Loose-fill insulation is used in attics and walls of many residential buildings. The International Residential Code and local building codes require insulation to pass high fire resistance testing. Loose-fill insulation can only be used in house construction with the appropriate certification.
Loose-Fill Insulation Fire Ratings
All insulation materials are tested to ASTM E84 for Surface Burning Characteristics. The test measures flame spread and smoke development. The material is given a fire rating based on the results. The ratings are Class 1/A, Class 2/A, and Class 3/A. Class 1/A indicates the highest fire resistance. Class 3/A has the lowest fire resistance.
Mineral wool, fiberglass, and cellulose are insulation types with a typical Class 1/A fire rating, with mineral wool offering excellent resistance due to its high melting temperature, fiberglass maintaining its rating when installed without facings, and cellulose requiring fire retardant treatment.
3 Most Popular Types Of Loose-Fill Insulation
Most types of loose-fill insulation do not burn. Some will smolder and some will melt in very hot fires.
Cellulose Loose-Fill Insulation
Cellulose insulation is typically at least 75% recycled paper. Paper burns easily. The final product is treated with boric acid to restrict fire. Borax also repels insects and rodents. Cellulose has a Class 1/A fire rating.
There are some reports of improperly treated cellulose. The boric acid may settle and leave the upper layers vulnerable to fire.
Fiberglass Loose-Fill Insulation
Fiberglass loose-fill insulation has a Class 1/A fire rating. It is manufactured from glass which does not burn. In very hot fires, fiberglass may melt. Most house fires are not hot enough unless they are fed by an accelerant like natural gas. Fiberglass contains no fire-retardant chemicals.
Mineral Wool Loose-Fill Insulation
Mineral wool insulation–also called rock wool or slag wool–is the loose fill that is most resistant to fire. It is manufactured from rock and iron slag spun into hair-like fibers. The fire rating is Class 1/A. Mineral wool has excellent sound abatement properties.
Additional Types Of Loose-Fill Insulation
Homeowners may encounter other types of loose-fill insulation–often in older homes.
- Asbestos. Asbestos insulation does not burn. Looks like cellulose. Heat from fire releases dangerous fibers. Classified as a known carcinogen.
- Vermiculite. Vermiculite insulation does not burn. Looks like chopped-up earthworms. Older product may contain asbestos that releases dangerous fibers in a fire.
- Perlite. Perlite insulation does not burn. Used for spray-on fire protection. Does not release any toxic substances when heated.
- Sheep Wool. Sheep wool insulation is naturally self-extinguishing. Class 1/A fire rating in the USA. May be treated with borates to repel pests which increases fire resistance.
Very old homes may have been insulated with sawdust or wood-shaving insulation. They have no fire rating and are highly combustible. When found in attics or walls, they should be replaced.
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