The fire resistance system for insulation is basically how long the material can withstand a standard fire resistance test. The definition of a class 1 fire resistance rating is the flame spread index and the smoke development index, which means how quickly a flame will spread when it enters the material and the amount of smoke it produces. Fiberglass insulation is made of glass combined with plastic polymers and is naturally fire resistant. However, you should be careful with blocks backed with aluminum foil or paper, as these materials can burn out quickly.
Cellulose is not fire resistant and will need to be treated with other flame retardants to help minimize burning. However, insulating materials such as mineral wool can be completely incombustible. RetroFoam has been installing injectable foam insulation on existing home walls for more than 35 years. Adding insulation to existing walls can save your home energy, but those savings won't mean as much if the material is flammable.
The results of this test depend not only on the thickness and characteristics of the insulation, but also on the characteristics of the protective coating and the pipe or equipment being insulated. In the case of insulating water pipes above a suspended ceiling that acts as a distribution chamber (for air distribution), the fire could get much worse if the insulating material spread the fire quickly and released a large amount of smoke. Pipe and duct insulation installed inside a building is different from wall and roof insulation, which is flat and continuous over a large area. More specifically, you don't want your wall insulation to be the reason the fire spreads throughout your house.
When choosing an insulating product, it's important to understand that there is no such thing as a “top insulator”.