The insulation on externally insulated ducts used for cooling should be covered with a vapor retarder that has a maximum permeance of what value?

The appropriate maximum permeance value for the vapor retarder on externally insulated ducts used for cooling is 0.05 perm. This specification is crucial because the primary function of a vapor retarder is to limit moisture transfer, which can be particularly problematic in cooling applications. When warm, moist air comes into contact with a cooler surface, condensation can occur, leading to potential issues such as mold growth, reduced insulation effectiveness, and even structural damage.

A vapor retarder with a low permeance, specifically at or below 0.05 perm, effectively prevents significant amounts of vapor from passing through, ensuring that the ducts remain dry and function efficiently. This low level of permeability is particularly important in cooling systems, where temperature differentials can cause moisture-related problems.

The other values listed are higher permeance limits that allow more moisture vapor to pass, making them less suitable for situations where controlling condensation is essential. A vapor retarder with a higher perm rating, such as 0.5 perm, 5 perm, or 0.005 perm, does not provide the same level of moisture control and could lead to the issues mentioned.