Set granite windowsill on Styrodur. Mortar or foam?

Hello everyone,

how are granite windowsills placed on Styrodur panels? In my case, the window recesses in the concrete basement are made of Styrodur, as Styrodur was used as formwork there. Are granite windowsills now set on the Styrodur in a mortar bed or with mounting foam?
Thank you very much

We have a reveal that is backed with Styrodur. Like all other windowsills, it is also in the mortar bed.

Roughen the Styrodur briefly again before applying the mortar (60-grit sanding mesh or similar) then it will fit.