What type of board should be used to protect the enclosed side of accessible space under stairs?

Using 1/2-inch gypsum board to protect the enclosed side of accessible space under stairs is appropriate because it serves multiple purposes. Gypsum board, or drywall, has fire-resistive properties, making it a suitable choice to comply with building codes that often require fire-rated materials in certain areas of a home, especially those near stairwells. This helps to slow the spread of fire, providing crucial time for occupants to evacuate safely.

Additionally, gypsum board provides a smooth finish that can be painted or treated, enhancing the aesthetics of the space while also contributing to sound attenuation between levels of a home. Its ability to resist moisture to some degree makes it better suited for enclosed spaces compared to other boards, which may not offer the same level of stability or safety against potential hazards.

While plywood, oriented strand board, and wood paneling may have their own applications in construction, they generally lack the specific fire-resistive qualities and code compliance often required in stairway or enclosed space applications. Thus, 1/2-inch gypsum board stands out as the most effective choice for this particular use case.