ᐅ Setting a granite windowsill on top of rigid foam insulation (Styrodur): mortar or foam?

Hello everyone,

How are granite window sills installed on foam insulation boards? In my case, the window reveals in the concrete basement are made of foam insulation boards because the foam was used as formwork there. Are granite window sills installed on a mortar bed on the foam boards, or with mounting foam?

Thank you very much

We have a reveal that is supported with Styrodur insulation. Like all other windowsills, it is also set in a mortar bed.

Roughen up the Styrodur briefly before applying the mortar (using 60-grit sandpaper or similar), then it will adhere properly.