This article was originally published in the May/June 1996 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.
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Home Energy Magazine Online May/June 1996
Knob and Tube Not a Fire Hazard In Retrofits We'd Rather Forget (Jan/Feb '96), you made reference to insulating over knob-and-tube wiring as being a fire hazard. This statement is incorrect.
Legislation was enacted in Washington state to allow insulating over knob-and-tube wiring per Bonneville Power Administration (BPA) specifications. This resulted because there were no documented cases of a fire being caused by knob-and-tube wiring, whether insulation covered it or not. Because the two conductors of knob-and-tube wiring circuits are spaced some distance apart, it is nearly impossible to short out. Even when covered with flammable materials such as wood shavings, the only way you could get a short was if the insulating materials were wet ... then they won't burn, so you couldn't start a fire anyway.
Overheating the wire would be the only method of ignition for knob-and-tube. Nonmetallic sheathed cable (NMC) such as Romex, on the other hand, can short as well as be overheated, increasing the potential for fire. NMC has started fires, and we insulate over it.
As an extra safety measure, it is a good idea to use Type S fuses or breakers sized properly for the wire size, the same as you would for NMC. An inspection by the Washington State Electrical Inspector and the installation of proper fusing (or breakers) is a requirement when insulating over knob and tube wiring in Washington and where allowed in other northwest states that follow BPA specifications.
Editor's reply: It's true that insulating over knob-and-tube wiring does not generally create a fire hazard and that the nonmetallic sheathed cable electricians currently use could cause fires. (Knob-and-tube was the most common form of home wiring until about the mid-1940s. See Knob-and-Tube Wiring Hang-ups, HE May/June '91, p. 7.)
Weatherization program policies concerning insulation in attics with knob-and-tube wiring vary, depending partly on electrical codes. Since 1987, the National Electrical Code has prohibited insulating walls, ceilings, and attics that contain knob-and-tube wiring, but several states have adapted their codes to allow insulation after a wiring inspecton. In the situation discussed in the article, the wiring was not inspected before insulating with cellulose.
Knob-and-tube wiring can be a fire hazard if the original fuses have been replaced with oversized fuses to handle larger electrical loads (an all-too-common practice). Type S fuses prevent homeowners from installing the oversized fuses, as long as the Type S fuses are not oversized. It's also good to check for bad connections or hot spots, as described in Assessing the Integrity of Electrical Wiring (HE Sept/Oct '95, p. 5). Once the wiring has been inspected and proper fuse sizing assured, it should be at least as safe after insulating as before. However, some weatherization programs require a sign to be posted to warn attic-goers of the danger of electrocution from concealed wires.
Noah S. Root
Barbara Shohl Wagner
Editor's reply: Sorry about the confusion. In the first article, author David Connelly Legg suggested that Tyvek could be used under a converted porch's floor insulation to protect it from wind. In the second article, Nancy Hurrelbrinck told the story of retrofitters Eddy Haber and Paul Knight, who attached Tyvek under a mobile home and then filled the gap with cellulose, with disastrous results.
Legg did not mean that Tyvek should be used to hold insulation against the floor, which the unfortunate crew in the later article found out rather quickly does not work. However, even if it's installed under floor insulation that is properly supported by other means, the manufacturer does not recommend horizontal use of Tyvek. Tyvek allows water vapor to pass through it, but can trap water inside, leading to problems with wet insulation, mold growth, and so on.
It's possible that a ground cover could provide adequate protection by preventing water vapor from the soil from permeating the Tyvek. However, as a general rule, it's probably best to avoid using Tyvek either structurally or horizontally. We all thank you for raising this question, which certainly needed clarifying.
Is this story accurate? If so, how are retrofitters handling fireplaces? Do you folks have any idea how many damperless fireplaces there are nationwide?
Editor's reply: The Uniform Mechanical Code (and the California Mechanical Code) state that [m]anually operated dampers shall not be placed in chimneys, vents, or chimney or vent connectors of liquid- or gas-burning appliances.... If the [gas log] fireplace is equipped with a damper, it shall be permanently blocked open to a sufficient amount to prevent spillage of combustion products into the room. Any jurisdiction that adopts the UMC will have the same prohibition.
The solution? An automatic damper with interlock. Other combustion equipment with dampers have an interlock preventing the burner from firing unless the damper is open. One could use a damper assembly with an end switch that would open a solenoid valve when open, but we don't know of any package designed for this application.
As for other options, fireplaces with glass doors and sealed-combustion air ducts don't need dampers (see Fireplaces: Studies in Contrasts, HE Sept/Oct '94, p. 27). Fireplace inserts (wood stoves that vent through the flue of the blocked-off fireplace) control the amount of heated room air lost up the chimney and provide hope of achieving positive efficiency. Standard fireplaces can have chain-activated chimney-cap style dampers installed at fairly low cost. Note that the code allows manual dampers on hand-stoked solid fuel appliances and installations (and only on these).
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