What is the thinnest high R-value insulation?

The thinnest high R-value insulation currently available for residential and commercial use is aerogel blanket or rigid vacuum insulated panels and phenolic foam boards. Aerogel, often called frozen smoke, provides about R-10 to R-10.3 per inch, so a half-inch aerogel blanket can provide R-5 in a space where traditional fiberglass would need 1.5 inches. Vacuum insulated panels or VIPs offer an extraordinary R-25 to R-50 per inch in the center of the panel because they contain a vacuum core with almost no conduction, but they are fragile, cannot be cut on site, and lose performance if punctured. More practical and widely available thin high R-value options are closed-cell spray foam at R-6.5 to R-7 per inch and polyisocyanurate rigid foam board at R-6 to R-6.5 per inch, and phenolic foam like Kingspan Kooltherm at R-8 to R-8.5 per inch, which is the highest R-value per inch among common rigid boards that can be cut and installed like foam. For most attics and walls where space is limited, 2 inches of closed-cell spray foam plus 1 inch of polyiso can provide R-19 in under 3 inches of total thickness.