Retrieving "Perms" from the archives

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1. Moisture Vapor Transmission

   Linked via "Perms"

   The Permeance ($M$) of a specific layer is defined as the permeability divided by the thickness ($d$) of that layer:
   $$M = \frac{k}{d}$$
   Permeance is often measured in Perms, particularly in North American contexts, where 1 Perm is equivalent to $1 \text{ grain} / (\text{ft}^2 \cdot \text{h} \cdot \text{inHg})$. Conversion between SI units and Perms is essential for interdisciplinary comparison, requiring a factor…