What type of air-impermeable insulation might be used above the roof sheathing?

Closed-cell spray foam insulation is the correct type of air-impermeable insulation that can be used above the roof sheathing. This insulation is highly effective because it creates a continuous air barrier, which helps to reduce air leakage and improve overall energy efficiency in a building. Its structure consists of tightly packed cells that are filled with gas, which not only provides high R-value but also contributes to moisture control, preventing issues related to condensation in the roofing system.

Additionally, closed-cell spray foam adheres well to surfaces, enhances structural integrity, and is suitable for applications where insulation is in contact with the exterior elements. While batt insulation and cellulose insulation are commonly used for thermal resistance, they are not air-impermeable, making them less effective in preventing air leaks in this specific application. Open-cell spray foam insulation, on the other hand, is more permeable to air and moisture, thus not providing the same level of air sealing benefits that closed-cell spray foam offers. Therefore, using closed-cell spray foam insulation above the roof sheathing is ideal for achieving both thermal resistance and air sealing in roofing applications.