All Duct Balloons are engineered to provide a tight seal after they become fully inflated. Based on the dimensional information you provide, it is engineered to expand larger than the actual duct work size. Our design permits small quantities of air supplied by the blower to bleed off at the seams so the balloon can never be over pressurized.  Being that the supplied blower remains on at all times while the Duct Balloon is installed, the air expanding inside the balloon pushes the perimeter edges outward in all directions against the interior wall of the duct. This “dynamic barrier” is what helps keep it in place and create a tight seal around the perimeter edges. Compared to homemade barriers like wood and plastic sheeting, this method works far better than those type of  “static barriers.”