Class E (integrity only) glazing is the cheapest one and usually consists of monolithic specially tempered glass. It is able to withstand the fire and prevent the spread of hot smoke and fumes for certain amount of time, but it does not block the radiating heat of fire, so that the surfaces on the other side of glazing may ignite, and the fire can spread further. Walking behind such glazing during the fire could be quite uncomfortable, so it is not the most appropriate glazing for escape routes;
class EW (integrity and radiation control) glazing is made of two or more sheets of glass with intumescent interlayer between them. In the event of fire the interlayer expands and turns into an opaque foam which keeps the level of radiating heat on the protected side below the certain level (15 kW/m² at a distance of 1 m from the glass surface), so that people can safely pass by the glazing and escape the building;
class EI (integrity and insulation) glazing composition principle is similar to EW glazing, but its insulation properties are even better, so that in the event of fire people can comfortably leave the building, sometimes even being not aware of the raging flames behind that opaque glazing.
Along with letters there are always figures provided, which mean the length of time in minutes, during which the protection is provided. For example, E30 means 30 minutes integrity; EW30 means 30 minutes integrity and radiation control; EI60 means 60 minutes integrity and insulation.
Sometimes the fire resistance properties of glass are given as a combination of two figures, like 30/30, which should be read as 30 minutes integrity and 30 minutes insulation, or 60/30, that is 60 minutes integrity and 30 minutes insulation, or 30/0, which means 30 minutes integrity and no insulation.
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