I am finishing my basement and am wondering how many supplies and returns I should put in the finished space. The finished area of the basement is about 14í x 35í ( about 490 sq ft, just under 4000 cubic feet). Itís a pretty well insulated space and Iím not even done insulating yet. I have about 1 inch foam insulation on the exterior, 1.5 inch thick EPS foam on the interior, and will eventually have R-13 fiberglass in the stud cavities for a total of R-24 on the exterior walls. The floor has 1 inch XPS insulation covered by 5/8Ē OSB. The rim joist has all been insulated and sealed with 2Ē XPS and spray foamed the edges. The room temperature as it sits right now is 65-68 degrees without any real supply of conditioned air. Itís been that temperature consistently over the past few weeks, even in the morning when it is below freezing right now in central Ohio. We normally run our upstairs heat in the 70-72 range, so we don't have far to go to stay comfortable in the basement... So I donít feel like Iíll need a bunch of supply ducts, but maybe Iím wrongÖ

I should note that it will be REALLY easy to put three supplies in. I can branch off each end of my bulkhead and create two supplies at each end of the finished space and one more near the middle of the room. Any more than that and it gets more difficult because I have to cut holes in the top or sides of the bulkheads elsewhere (which are difficult to get to). Is three supplies going to be enough for this space? Or should I go more?

As for return air I have direct access to an existing return duct that is practically right in the middle of the finished area. What is the rule of thumb for how big the return should be in relation to the supply? From what I've read does the square area of the return grill have to equal the area of the supply duct work? So if I put in three supply ducts at 6 inches in diameter each I would need close to 90 square inches of return grill or is it 90 square inches of return duct also? (I calculated the 90 square inches from radius squared * pi. Or about 28-29 square inches per 6 inch supply duct)

Any help would be greatly appreciated. Thanks!