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Question DetailsAsked on 7/17/2014

Which type of insulation is the least combustible

Looking for an insulation that will not easily start on fire, even after 15 years

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3 Answers


From most fire resistant to least, roughly (some overlap between some products and partly depends on brand and density):

Asbestos - plain indestructable, will survive a complete burn-down of a house.

Rock wool - used around fireplaces and chimney flues, will not burn - period - made of rock fibers like asbestos, but not the cancer issue associated with it. Only paper backing, if any, will burn

Fiberglass - only the paper covering (if bought with that) and the resin glue used as a binder can burn - does not support combustion, so will not "start on fire", but does degrade some in a fire, so acts as a fire retarding layer, not fireproof.

Cellulose treated with borate - smolders but does not "burn" unless in a hot fire from another material

Fire-resistant foam - melts and burns some in a fire, but fairly hard to start

Fiberboard, untreated - burns mildlywith low flame unless asphalt impregnated type, smokes a lot, flame will spread if not borate treated

Foam - either board or foamed in place - generally burns about like gasoline or a tire - quite combustible and produces nasty fumes

Cellulose / straw / seaweed - untreated - lights and burns well, as you would expect

Paper - actually burns less hot and nasty than foam but generally starts easier and spreads faster

Shredded polyethylene and similar plastics - takes flame and burns like a torch, one would have to be an idiot to use as insulation, along with everything below fiberglass in my opinion

You should consider what your use is and use the material specified by code, or a higher class material - if you have a specific application in mind, you can ask for job-specific input using the Answer This Question button right below your question.

Answered 5 years ago by LCD


Assuming that you are asking regarding attic insulation, the best is closed cell isulation 3-4", then coated with a thin layer of fire-rated (orange or red) foam to protect it.

This will give you fantastic insulation that will give you very good energy savings over time and yet quite well protected should there be a fire.

Good luck!


Answered 5 years ago by Jefferson


Just to finish out the blanks, for those who might read this in the futre, wool and cotton insulation (a BAD idea in my mind, along with straw and cellulose) probably falls in about between foam and fiberboard in terms of fire resistance. May catch a bit easier than foam, but generally foam flame spread is going to outrun it as cotton and wool tend to char and the charred material inhibits the fire to some extent - especially in a wall, where it will tend to smolder more than actively burn.

BTW - you inferred that you thought some insulations bcame more fire resistant wih age - i don't know that I have eer sen anythingto indicsate that. Some may become LESS combustible or have a lower flame spread with age due to picking up moisture and packing and clumping, but I have not heard of any becoming more combustible - unless you are thinking of spontaneous combustion in organics like wool, cotton, straw, etc.

Answered 5 years ago by LCD

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