Reference
Cellulose Insulation
Burning Down the House
By Mark Hughes, P. Eng.
© 1999 Sintra Engineering Inc.
Blown-in insulation is used in attics to provide additional "R" value for older houses. Blown-in, or cellulose insulation consists of shredded newspaper that has been chemically treated with a fire retardant. When people refer to the "R" value of an insulating material it actually describes the insulating property (resistance to heat transfer) as it relates to a common standard. In actuality, the insulating material for most types of insulation is air as it is a poor conductor of heat. The insulation just traps the air in a way that prevents circulation. This stagnant air is the actual insulator.
Cellulose insulation is an interesting construction material for how it relates to fires. Cellulose insulation has unusual burning properties that can result in fires if improperly installed. Cellulose insulation also suffers from a number of misconceptions regarding its safety.
Cellulose insulation, according to the Cellulose Insulation Manufacturers Association (CIMA), is perfectly safe and not capable of sustaining a fire. The CIMA states that older forms of cellulose insulation was once a problem, but that newer, chemically treated (more fire retardant) forms are no longer a problem. In fact, CIMA states that anti-cellulose insulation information is propaganda put forth by the fiberglass manufacturing industry. Unfortunately for CIMA, the source of the false propaganda regarding cellulose insulation comes primarily from CIMA.
Although cellulose insulation has vastly improved in quality and is significantly less likely to cause a fire than earlier incarnations, cellulose insulation can still start fires. Of primary concern is how the material is installed particularly around flues and electrical components. Before we look at causes of fires, let's examine the controversy.
The CIMA claims that the Underwriter's Laboratories (UL) performance tests for cellulose insulation (the minimum standard that must be achieved to allow it to be sold) as an indication that this form of insulation does not sustain fire. The UL test involves an assessment of the flame sustaining of a tight ball of the insulation. In these tests, cellulose insulation does indeed not sustain fire, however, the test does not accurately represent the real world environment. The installation procedure for cellulose insulation requires the entrainment of air to move the insulation and provide some of the insulating properties. Once blown into place, the insulation no longer resembles the UL test and is capable of sustaining a fire. Any firefighter who has overhauled an attic is familiar with the ability of cellulose insulation to sustain a fire.
CIMA also relates the flammability of cellulose insulation to being just another combustible building product in houses with wood frame construction. This unfortunately seems to contradict the information from CIMA claiming that cellulose insulation cannot sustain fire.
Cellulose insulation has two properties that are of particular concern when investigating fires. The first feature is that the insulation is capable of smoldering for hours before transitioning to flaming combustion and path the fire will travel tends to tunnel under the surface. This tunneling of the burning insulation results in unpredictable fire patterns and the high potential for rekindling of fires that have reached the attic space. These properties have caused most fire departments in North America to require the complete evacuation and overhaul of cellulose insulation when a fire has spread into the attic space.
The second feature of cellulose insulation concerns how this material is installed. Most installers just blow the installation into the attic. This material is very easy to install. How can this simple commonly used procedure lead to possible fires?
The problem, from a fire cause perspective, usually revolves around the placement of the blow in material adjacent to hot surfaces. In an attic these surfaces are usually either flues (chimneys) or recessed lights. To protect the insulation from these hot surfaces, trained installers will install shields around flues and make sure no insulation surrounds recessed lights. Even untrained installers are warned as the instructions are usually placed on the exterior of the bags of insulation.
In Alberta, the building code does not directly address all the limitations that should be placed on cellulose insulation installers. The building code requires that there be a minimum clearance between combustible material and flues. However, the requirement for electrical equipment (such as recessed lights) is not found in the building code but the electrical code, something the installers may not be familiar with. The electrical code requires a minimum clearance between cellulose insulation and all electrical junctions. There is also a specific requirement for recessed luminaires (recessed lights).
To protect recessed lights, many more modern models have built in thermostats to protect the fixtures from excessive heat buildup (the result of the insulation) or overbulbing situations (a bulb that has too high a wattage). Unfortunately this protection is not available in older fixtures.
Of interest to the fire investigator is the assessment of whether a fire was started in the insulation or was caused by some other ignition source. The two most common forms of fire started in cellulose insulation both involve installations where either heat shields were not installed or insulation was installed without providing adequate clearance. A fire investigator while trying to determine the cause of a fire must search debris for the remains of a heat shield around the flues (usually unburned in a fire) or for the thermostat in the recessed luminaire.
If you have any questions regarding sampling, please feel free to call, I may be able to point you in the right direction.
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