What types of insulation can be installed below the roof sheathing?

The correct answer is that insulation board or closed-cell spray foam insulation can be installed below the roof sheathing. This type of insulation is particularly effective at reducing heat transfer and providing a continuous insulation layer, which helps to improve energy efficiency in the home.

Insulation board is a rigid insulation that can be placed between rafters or over the roof sheathing itself. It not only provides thermal insulation but also can help protect against moisture when properly installed. Closed-cell spray foam insulation also offers high insulating values and creates an airtight seal, which is advantageous in preventing air leakage and moisture infiltration.

Other types of insulation, such as fiberglass bats or cellulose insulation, can be effective in various applications but are typically not recommended for installation directly under roof sheathing due to the potential for moisture issues and their inability to provide an airtight barrier. Similarly, mineral wool or loose fill insulation may not perform as well in this specific application, as they typically do not form a continuous barrier and may allow for air movement that affects insulation performance. Radiant barriers or reflective foil, while effective in specific situations for reducing heat gain, are designed for other applications and do not serve as traditional insulation below roof sheathing.