The 2'x4' panels of rigid fiberglass are highly compressed layers of regular insulation, formed by pressure into semi-rigid panels. They're often used in office wall and vaulted ceiling construction. They can be difficult to find, but here in the Pacific Northwest Paragon Pacific Insulation (in Kent near Southcenter Mall, 253-872-0800) has almost every kind of panel in stock, and they welcome walk-ins!