One of the main mistakes made when evaluating the reaction to fire of building materials is to consider them individually without obtaining an overall perspective of the structure. As it has been shown in different studies, such as the one carried out by ANPE and PU EUROPE at the LS Fire Testing Institute laboratory, it is important to value the complete constructive solution and to reproduce the final application of use of the products, in order to obtain values adjusted to reality.
Reproduction of the actual behaviour of materials in case of the fire
The B-Roof test is intended to comply with international standards regarding the behaviour of roofs and their coatings against external fire. The objective of the classification obtained from the B-Roof test is to offer more realistic information about the reaction of the materials in case of fire.
Unlike other classifications, the B-Roof test simulates the effect of air (determinant for fire propagation on decks) and an organic impermeable membrane is placed on top of the material (to simulate its condition in a building). If we only take into account the classifications carried out with tests of reaction to fire on naked products instead of in their final application of use, we are approaching the problem incompletely and it is not adapted to the reality of the constructions.
Differences in behaviour between mineral wool and polyurethane deck roofs
If we take into account the fire safety classification in the regulations, mineral wool obtains a higher rating (A1) than polyurethane (B-s1, d0); this is mainly due to the fact that polyurethane systems are classified as a combustible material.
However, by subjecting both materials to the B-Roof test, polyurethane deck roofs were kept below the required limit, and thus had a fire-safe behaviour, whereas mineral wool deck roofs did not prevent the spread of fire, but they even favoured the persistence of flames on the deck. This is largely due to the “chimney effect”.
The danger of the chimney effect
Mineral wool products are non-combustible but fibrous, so that when they are subjected to the B-Roof test and the air currents are simulated, their structure lets the air circulate through the frets of the deck, which generates the chimney effect that feeds the fire and propagates it through the impermeable membrane. This phenomenon explains the poor performance of mineral wool decks compared to polyurethane systems.