The glass wool is incombustible and it doesn't work to the propagation and developpment of a fire
There is often confusion between the two concepts of "Reaction to fire" and "Fire resistance"."
Reaction to fire" determines:
”...whether a material fuels a fire. In relation to this, insulating materials (among which, glass wool) are classified as Al, A2, B, C, D, E etc. according to their contribution to fire. Those in class Al are considered non combustible."
Therefore, the reaction to fire classification is a measure of the inflammability of an insulating material, e.g. glass wool, and not of a building element, such as concrete blocks or bricks.
"... how well a building element, such as component or structure, can withstand – for a stated period of time – its load-bearing capacity (R), keep its integrity (E) and keep its insulation (I) in case of a fully developed fire."
More specifically, the regulation states the following:
A. load-beating capacity (R) is the capability of a building element to keep its strength and stability;
B. integrity (E) is the capability of a building element to prevent fire, fumes or hot gases from penetrating from one side to another;
C. insulation (I) is the capability of a building element to reduce, within certain limits, heat transmission. The REI value, which is the value measuring the fire resistance, is measured in minutes and represents the time below which a building element is able to keep and guarantee its load-bearing capacity, integrity and insulation.