1 perm equals which of the following?

• A. 1 grain
• B. 10 grains
• C. 1 pound
• D. 1 ounce

Answer: (A)

The concept being tested is what a perm measures and how it’s defined. A perm is a unit of vapor permeance used in building science, describing how much water vapor can pass through a material per hour per square foot under a given pressure. By definition, one perm equals one grain of water vapor passing through one square foot in one hour when the driving pressure is one inch of mercury. So the correct match is that single grain per hour per square foot per inch of mercury. The other options are masses (ounces and pounds), not a measure of vapor transmission, and ten grains would correspond to ten perms, not one.