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In a recent two-week period, the Syracuse, NY, Fire Department had five fires involving blown-in cellulose insulation. It surely is a case where homeowners trying to save money ended up spending more in the long run. What's going on?
Cellulose is "green." This insulation is 80% post-consumer recycled newsprint. Most often, it is chemically treated with non-toxic borate compounds to resist fire, insects, rodents and mold. For those whose homes were built in the 1950s and who are looking to increase the R-value of their homes' insulation blanket, it appeared that adding blown-in cellulose is just the ticket. (According to the U.S. Department of Energy/Oak Ridge National Laboratory, "Insulation is rated in terms of thermal resistance, called R-value, which indicates the resistance to heat flow. The higher the R-value, the greater the insulating effectiveness. The R-value of thermal insulation depends on the type of material, its thickness, and its density." See http://www.ornl.gov/sci/roofs+walls/insulation/ins_02.html.)
So why all the fuss? In total, we have had seven incidents involving blown-in cellulose insulation within two months. In defense of the product, I will admit that it was not the cause of the problem, but it definitely was a contributing factor. In all cases, the homes were at least 50 years old. The original insulation was fiberglass batting with paper backing. The wiring at each structure was the original cloth-covered non-metallic two-strand copper. And at each incident, blown-in cellulose insulation completely blanketed the attic space. This insulation was added many years after the home was constructed to help increase the insulation blanket.
Another common factor at the fires was the existence of the original 60-amp fuse box in the basement. With these old services, it was possible to circumvent the protection by screwing in a larger-amp fuse. Back then, the fuses (known as Edison base) were interchangeable. The newer safety fuses have different threads for each amperage. Higher amp fuses cannot be screwed into a lower amp circuit.
Recipe for Disaster
Overloading of electrical circuits is the human element of the equation. Summer comes and occupants install a window air conditioner. Perhaps it's cold and they use an electric blanket. In the next room, junior has his computer work station with scanner, printer and monitor plugged into a wire strip. All of a sudden, the occupant sees that the fuses seem to blow more often. The inconvenience and cost of fuse replacement becomes bothersome. Well, with only four fuses to serve the entire house, that one fuse is probably covering two bedrooms and the bathroom. Perhaps (as in one of my incidents) the fuse covered three bedrooms and the basement. No wonder it kept blowing! So, the ill-advised human screws in a 20-amp fuse instead of the requisite 15-amp.
Now let's return to the concealed attic space above. For 45 years, that non-metallic wiring was exposed to the air. It was originally installed above the batt insulation and any heat generated by flowing electrical current dissipated readily into the attic air. The homeowner, needing to save energy, paid a contractor to blow in cellulose insulation over the entire attic space. To obtain the greatest R-value for the money, the homeowner gets 16 inches of the stuff. This, by the way, is exactly how every reference document describes proper installation.
The original wiring is now buried deep under the cellulose. There is no convective method for the wire to dissipate heat. Today's appliances and electronics create a greater electrical load. This large current draw heats up the electrical lines more often. And the icing on the proverbial cake is the installation of a fuse rated much higher than what the circuit was designed for. Voila! We have the perfect mix for an attic fire. The overheated wires burn down into the ceiling joists.