Insulation applied to an exterior surface of ducts in buildings other than dwelling or apartments shall have a flame spread rating of not more than?

The correct answer is that insulation applied to an exterior surface of ducts in buildings other than dwellings or apartments should have a flame spread rating of not more than 25. This relatively low flame spread rating is crucial for enhancing fire safety in commercial and industrial settings.

Materials with a flame spread rating of 25 or less are considered to have limited combustibility. This is particularly important for ducts, which can carry air (and potentially smoke and heat) throughout a building. By ensuring that the insulation on these ducts has a low flame spread rating, the potential for fire spread is minimized, thereby providing greater safety for occupants and protecting property.

In this context, a flame spread rating greater than 25 would indicate that materials could contribute to the rapid spread of flames or smoke, which is undesirable in non-residential buildings that often house more people and valuable equipment. Therefore, specifying a flame spread rating of 25 or lower aligns with safety codes and best practices in fire protection engineering.