Will rigid air conduit improve flow, over flexible ones

Question

My top room (3rd floor) has poor airflow, from a central air (cool and heat). Looking in the attic, I found flexible ducts that go over many curves. I feel like the duct are getting compressed/bent at many place and wonder the impact this has on airflow. See below picture.

The central air unit is oversized and should be plenty, given the total floor space.

• Would I improve airflow by converting to rigid ducts and minimizing the number of elbows ?
• How would I insulate those ducts ?

Would it make sense to use 6-inch rigid ducts, and have those rigid duct run inside flexible insulated ducts like the one I currently have ? I feel the rigid duct would keep the flexible one in the correct shape, thus allowing more air through, while the insulated one would still provide insulation. (This attic goes to 40c in the summer and -15c in the winter, insulation is not optional.)

Edit from questions:

Air circulation is on 24/7, has always been.

There used to be a booster fan I installed myself. But it's loud and helped very marginally. We're currently renovating, we reduced the attic run (pictured) by half. I might end up putting the booster fan back.

There is a return intake, it will get a nicer cover later, but for now it allows as much air as possible :-) second pic.

(not native english, sorry, "isolation" on picture was meant to be "insulation" :faceplam: ) )

Answer 1

The flexible ducts should have something solid or semi solid inside to prevent them from folding or collapsing as long as the curves and bends are according to design and their installation instructions. You should not have to switch to solid ducts to solve this problem. If the flexible ducts had no structure they would be like balloons and collapse when the system is off. There are ducts like that but i think not yours

Solutions:

• if bends are too sharp, make them bigger.
• you show a return duct. Is there a return opening on the third floor? If not, install one.
• The rooms on the third floor need more ducts and vents per square foot than the ones on the second floor in order for A/C to balance well. But usually the third floor is very under served. For example 15 vents serving the second floor and only one or two for the third. So: Install more ducts and more vents, at least two per room, on the third floor.
• If one system serves both the second and third floors, use a thermostat that can circulate air on a timer, even when the temperature at the thermostat is good. Run this all year. It helps keep things balanced.

Thermostat with fan Circulate mode

10 inch return for third floor

Small third floor room with second register for more share of air