What insulation gives the best R-value?

The insulation that gives the best R-value per inch among practical, field-installable materials is closed-cell spray foam at R-6.5 to R-7 per inch, phenolic foam board at R-8 to R-8.5 per inch, and polyisocyanurate rigid board at R-6 to R-6.5 per inch. If you include exotic and specialty products, vacuum insulated panels give R-25 to R-50 per inch and silica aerogel blankets give R-10 to R-10.3 per inch, but these are very expensive, fragile, and not commonly used in standard homes. Among affordable blown insulations, closed-cell spray foam gives the highest R-value, followed by cellulose at R-3.2 to R-3.8 per inch, mineral wool at R-3.0 to R-3.3 per inch, and fiberglass at R-2.5 to R-2.8 per inch blown or R-3.1 to R-3.4 per inch in batts. However, best R-value does not always mean best choice, because air sealing, moisture management, cost, health, and environmental impact matter. For example, cellulose has lower R-value per inch than spray foam but is much cheaper, made from recycled content, and excellent at reducing air flow when dense-packed, so it often gives the best overall value for attics and walls despite not having the highest lab R-value.