Concrete ceilings below unheated spaces should have a maximum U-value of 0.07, requiring insulation rated R-11 or greater. The following methods are suggested for insulating concrete ceilings:
1. Attach preformed rigid insulation to either surface with adhesives.
2. Apply sprayed-on rigid insulation to either surface.
3. Make a sandwich construction with preformed rigid insulation formed around a concrete core.
4. Apply rigid insulation to the top surface where the ceiling also serves as the roof of the structure.