Roof mount A/C is inherently noisy as to move air from a central location
to the ends of a large trailer (or MH) requires a high speed blower. A
ducted system has pipes (usually flex ducting) that carry conditioned air
to several outlets spaced throughout the living space, so the speed of
the air can be low enough that it generates little noise. Ducts for
refrigerated air need to be of materials that do not encourage
mildew-fungus growth and are easy to clean. Rigid conduit is therefore
superior for cool air than flexible ducting. Its smooth interior has less
surface area, few irregular surfaces for dust and mildew to accumulate,
and is easier to clean than flex. It also has much less resistance to
airflow.
Al