CNZ PIR Insulation Board 90mm Glass Fabric Faced R3.91 2.4m x 1.2m sheet - sold per sheet
Aged R-Values as per AS/NZ4859.1 part 2 section 5.2

Polyisocyanurate (PIR) is a closed cell rigid polyurethane foam made by mixing two main chemical streams (isocyanate and polyol) with appropriate catalysts and a blowing agent.​ This mixture undergoes a number of processes including foaming and expansion, heating, curing, trimming and cutting followed by quality control to produce the panel as you know it. 

These process produce a cross-linked and closed cell structure, which contributes to the rigidity, stability, compressive strength and insulation properties of the panel, by reducing heat transfer through convection.