Cold air drafts up from first floor returns when furnace is off. Located in Massachusetts.

New furnace and ducts installed in a 2 story house with basement (retrofit). Furnace and ducts are properly sized. Second floor returns are in the ceiling with trunk in the attic. First floor returns are on floor. Trunk runs from attic to basement through a chase located along exterior side of house. In the basement, the first floor returns connect to the return trunk and the trunk the extends to the furnace. All joints are sealed and all connections are tight. All ducts outside of conditioned space are insulated. Basements walls are insulated.

Do you think warm air rises into the second floor ceiling returns, cools, and travels down the attic trunk return and to the basement? Cool air is heavy. The first floor return, which are connected to the return trunk in the basement, allows the cooled air to enter the first floor. Basically air is circulating through this path.

Have you heard of this before? How to stop? A damper in the return trunk where it enters the basement would stop airflow when the furnace is off. Motorized dampers are more efficient and costly. Baromatric dampers are passive and cheaper, but they reduce the flowrate through the return (high resistance).

I installed an attice return because I did not have enough return on the second floor, hence retrofit. The work was expensive and I don't want to reduce the attic return capacity with a barometric damper (if barometric dampers create high resistance.)

Any thought anyone?