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Sealing the air barrier. Caulk forms a dubious bond on most air barriers, such as Tyvek and Typar. So when sealing the outside of the window, it's best to caulk the flange (if there is one), or the exterior trim, directly to the sheathing. The seal along the top of the window, of course, is the most important. The house is usually wrapped after it's sheathed but before the windows are installed. And if the house wrap is run right across the window openings and simply X-cut, it is difficult to caulk to the plywood. Instead, cut the barrier about 6 inches horizontally at the top of