Comparing insulation types on more than one axis. Fibreglass blanket and batt is the default for metal buildings because it is cheap, light and installed with the sheeting, but it has the lowest resistance per unit thickness of the common options and is highly sensitive to compression. Mineral wool performs comparably per inch while offering better fire and acoustic behaviour, which is often why it is chosen. Rigid boards including EPS, XPS and polyisocyanurate offer higher resistance per inch and, more importantly, can be applied as a continuous layer outside the frame; polyisocyanurate publishes the highest figures of the three, though its performance varies with temperature in a way that matters in cold climates. Closed-cell spray polyurethane foam has the highest resistance per inch of the common options and air-seals in the same operation, at the highest cost and with one consequence that is rarely discussed: it permanently encases the structure, so the frame behind it cannot be inspected or its coating repaired. Insulated metal panels sidestep the whole argument by arriving as a factory-made sandwich, which removes most of the on-site variability that undermines the other systems. The wider comparison of systems and where each belongs is covered on our [steel building insulation options](/steel-building-insulation-options) page.