The thermal coating on the underside of my attic roof is rated at 0.147 transmissivity or conversely, 84.3% reflectivity. Reflective sheeting is rated at 97%, but it will degrade as dust collects on its surface. If reflective sheeting were put directly on the underside of attic roofing OSB, would it be as effective as Lo/Mit-II Max? What is the best long-term solution here in Phoenix, AZ?
A material whose surface has the lowest emissivity (highest reflectivity) will always outperform with higher emissivity (lowest reflectivity). A radiant barrier, reflective insulation or IRCC that is applied to the underside of the roof will not collect dust — its reflective surface would be facing down.