What is the best insulation for a flat roof?

The best insulation for a flat roof typically includes Polyisocyanurate (Polyiso), which offers high thermal resistance and energy efficiency, making it a top choice for many installations. Other good options include Extruded Polystyrene (XPS) for its moisture resistance and durability, and Expanded Polystyrene (EPS) for a more affordable solution with decent thermal performance. Spray Foam Insulation (SPF) is also effective for creating an airtight seal but requires professional installation. Polyiso is often considered the best overall due to its high R-value and cost-effectiveness.