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Old knob-and-tube wiring could cause insulation woes

QuestionMy 1906 1,000-square-foot home has 4 inches of sawdust insulation in the attic. Does sawdust have a good R-value? Is it safe? If not, what's the best way to remove it? Can I insulate over the sawdust?

AnswerThanks for submitting your unusual question. Over the years, many products have been used to insulate homes including loose-fill fiberglass, rock wool (a mixture of basalt, slag, a byproduct from steel furnaces, and limestone), cellulose (shredded newspapers), vermiculite (some mined near Libby, Montana, where asbestos contaminated the vermiculite), perlite (a generic term for naturally occurring siliceous volcanic rock), asbestos (which may be mixed with other materials), and cellulose-based products including sawdust, redwood bark, balsa wood, corncobs, straw, cotton, and hemp.

Now, my answers to your questions could open up a can of worms, and you should know that insulating can be a nasty project, so I recommend that you hire a pro.

Here are a few things to consider: Does sawdust have a good R-value? No, with an R-1 or R-2 rating per inch, it's not very efficient.

Is sawdust safe? Sawdust is combustible and certainly would help fuel a fire, but generally speaking, your 100-year-old home most likely has very dry wood and varnishes throughout so the entire home would burn very quickly if a fire started.

What's the best way to remove the sawdust? A wet and dry vacuum would work, but vent the exhaust port to the outside so contaminants exit your home. I highly recommend that you wear a respirator (not an inexpensive mask) while you work, especially one with replaceable cartridges.

Insulation removal may be hazardous in terms of rodent droppings, woodworking insects, dust, and even old electrical wires.

If you wish, you can insulate over the sawdust. Either blow in an R-38 or install batt insulation (no face) with an R-38 rating.

A building permit is not required to remove old attic insulation and install new. However, if a wall is opened or a ceiling is removed (e.g., lath & plaster or wallboard is removed), then an electrical permit is required and the entire wall must be brought up to electrical code standards. Additionally, a building permit is required to install new wall and/or ceiling covering.

Before you install insulation over the sawdust, check the attic for old knob-and-tube wiring, commonly installed from the late 1800s until the late 1950s.

Knob-and-tube wiring is made of two individual wires that run through and are connected to porcelain insulators. The system lacks a ground wire and is associated with fuses instead of circuit breakers. That grounding wire in modern systems reduces the chances for creating a fire hazard. Knob-and-tube wiring also has a lower capacity, so an electrical overload is easy to achieve with today's appliances.

Especially dangerous is that the coating on knob-and-tube wiring deteriorates and flakes off. The wire used before about 1930 is especially vulnerable.

Finally, because knob-and-tube wiring is readily accessible in attics and basements, homeowners have often made incorrect and dangerous alterations.

For all these reasons, the National Electric Code (NEC) requires that the wiring be disconnected and removed if there is any insulation around it.

If you find knob-and-tube wiring, check the terms and conditions of your homeowner's insurance policy to determine if you are covered; your policy may specifically exclude knob-and-tube wiring. A number of insurance companies drop fire insurance if there is any such wiring in the house, mainly because of the deterioration of the electrical system as a whole. Some insurance companies base that decision on the results of an inspection of the specific home.

If you find knob-and-tube wiring, consider hiring an electrician to inspect and evaluate it.

Back to the insulation question: if you have knob-and-tube wiring, carefully remove the sawdust around the wires so as not to damage the coating on the wires. This will ventilate the wires to help keep them from heating up.

If you want to insulate around the old wiring, the NEC requires that the knob-and-tube wiring be disconnected and replaced with new wiring before insulation.

Of course, an electrical permit will be required for any rewiring. With the knob-and-tube wiring disconnected, you'll have lots of choices for insulating the attic.

Copyright © 2005, 2006, & 2007 LAF/C.R.S., Inc. All rights reserved.
Question answered by Leon A. Frechette.

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