I am remodeling my home, and I am adding more insulation to the walls and sealing all the air leaks. Basically, some rooms get more air than other rooms. I am wondering if it is possible to install a manual damper inside the duct or is it better to replace the branch duct with a smaller one.

For example, in the bathroom on the main floor, the current duct is a 6" round duct, which delivers about 100 CFM. However, the room is only 40 sq. feet and really only needs 30CFM, or a 4" round duct.

I plan on having a variable speed furnace installed that is sized correctly, because the current unit is a single stage and it basically dumps about half of the rated CFM into the basement, which makes it feel like an oven in the winter and a freezer in the summer.