Replies

1. 
   cap | Aug 26, 2006 07:16am | #1

   Well, the National Electrical Code prohibits contact of thermal insulation with knob-and-tube wiring.  The issue is twofold: the K&T was designed to operate in open air, and burying it in thermal insulation traps heat.  This can accelerate degredation of the electrical insulation on the wires.  Bare wiring is not good.  The other issue is with the K&T buried in thermal insualtion, someone doing work in the attic might not be aware of the hidden wires, and damage them.

   Now, the California Electrical Code has for years differed from the NEC, in that the CEC allows K&T to be buried in thermal insualtion, if two conditions are met:

   1) a Calif licensed electrical contractor must inspect the exposed K&T to be buried in thermal insualtion, and certify in writing that the K&T is in good condition (the CEC gives no specific criteria as to what this means); and,

   2) there must be a sign posted at the attic access point saying that there's hidden K&T wiring.

   The anecdotal info I have is that the CEC differs from the NEC because the energy efficiency camp wanted to allow use of thermal insualtion in old houses, and a State Fire Marshall analysis of records of several years of residential fires found no cases where K&T wiring in thermal insulation caused a fire.

   The NEC and the CEC have no legal standing as enforceable regulations unless they are adoted by local building jurisdictions (in California, that's each of the 58 counties).  So, you need to check San Diego County requirements.  The thermal insulator guy might be basing his stance on the NEC prohibition.  Or the county may have adopted the NEC prohibition as apart of the county electrical code.

   My take on this is that the California approach is reasonable.  If the electrical insulation on the K&T wiring and the knobs and other parts of the wiring are in good condition, burying the wires in insulation isn't going to create an undue hazard.  I've certified a good bit of K&T pursuant to the CEC requirement.  And I've found some in horrible shape--almost all the rubber insulation fallen off of the wiring, bad splices, bad modifications, and so on.

   So it depends on how you weigh risk against cost and benefit.

   Cliff

   
   

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   Edited 8/26/2006 12:21 am ET by CAP

2. 
   dutchblue | Aug 26, 2006 07:29am | #2

   CAP nailed it all and with common sense.  Washington state however has read the absurd into the NEC.

   Per WAC 296-46B

   Article 394 NEC does not prohibit the installation of loose or rolled thermal insulating material in spaces containing existing knob-and-tube wiring provided that all the following conditions are met:

        (1) The wiring must be surveyed by an appropriately licensed electrical contractor who must certify in writing to the department that the wiring is in good condition with no evidence of improper overcurrent protection, conductor insulation failure or deterioration, and with no improper connections or splices. The electrical inspector must inspect all repairs, alterations, or extensions to the electrical system.

        (2) The insulation must meet Class I specifications as identified in the Uniform Building Code, with a flame spread factor of twenty-five or less as tested using ASTM E84-81a. Foam insulation may not be used with knob-and-tube wiring.

        (3) All knob-and-tube circuits must have overcurrent protection in compliance with NEC Table 310.16, 60 degree centigrade, Column C. Overcurrent protection must be either circuit breakers or Type S fuses.

    

   As CAP suggested check with your local codes.