The qualifying criterion for the EW glass (insulated glass) is a maximum radiant heat level measured in a standard furnace test at a distance of 1 metre of 15kW/m2.
Insulated Fire Rated Glass is a laminated glass. Between each layer of glass is an intumescent material that is either in solid or gel form depending on the brand. It is this intumescent material that gives the sheet of laminated glass its fire resistance.
As the laminated glass heats up, the intumescent material will harden and when the first layer of glass in the laminate reaches approximately 120oC the glass will shatter exposing the intumescent material. As the fire burns through this material the next layer of glass will heat and shatter exposing the next layer of laminate and so on. The higher the fire rating needed the more laminates are used, so the thicker the piece of glass will become. (Insulated fire rated glass with a -/30/30 fire rating will be 16mm thick and a -/120/120 fire rating will be 53mm thick.)