A perm is a measurement of what?

A perm is indeed a measurement of the permeability of a material to water vapor. More specifically, it quantifies how much water vapor can pass through a given area of a material over time. This measurement is crucial in building science, especially for understanding moisture control and ensuring adequate ventilation in multifamily buildings. Materials with high perm ratings allow significant moisture transfer, which can influence decisions on insulation, air barriers, and overall building envelope design to prevent moisture-related issues.

In contrast, the other options represent different material properties: electrical resistance relates to how well a material inhibits electric current; weight capacity pertains to the structural integrity and load-bearing abilities of materials; and thermal conductivity describes how well a material can conduct heat. Understanding these distinctions is critical when analyzing building materials and their performance in various applications.