My new insulation dam is also an air dam. I used left over rigid insulation because it is easy to cut with my Leatherman and it is easy to handle and shape. The bottom edge is caulked to the osb wall sheathing. The joints between the dam and the framing and roof deck are sealed with low expanding foam. My intent is to block all potential air flow from the ceiling surface and to direct it up into the attic itself, where it will be drawn out by the ridge vents. Much later, when I am working on the inside of the attic, I will seal any air leaks I can see as I seal the nearby wall plates.