R8 insulation is used mainly on heating and cooling ducts. The residential code requires R-8 on supply and return ducts that run through an attic, as long as they’re 3 inches across or bigger. That’s why most flex duct and duct wrap is sold as R-8. You’ll also find R8 in thin spots where thick …

Polyisocyanurate insulation, or polyiso, is rated around R-6 per inch at room temperature. When the board itself gets colder than about freezing, it loses some of that R-value. The gas trapped in its cells starts to condense and carries heat more easily. In cold places like Colorado, roofing pros plan around R-5 per inch instead …