6 Inch Vs 8 Inch Duct . I need to decide between: R8 ducting is more efficient at heat insulation than r6 ducting. 4" to 5" 4"x5" Stove Pipe Reducer eBay from www.ebay.com Self help video on ducting your vent a hood kitchen ventilation unit. The area of rectangular ducts is calculated as x*y where x and y and the length of the sides of the ducts. After much research and a few suggestions here is my game plan for the lights.