I have opted to seal the 8″ duct work for our ventilation system from the inside. Normally, sealant is applied on the outside seams but I think it results in a rather unattractive installation. I also chose to use rivets rather than sheet metal screws to secure sections of ducting. Screws, in my experience, can loosen up over time, something I have never seen with a properly set rivet. These details take extra time and no one will ever notice them. Creating innovative works of enduring value is one component of my personal mission statement. I think it is fair to say that my approach to ductwork falls under that banner.