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Potential Safety Hazards Associated with Weatherizing
Last Updated: 08/18/2010
Potential Safety Hazards Associated with Weatherizing

Part 1: Insulating Homes with Knob and Tube Wiring

Should homes with "knob and tube wiring" add additional insulation?           In many states there is no issue over whether it is okay to insulate homes with "knob and tube" wiring.  Whether or not it is okay depends on the state's building code agency (or equivalent) chief electrical inspector's interpretation of the National Electrical Code (NEC).  Each state has the flexibility to adopt all or part of the code.  At issue is the potential for overheating and subsequent fires when insulation is installed around knob and tube wiring.  That is because knob and tube wiring in older homes was not designed for loads as great as with newer conventional wiring and can overheat if the circuit is overamped.  Overamping of fused circuits in low income homes is relatively common.  If the circuit amperage doubles as a result of overamping, the heat output quadruples.  Surrounding the overheated wiring with insulation could potentially lead to a fire.

Prior to about 1985 the NEC allowed knob and tube wiring to be covered with retrofitted insulation.  However, in 1988 the NEC was successfully lobbied by electricians to prevent knob and tube wiring from being covered, ostensibly because of concerns over fire safety.  In 1990 the NEC decided to back off and allow insulation to be placed on one side of the wiring, but one side had to be exposed to air.

In 1988, Mike Piper of the Washington Department od Community Development studied the issue in detail and found that there were no documented cases of fire caused by the addition of insulation.  Chages were made requiring two conditions to be met prior to insulating homes in Washington with knob and tube wiring.  First the wiring had to be inspected by an electrician, and second, fuses had to be changed to type "S" fuses.  S-type fuses are a special screw-in fuse with an insert that prevents overamping and consequent overheating.  They cost less than a dollar apiece.

It should be stressed that insulating homes with knob and tube wiring in most other states is not an issue.  Nonetheless, the potential for problems does exist.  Often knob and tube wiring has been modified by a home owner or occupant such that the wiring is dangerous.  One insulator got quite a jolt when crawling in an attic.  If modifications to knob and tube wiring are noted, then the wiring ought to be inspected by an electrician and fixed if necessary, prior to adding insulation.  In addition, inspection of all fuse boxes is prudent since overamping is so relatively common in low income housing.  Electrical inspection could be built-in as part of the job cost.  If overamping is noted, S-fuses should be installed and the potential fire problem explained to the occupants.

From George Tsongas,Ph.D., P.E. Portland State University  "Indoor Air Quality - Issues in Weatherization"

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