Adding BR A/C to Styrofoam insulated roof

I believe you need to cut back the Styrofoam and build/insert a wood frame in-between the roof and ceiling. When the AC is mounted the bolts go up through the roof and are tightened from below. They need to be tight and you can't compress that Styrofoam enough to get it tight. You need to be able to clamp down on wood. If it is loose, water will come in where the AC contacts the roof (at the gasket).