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– Housewrap to create an exterior air barrier (Tyvek is popular) – Polyethylene as an internal air barrier – Exterior sheathing for an external air barrier (Zip sheathing is the leading manufacturer)
A home will have more moisture inside during winter in cold climates than outdoors. Since heat moves from hot to cold, warm moisture-rich air will move into the wall cavity on its way outdoors.
Installing continously is much easier on the exterior of a home. Although, there are still challenges, such as where the exterior wall meets the roof framing.
In theory, you only need one air barrier; installing two makes sense. Two air barriers are common in Europe (where they’re ahead of the U.S. regarding home energy efficiency).