If an insulated ceiling has a vapor diffusion-retarder, what should be the ratio of net square footage per vent opening?

The correct answer reflects a common guideline for ventilation in attics, particularly when an insulated ceiling has a vapor diffusion retarder. In this scenario, the recommended ratio of net square footage to vent opening is 300:1. This ratio helps ensure proper ventilation, which is crucial for controlling moisture levels in the attic space.

When there is a vapor diffusion retarder in place, it helps prevent moisture from passing through the ceiling assembly, thus reducing the risk of condensation forming in the attic. However, proper ventilation is still necessary to allow for any moisture that does accumulate to escape, thereby preventing mold growth and damaging effects on structural materials.

The 300:1 ratio is based on code requirements and best practices that have been established to maintain an effective airflow while also considering the insulation materials and vapor barriers used in attic spaces. Adequate ventilation helps to regulate temperature within the attic, reducing the chance of ice damming in colder climates while also improving energy efficiency.

This guideline is more stringent than the ratios suggested by the other options, which are typically reserved for situations where there is less concern about moisture management. Therefore, the ratio of 300:1 effectively balances the need for adequate attic ventilation without compromising the function of the vapor retarder.