The pre-insulation layer is installed between the expansion joint bellows and the flow liner. It consists of individual, loosely superimposed layers of wire mesh, ceramic felt and mineral wool, or of prefabricated insulation pillows. Pre-insulation is added either to the flow liner or laterally to the construction angles using insulation spikes. In rare cases, insulating pillows are fixed together with the expansion joint belt.

For very large movements, the insulation pillow is affixed laterally to the construction angles. In the event of movement, the pillow is compressed, and returns to its original shape when the system cools. Thus, the insulation will performs its function when the temperature begins to rise again.

In the temperature range between 150 °C and 200 °C, the thickness of the pre-insulation layer must be planned very carefully, since there is a risk that the temperature will drop below the dew point in the pre-insulation layer and that condensate will form.