Polyisocyanurate, commonly called polyiso, is a high-performance, rigid foam insulation widely used in construction for its excellent thermal resistance and fire protection. It's often used in roofs and walls, both in commercial and residential applications. https://organiqo.com/index.php/forum/thread/8397/on-polyisocyanurate-%C2%A0the-unsung-hero-of-modern-insulation/