XPS itself acts as a highly effective vapor retarder due to its closed-cell structure, with a very low water vapor transmission perm rating (often less than 1.0 perm). In many wall and roof assemblies, especially in cold climates, XPS can serve as the primary vapor retarder on the exterior (cold) side of the insulation. This placement helps prevent warm, moist indoor air from condensing within the wall cavity as it moves outward and meets the cold sheathing.
Important: Building science must guide its use. In hot-humid climates or mixed climates, the placement and need for a vapor barrier/retarder change. It is essential to follow local building codes and the specific assembly design to avoid trapping moisture, which could lead to mold or rot. Professional consultation is recommended.
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