I have a brick three-bedroom rancher built in 1974. We blew insulation into the attic when we moved in, and last year installed brand new windows throughout the entire house. Yet when I close the doors to two bedrooms they get warmer in the summer and cooler in the winter. A lot of air that comes from the registers. There are also 31-inch air returns on the walls in both rooms. Is it possible that these are too big for the rooms and causing this problem? Or could it be something else?
If registers are letting air into rooms from unconditioned spaces, like an unfinished basement or crawlspace, it could be the cause - or at least a contributing factor. Hire a HVAC contractor who can measure airflow through those registers. If the registers are damanged or perhaps not even fully open, it could account for differences.