Buy Rigid Foam Insulation -

Not all foam boards are equal. The "best" one depends entirely on where you’re putting it: Types of Insulation - Department of Energy

Buying rigid foam insulation is a smart move for long-term energy savings and moisture control in areas like basements, roofs, and exterior walls. Unlike fiberglass, these solid panels don't sag or shift, and they provide up to double the thermal resistance for the same thickness.