Supply ducts running through unconditioned spaces should be insulated and have what?

Insulating supply ducts that run through unconditioned spaces is essential for maintaining energy efficiency and comfort within a building. In addition to insulation, the presence of a vapor barrier is crucial because it prevents moisture infiltration from the unconditioned space into the ducts. Moisture can lead to mold growth, reduced air quality, and damage to the ductwork and surrounding areas.

The vapor barrier acts as a protective layer that limits the potential for condensation to form on the ducts due to temperature differences between the conditioned air inside the ducts and the colder, unconditioned air surrounding them. Proper use of a vapor barrier helps ensure that the conditioned air remains at the desired temperature and that the insulation remains effective.

Other options, while they play roles in air distribution and efficiency, do not address the moisture control needed for ducts located in unconditioned spaces. For example, excessive air velocity can lead to increased noise and energy loss, while transition backing and equal friction methods focus more on the airflow dynamics rather than the insulation and moisture concerns associated with unconditioned spaces.