspection and need to be replaced.
You are correct I made such a broad statement, and it is true, the last time I checked the National Electrical Code (NEC), that you do not have to replace knob-and-tube wiring in existing construction.
Also consider the fact that many insurers will not renew a homeowners policy if the home has knob-and-tube wiring.
The 2008 NEC states that knob-and-tube wiring systems are not to be covered by insulation.
What worries me are the number of fires reported each year because someone insulated over the wiring, improperly spliced the wiring or simply overloaded the circuit.
Knob-and-tube wiring was never designed to handle the loads that a modern home requires, plus there is no grounding system for a modern three-prong plug. With all the added appliances used by today's homeowner, such as televisions, air conditioners, computers, etc., the added load can, and most likely will, cause the wiring to overheat. In such a case, a fire can result.
Knob-and-tube wiring was originally designed to be air-cooled; therefore, the two wires were always separated by a few inches inside a wall, attic or basement. If you add insulation of any kind, you eliminate the cooling of the airflow around the wiring.
If the fuse size of each circuit is reduced to protect the wire from overheating, you then experience the problem of having to replace blown fuses more often because of the extra loads placed on that circuit.
Homeowners soon learn to replace a 15-amp fuse that blows continually with a 20-amp fuse, not knowing they are placing their homes in danger.
Changing a panel to Fustats to replace the Edison-based fuses or to a breaker panel also is costly, but this will prevent the homeowner from over-fusing a circuit.
Regardless of amendments to local or state codes, I'll stick with the premise and I will continue to advise my clients it is not safe to insulate over knob-and-tube wiring.
Dwight Barnett is a certified master inspector with the American Society of Home Inspectors. Write to him with home-improvement questions at d.Barnett@insightbb.com. Sorry, no personal replies. Always consult local contractors and codes.