Sorry for the length, I am looking for feedbacks and opinions here. I want to enclose a 12x12 room (with two large open walls now) currently open to the rest of the house, and convert it to a den. It is a very easy task from a framing perspective. The problem is, one of the (2) central return air vents/ducts for this zone is located in this room. The Return grill in this room is 20x20, with I am guessing at least a 15 inch round duct. There is a filter at the return grill. (The other filtered return located in another common room, is larger, at least 20x30 with 2 15 inchh ducts.) They feed a 5 ton unit. Current design makes extending that return to outside the newly enclosed room problematic. My solution is to install some pass through grilles in the new (2x4) wall I am erecting. My question is, how do I compute the total pass through grills I will need to add to the new wall, if I assume that there will be a one inch undercut beneath a set of (4 foot?) double doors and a 30 inch door as well. Will it even work to add these grilles, which I plan on offsetting to help with noise and light, in a 2 x 4 wall, or do I need to go to a 2x6 frame for more pass through space? Aesthetically, I can not put in 5 or 6 smaller grilles in the new walls, but I can probably get away with 3. Hope this was clear, help is appreciated.